Physics Briefings https://atlas.cern/ en Studying the Higgs boson in its most common – yet uncommonly challenging – decay channel https://atlas.cern/updates/briefing/studying-Higgs-decay-b-quarks <span>Studying the Higgs boson in its most common – yet uncommonly challenging – decay channel</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Tue, 12/01/2020 - 11:41</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">True</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/higgs-boson" hreflang="en">Higgs boson</a></div> <div class="field--item"><a href="/tags/higgs2020" hreflang="en">Higgs2020</a></div> </div> <div class="field field--name-field-image-caption field--type-string-long field--label-hidden field--item">Figure 1: ATLAS event display of a Higgs boson candidate produced through vector boson fusion in association with a high-energy photon, and then decaying to b quarks. The two b-quark jets from the Higgs boson decay are represented by cyan cones. The two muons inside the jets are shown with red lines. Other particle jets are shown as purple cones, and the high-energy photon is represented by a yellow cone. (Image: ATLAS Collaboration/CERN)</div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p>According to the Standard Model of particle physics, more than half of the Higgs bosons produced at the LHC decay to a pair of bottom (b) quarks. Despite that, measuring these decays was a major challenge that experimental physicists spent many years chasing. It was finally accomplished in the summer of 2018, when the ATLAS Collaboration <a href="https://atlas.cern/updates/press-statement/observation-higgs-boson-decay-pair-bottom-quarks">reported the observation</a> of Higgs bosons decaying to b-quark pairs (H→bb). The trick was to reduce the overwhelming background from the strong-interaction production of b-quark pairs by requiring the Higgs boson to be produced in association with a leptonically-decaying W or Z boson. </p> <p>Since then, measurements with more data have provided greater precision and deeper probes of this important interaction. Recent results from the ATLAS Collaboration, released for the Higgs 2020 conference, focus on different production modes of the Higgs boson decaying into b-quarks. These new analyses capitalise on the power of machine learning to better discriminate this particular process from other proton collision events.</p> <h3><strong>Fusing bosons</strong></h3> <figure class="right mobile-float img-60"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-045-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-045-1/file?size=large" /></a><figcaption>Figure 2: The reconstructed mass distribution for b-quark pairs produced in signatures consistent with vector boson fusion, after the expected Standard Model background has been subtracted from data. The contribution from Higgs boson decays is shown as a red histogram, and the contribution from Z boson decays is shown as a grey histogram, as fit to data. The hatched band shows the size of the background uncertainties. Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>The Higgs boson can be produced through vector boson fusion (VBF), where quarks from colliding LHC protons radiate W or Z bosons, which in turn annihilate (or “fuse”) to create a Higgs boson. VBF events have a distinctive signature in the ATLAS experiment, resulting in high-energy particle jets in the forward regions of the detector and a lack of activity in the central region.</p> <p>The ATLAS Collaboration has released two new results for VBF production of Higgs bosons that decay into two b-quarks. The results use advanced methods to enhance the Higgs boson signal relative to the background. In the <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2020-14/">first case</a>, as the fusion of two W bosons can result in the creation of photons, physicists looked for proton collision events with a photon present in addition to the VBF Higgs-boson signal. As Z bosons do not radiate photons, this measurement is uniquely sensitive to the difference between W-boson fusion and Z-boson fusion. </p> <p>For their <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2019-04/">second VBF analysis</a>, ATLAS physicists used a machine-learning technique called an adversarial neural network (ANN) to gain the advantage over the main source of background (events with b-quarks from sources other than Higgs bosons). The Higgs boson signal strength – defined as the ratio of the measured rate to the Standard Model prediction – could be determined by examining the invariant mass of the two b-quarks in different bins of the neural network output.</p> <p>When combined, the two VBF analyses give a signal strength of 0.99 ± 0.35, exactly in line with the Standard Model (1).</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">These new studies of the H→bb decay focus on different Higgs-boson production modes: via vector boson fusion (VBF) and in association with a pair of top quarks (ttH).</h3> <hr class="divider"> <div class="narrow"> <h3><strong>Most uncommon of the common</strong></h3> <figure class="right mobile-float img-60"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-045-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-045-2/file?size=large" /></a><figcaption>Figure 3: Summary of Higgs boson measurements in the ttH production mode, with subsequent decay to b-quark pairs. The results are presented in different intervals of Higgs boson momentum, and the sensitivity to new particles increases with the Higgs boson momentum. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>ATLAS physicists also tried their hand at uncommon searches. The associated production of the Higgs boson with a pair of top quarks (ttH) is the rarest Higgs production mode observed so far, a hundred times smaller than the dominant Higgs boson production through the fusion of two gluons. It gives direct access to the coupling between the Higgs boson and the top quark. </p> <p>A <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-058/">new result</a> from the ATLAS Collaboration studies proton collision events in which Higgs bosons decay to a pair of b-quarks, and top-quark pairs decay into a final state with one or two leptons and particle jets. Again here, the biggest challenge is the separation of the signal from the background events. ATLAS researchers used multivariate analysis techniques, including a Deep Neural Network, in both the “resolved regime” – where the Higgs boson momentum is small and the particle jets arising from the two b-quarks are distinct from each other – and in the “boosted regime” – where the Higgs boson momentum is large and the two b-quark jets merge together.</p> <p>The combination of these two regimes allowed ATLAS physicists to probe – for the first time – the ttH(bb) signal strength as a function of the Higgs boson transverse momentum. The measurements were performed in mutually-exclusive regions called “Simplified Template Cross Section (STXS) bins”. They allow researchers to constrain physics beyond the Standard Model and obtain sensitivity to the <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2019-049/">Higgs boson self-coupling</a>.</p> <p>The inclusive signal strength measurement in the ttH production mode is 0.43 ± 0.35, lower than the Standard Model prediction of 1.0 but still compatible with it. Five independent signal strengths were measured for the STXS analysis, all are compatible with the inclusive result.</p> <h3><strong>Combination for new insight </strong></h3> <p>These new results join a <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2018-52/">recent study</a> of specific high-momentum (“boosted”) Higgs boson production in association with a leptonically-decaying W or Z boson. Here the Higgs boson decays into two close-by b-quarks, reconstructed as a single large-radius hadronic jet with a transverse momentum greater than 250 GeV. Effects beyond the Standard Model could modify the production rate of Higgs bosons at high momentum. The measured signal strength was found to be 0.72 ± 0.38, in agreement with the Standard Model prediction. </p> <p>ATLAS researchers will continue to pursue Higgs boson studies in its dominant decay mode. These are particularly relevant in kinematic regions where Higgs boson production is limited by the available data statistics, such as at high momentum. By combining and comparing these new results with earlier results, the interaction of the Higgs boson with the other elementary particles is being elucidated with ever-increasing precision.</p> <hr class="divider"> <h3>Links</h3> <ul> <li>CERN Seminar by Matt Klein: <a href="https://indico.cern.ch/event/941485/">Latest ATLAS results on H-&gt;bb decays and interpretation of combined Higgs measurements</a></li> <li><a href="https://arxiv.org/abs/2010.13651">Search for Higgs boson production in association with a high-energy photon via vector-boson fusion with decay into bottom quark pairs at 13 TeV with the ATLAS detector</a> (arXiv: 2010.13651, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2020-14/">see figures</a>)</li> <li><a href="https://arxiv.org/abs/2011.08280">Measurements of Higgs Bosons Decaying to Bottom Quarks from Vector Boson Fusion Production with the ATLAS Experiment at 13 TeV</a> (arXiv: 2011.08280, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2019-04/">see figures</a>)</li> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-058/">Measurement of the Higgs boson decaying to b-quarks produced in association with a top-quark pair in proton–proton collisions at 13 TeV with the ATLAS detector</a> (ATLAS-CONF-2020-058)</li> <li><a href="https://arxiv.org/abs/2008.02508">Measurement of the associated production of a Higgs boson decaying into b-quarks with a vector boson at high transverse momentum in proton–proton collisions at 13 TeV with the ATLAS detector</a> (arXiv: 2008.02508, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2018-52/">see figures</a>)</li> <li><a href="http://atlas.cern/updates/physics-briefing/measuring-beauty-higgs-boson">Measuring the beauty of the Higgs boson</a>, <em>Physics Briefing, </em>7<em> </em>April 2020<em> </em></li> <li><a href="https://atlas.cern/updates/press-statement/observation-higgs-boson-decay-pair-bottom-quarks">ATLAS observes elusive Higgs boson decay to a pair of bottom quarks</a><em>, Press Statement, </em>28 August 2018</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Tue, 01 Dec 2020 10:41:00 +0000 Steven Goldfarb 34361 at https://atlas.cern Refining the picture of the Higgs boson https://atlas.cern/updates/briefing/refining-picture-higgs-boson <span>Refining the picture of the Higgs boson</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Thu, 11/19/2020 - 12:22</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">True</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-subtitle field--type-text field--label-hidden field--item">Investigating properties of the Higgs boson using correlations between particle jets</div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/higgs-boson" hreflang="en">Higgs boson</a></div> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/higgs2020" hreflang="en">Higgs2020</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p>To explain the masses of electroweak bosons – the W and Z bosons – theorists in the 1960s postulated a mechanism of spontaneous symmetry breaking. While this mathematical formalism is relatively simple, its cornerstone – the <a href="https://atlas.cern/updates/atlas-feature/higgs-boson">Higgs boson</a> – remained undetected for almost 50 years! </p> <p>Since its discovery in 2012, researchers of the ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) have tirelessly investigated the properties of the Higgs boson. They’ve measured its mass to be around 125 GeV – that’s about 130 times the mass of the proton at rest – and found it has zero electric charge and spin. </p> <h3><strong>The mirror image</strong></h3> <figure class="right mobile-float img-60"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-043-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-043-2/file?size=large" /></a><figcaption>Figure 1: The weighted distribution of the azimuthal angle between two jets in the signal region used in the CP measurement. The signal and background yields are determined from the fit. Data-to-simulation ratios are shown at the bottom of the plot. The blue histogram represents measured signal; the shaded areas depict the total uncertainty. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Researchers set out to determine the Higgs boson’s parity properties by measuring its decays to pairs of W bosons (H → WW*), Z bosons (H → ZZ*) and to photons (H → γγ). Through these measurements, they confirmed that the Higgs boson has even charge-parity (CP). This means that – as predicted by the Standard Model – the Higgs boson’s interactions with other particles do not change when “looking” in the CP mirror.<strong> </strong>As any distortions in this CP mirror (or “CP violation in Higgs interactions”), such as CP-odd admixture, would indicate the presence of as-yet undiscovered phenomena, physicists at the LHC are scrutinising the strengths of Higgs-boson couplings very carefully. A <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-055/">new result from the ATLAS Collaboration</a>, released for the Higgs 2020 conference, aims at enriching the Higgs picture by studying its WW* decays. </p> <p>One new ATLAS study examines the CP nature of the effective coupling between the Higgs boson and gluons (the mediator particles of the strong force). Until now, the gluon-fusion-induced production of a Higgs boson, in association with two particle jets, had not been studied in a dedicated analysis. The study of this production mechanism is an excellent way to search for signs of CP violation, as it affects the Higgs-boson kinematics, leaving a trace in the azimuthal angle between the jets measured by ATLAS.</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The new result from the ATLAS Collaboration, released for the Higgs 2020 conference, aims at enriching the Higgs picture by studying its WW* decays.</h3> <hr class="divider"> <div class="narrow"> <h3><strong>Polarisation filter </strong></h3> <p>At high energies, the weak and electromagnetic forces merge into a single electroweak force. Yet at low energies, electromagnetic waves (such as light) can travel an infinite distance, while weak interactions have a finite range. This is because unlike photons (the carriers of the electromagnetic force), W and Z bosons are massive. Their masses originate from interactions with the Higgs field. </p> <figure class="left mobile-float img-60"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-043-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-043-1/file?size=large" /></a><figcaption>Figure 2: The weighted distribution of the azimuthal angle between two jets in the signal region used in the polarisation measurement. The signal and background yields are determined from the fit. Data-to-simulation ratios are shown at the bottom of the plot. The red histogram represents measured signal; the shaded areas depict the total uncertainty. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Another difference is that electromagnetic waves are transverse; oscillations in the electromagnetic field only occur in the plane perpendicular to its propagation. W and Z bosons, on the other hand, have both longitudinal and transverse polarisations due to their interactions with the Higgs field. There is a subtle interplay between these longitudinal polarisations and the boson masses that ensures that Standard Model predictions remain finite. </p> <p>Should the Higgs boson not be a fundamental scalar particle, and instead an entity arising from new dynamics, a different (more complicated) mechanism would have to give mass to the W and Z bosons. In such a case, the measured Higgs-boson couplings with electroweak bosons may deviate from the predicted Standard Model values. </p> <p>The ATLAS Collaboration has released its first study of individual polarisation-dependent Higgs-boson couplings to massive electroweak bosons. Specifically, physicists examined the production of Higgs bosons through vector-boson fusion in association with two jets. Just as a polarising filter helps you to take a sharper picture at a seaside by selectively absorbing polarised light, this new ATLAS study investigated individual Higgs-boson couplings to longitudinally and transversely polarised electroweak bosons. Further, similar to the study of the Higgs-boson coupling to gluons, the presence of a new mechanism would impact the kinematics of the jets measured by ATLAS.</p> <h3><strong>Follow those jets! </strong></h3> <p>The main challenge of these analyses is the rarity of the Higgs-boson events being studied. For the signal selections studied in the new ATLAS result, only about 60 Higgs bosons are observed via gluon fusion and only 30 Higgs bosons via vector-boson fusion. Meanwhile, background events are almost a hundred times more abundant. To tackle this challenge, both analyses not only counted events but also looked into the shapes of the azimuthal angle (the angle transverse to the direction of the proton beams) between the two jets. The correlation between these jets has helped resolve properties of Higgs-boson production. </p> <p>Researchers used the technique of parameter morphing to interpolate and extrapolate the distribution of this angle from a small set of coupling benchmarks to a large variety of coupling scenarios.The fitted distributions of the azimuthal angle between the jets are shown in Figures 1 and 2. </p> <p>So far, both distributions show no sign of new physics. Once more LHC data is analysed (these studies only include data collected in 2015 and 2016), the shaded areas in the plots that represent the measurement’s uncertainty should decrease. This will provide an even sharper picture of the Higgs boson.</p> <hr class="divider"> <h3><strong>Links</strong></h3> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-055/">Constraints on Higgs boson properties using WW*(→eνμν)jj production in 36.1fb<sup>−1</sup> of 13TeV proton-proton collisions with the ATLAS detector</a> (ATLAS-CONF-2020-055)</li> <li><a href="https://cds.cern.ch/record/2066980">A morphing technique for signal modelling in a multidimensional space of coupling parameters</a> (ATL-PHYS-PUB-2015-047)</li> <li><a href="https://arxiv.org/abs/1410.7388">Interpolation between multi-dimensional histograms using a new non-linear moment morphing method</a> (arXiv:1410.7388)</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Thu, 19 Nov 2020 11:22:00 +0000 Steven Goldfarb 33268 at https://atlas.cern ATLAS uses the Higgs boson as a tool to search for Dark Matter https://atlas.cern/updates/briefing/higgs-boson-search-dark-matter <span>ATLAS uses the Higgs boson as a tool to search for Dark Matter</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Thu, 10/29/2020 - 09:56</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">True</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/new-physics" hreflang="en">new physics</a></div> <div class="field--item"><a href="/tags/dark-matter" hreflang="en">dark matter</a></div> <div class="field--item"><a href="/tags/higgs-boson" hreflang="en">Higgs boson</a></div> <div class="field--item"><a href="/tags/higgs2020" hreflang="en">Higgs2020</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p>One of the great unexplained mysteries is the nature of <a href="http://atlas.cern/updates/atlas-feature/dark-matter">dark matter</a>. So far, its existence has only been established through gravitational effects observed in space; no dark-matter particles with the needed properties have (yet) been detected. Could the <a href="http://atlas.cern/updates/atlas-feature/higgs-boson">Higgs boson</a> be the key to their discovery?</p> <h3><strong>Looking for dark matter in particles of light </strong></h3> <p>The high-energy proton–proton collisions of the LHC could be the perfect environment to study dark-matter particles. In particular, if these elusive particles are produced in association with the Higgs boson, the ATLAS experiment should be sensitive to their presence. Although the dark-matter particles would escape the direct detection – as they would not interact with the sensitive material of the detectors – they could result in collision events with a substantial amount of missing transverse momentum. The experiment would detect a large momentum imbalance among the reconstructed particles in the plane perpendicular to the LHC beam.</p> <p>At this week’s <a href="https://indico.cern.ch/event/900384/">Higgs 2020 conference</a>, the ATLAS Collaboration presented a new search for dark-matter particles produced in association with a Higgs boson decaying to two photons. ATLAS researchers studied the full Run 2 dataset collected between 2015 and 2018, selecting for events with two high-quality photons and large missing transverse momentum. Their search focused on three types of dark-matter models – namely the Z<sub>’B</sub>, Z’-2HDM, and 2HDM+a models – all of which include new vector or pseudoscalar mediator particles. </p> <p>To identify a potential dark-matter signal, the ATLAS physicists used a machine-learning algorithm (<em>boosted decision tree</em>) to spit events into categories with increasing sensitivity by analysing the kinematic information of the selected photon pair and the event’s missing transverse momentum. In each event category, they looked at the invariant-mass spectrum of the two photons, where a Higgs boson produced in association with dark-matter particles would show an enhanced peak at around 125 GeV over a smooth, non-Higgs-boson background. </p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-042-2" title="View on CDS"><img alt="Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-042-2/file?size=large" /></a><figcaption>Figure 1: Diphoton invariant mass spectrum in the most sensitive event category (“high MET BDT tight”), comparing the data (black) with the sum of the fitted signal and background (blue). The fitted signal (red) and the background (green) are also shown respectively. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Figure 1 shows the diphoton invariant-mass spectrum in the most sensitive event category, comparing the data with the fitted number of expected signal and background events. The data has a small mass peak at around 125 GeV that is fully consistent with a fluctuation of the Standard Model background. </p> <p>The ATLAS Collaboration observed no significant excess of signal events in any of the categories, and thus set limits on three dark-matter models. In the context of the Z’B model, for dark-matter particles of 1 GeV, the new result extends the limit on the Z’<sub>B</sub> mediator mass to 1150 GeV and significantly improves upon <a href="https://journals.aps.org/prd/abstract/10.1103/PhysRevD.96.112004">previous ATLAS results</a>. </p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The new results from the ATLAS Collaboration set strong constraints on many dark-matter models, making important progress in the ongoing search for new physics.</h3> <hr class="divider"> <div class="narrow"> <h3><strong>Seeking out “invisible” particles</strong></h3> <p>Another ATLAS search for dark matter looked for Higgs-boson decays into particles invisible to the detector, leading to missing transverse momentum. Such decays are actually possible in the Standard Model; the Higgs boson can decay via two Z bosons into four neutrinos, which escape the detector undetected and thus resemble dark matter particles. However, this only happens in ~0.1% of Higgs-boson decays and is thus negligible; if ATLAS were to see such events, it would be a clear sign of new physics. </p> <figure class="right mobile-float img-60"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-042-1" title="View on CDS"><img alt="Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-042-1/file?size=large" /></a><figcaption>Figure 2: Expected (dashed) and observed (solid) limits on the invisible decay branching fraction for the production channels of a Higgs boson in association with a top-antitop quark pair (ttH) and through vector boson fusion (VBF) as well as the combined result from Run 2, from Run 1, and the combined Run 1+2 result, respectively. The one (two) standard deviation uncertainty band on the expected limit is shown in green (yellow). (Image: ATLAS Collaboration/CERN) <h3></figcaption></figure></h3> <p>Since all known massive elementary particles interact with the Higgs boson, it is reasonable to assume that as-yet undiscovered dark-matter particles would behave the same way. New-physics models where dark matter interacts with known particles through the Higgs boson are called “Higgs portal” models. If the dark matter particles have a mass of less than half the Higgs-boson mass, the Higgs boson could decay into a pair of dark-matter particles. </p> <p>ATLAS physicists considered different production mechanisms of the Higgs boson to search for such events in different event topologies; to increase the overall sensitivity, these searches can be statistically combined. Such a combination was <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.231801">conducted by ATLAS</a> in 2019 with partial Run 2 data, resulting in an upper limit of 26% (17% expected) on the Higgs-to-invisible decay probability (<em>branching fraction</em>).</p> <p>The ATLAS Collaboration has now released a new combination using the full LHC Run 2 dataset, further constraining the invisible decay channel. The result combines several measurements, including the recent search for invisible particles via Higgs bosons produced through <a href="https://atlas.cern/updates/physics-briefing/probing-dark-matter-higgs-boson">vector-boson fusion</a> – a result which, by itself, already sets an upper limit of 13% on the Higgs-to-invisible branching fraction. The new combination also incorporates studies of the Higgs boson produced in association with a top–antitop pair, focusing on final states with <a href="https://link.springer.com/article/10.1140/epjc/s10052-020-8102-8">zero</a> or <a href="https://cds.cern.ch/record/2728056">two leptons</a>. Physicists were able to reinterpret these results – originally conducted outside the Higgs-to-invisible context – to look for dark-matter particles.</p> <p>ATLAS researchers then went one step further, combining their analysis of the full Run 2 dataset with a similar Higgs-to-invisible analysis using the full <a href="https://link.springer.com/article/10.1007/JHEP11(2015)206">Run 1 dataset</a>, which examined the Higgs-boson production via vector-boson fusion and its associated production with a W or Z boson. Their final result found no evidence for invisible particles, and thus physicists set a new upper limit of 11% (11% expected) on the Higgs-to-invisible branching fraction, at a 95% confidence level. This is the strongest direct limit on this process up to date.</p> <h3><strong>Making important progress </strong></h3> <p>The new results from the ATLAS Collaboration set strong constraints on many dark-matter models, making important progress in the ongoing search for new physics. In the future, ATLAS physicists will further improve the discovery potential of using the Higgs boson as a tool to search for dark matter and other new physics phenomena. </p> <hr class="divider"> <h3>Links</h3> <div class="span1of2"> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-054/">Search for dark matter in events with missing transverse momentum and a Higgs boson decaying to two photons in proton–proton collisions at 13 TeV with the ATLAS detector</a> (ATLAS-CONF-2020-054)</li> <li>CMS Collaboration: <a href="https://arxiv.org/abs/1806.04771">Search for dark matter produced in association with a Higgs boson decaying to γγ or τ<sup>+</sup>τ<sup>-</sup> at 13 TeV</a> (arXiv:1806.04771)</li> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-052/">Combination of searches for invisible Higgs boson decays with the ATLAS experiment</a> (ATLAS-CONF-2020-052, <em>link coming soon</em>)</li> <li>CMS Collaboration: <a href="https://arxiv.org/abs/1809.05937">Search for invisible decays of the Higgs boson produced through vector boson fusion at 13 TeV</a> (arXiv:1809.05937)</li> <li>Higgs 2020 presentation by Samuel Ross Meehan: <a href="https://indico.cern.ch/event/900384/contributions/4064241/">Dark Matter produced in association with Higgs at ATLAS and CMS</a></li> <li>Higgs 2020 presentation by Prasanna Kumar Siddireddy: <a href="https://indico.cern.ch/event/900384/contributions/4064248/">LFV, rare decays and invisible Higgs decays at ATLAS and CMS</a></li> <li><a href="https://atlas.cern/updates/physics-briefing/probing-dark-matter-higgs-boson">Probing dark matter with the Higgs boson</a>, Physics Briefing, April 2020</li> <li>See also full lists of the ATLAS Conference Notes and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div> <div class="span1of2 last"> <p><iframe allow="encrypted-media" allowfullscreen="true" allowtransparency="true" frameborder="0" height="300" scrolling="no" src="https://www.facebook.com/plugins/video.php?height=300&amp;href=https%3A%2F%2Fwww.facebook.com%2FATLASexperiment%2Fvideos%2F664236561130501%2F&amp;show_text=false&amp;width=300" style="border:none;overflow:hidden" width="300"></iframe></p> </div><div style="clear: both; height: 0;"></div> </div><div style="clear: both; height: 0;"></div> </div> Thu, 29 Oct 2020 08:56:00 +0000 Steven Goldfarb 33198 at https://atlas.cern Higgs boson probes for new phenomena https://atlas.cern/updates/briefing/higgs-boson-probes-new-phenomena <span>Higgs boson probes for new phenomena</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Wed, 10/28/2020 - 11:09</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-subtitle field--type-text field--label-hidden field--item">ATLAS sets constraints on effective field theory and supersymmetric models</div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/new-physics" hreflang="en">new physics</a></div> <div class="field--item"><a href="/tags/supersymmetry" hreflang="en">supersymmetry</a></div> <div class="field--item"><a href="/tags/higgs-boson" hreflang="en">Higgs boson</a></div> <div class="field--item"><a href="/tags/higgs2020" hreflang="en">Higgs2020</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p>LHC physicists are on the hunt for many different forms of phenomena beyond the Standard Model. Some theories predict an as-yet undiscovered particle could be found in the form of a new resonance (a narrow peak) similar to the one that heralded the discovery of the Higgs boson in 2012. </p> <p>However, Nature is not always so kind and new resonances may be so massive that their production requires collision energies beyond that of the LHC. If so, all is not lost. Just as gently sloping terrain may indicate the presence of a mountain peak ahead, LHC data may contain some hints that interesting phenomena are present at higher energy scales. </p> <h3><strong>A very effective model</strong></h3> <figure class="right mobile-float img-50"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-041-1" title="View on CDS"><img alt="Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-041-1/file?size=large" /></a><figcaption>Figure 1: Allowed ranges for the coupling coefficients of new EFT interactions. The coefficient cHq(3), for example, describes the strength of an effective four-particle interaction between two quarks, a gauge boson and the Higgs boson – which is not present in the Standard Model. The Standard Model prediction for these coefficients is zero. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Instead of looking for a new particle, physicists can look for new types of interactions, not present in the Standard Model. Since their underlying mechanisms are unknown, these interactions are called “effective” interactions, and their framework “effective field theory” (EFT). Almost all types of new physics give rise to these new interactions, with different theoretical models leaving different footprints on the EFT. However, the effects can be subtle, especially if the high-mass phenomena are far beyond the reach of the LHC’s collision energy. </p> <p>Since these additional interactions would affect all physics processes, ATLAS scientists are implementing a new search strategy that combines measurements across the full spectrum of their research programme. A <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-053/">new ATLAS analysis</a> released today uses combined measurements of the properties of the Higgs boson to search for signs of new phenomena using this EFT framework. As no such new phenomena have been seen, physicists set constraints on their magnitude. Out of all the possible new interactions between Standard Model particles, only a subset related to the Higgs boson could be tested (those studied in the original combined measurement, which includes Higgs-boson decays to two b-quarks, two photons, and four leptons). </p> <p>Figure 1 shows the allowed ranges for the coupling coefficients of new EFT interactions to which the ATLAS analysis is sensitive. The Standard Model requires all of these coefficients to be zero, as the interactions are not present. Significant positive or negative deviations would indicate new phenomena. </p> <p>All of ATLAS’ measurements are compatible with the Standard Model indicating that – if new physics is present – it is either at energy scales larger than 1 TeV (the reference mass scale for which these results are reported) – or manifests itself in other interactions not probed by this study. In the meantime, thanks to the design of the analysis, the results can be added to wider combinations, with EFT measurements obtained in other measurement channels and even in other experiments.</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">ATLAS scientists are implementing a new strategy in the search for physics beyond the Standard Model – one that combines measurements across the full spectrum of the Collaboration's research programme. </h3> <hr class="divider"> <div class="narrow"> <h3><strong>A super model</strong></h3> <figure class="right mobile-float img-50"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-041-2" title="View on CDS"><img alt="Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-041-2/file?size=large" /></a><figcaption>Figure 2: Exclusion ranges for the Mh125(χ) scenario, in terms of the two model parameters: the mass of the pseudoscalar A and another model parameter, tan β, which together and in first approximation determine the extended Higgs boson sector of the MSSM. The blue-dashed and purple areas are excluded by the direct searches and the yellow area is excluded by the new measurement based on the Higgs boson properties. The gray area is excluded since the resulting MSSM Higgs boson mass would not be compatible with the measured value of 125.09 GeV. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>The Minimal Supersymmetric Standard Model (MSSM) is an extension of the Standard Model which predicts (in addition to a plethora of other new particles) a total of 5 Higgs bosons – two scalar (<em>h</em> and <em>H</em>), a pseudoscalar (<em>A</em>), and two charged Higgs bosons (<em>H<sup>+/-</sup></em>) – as well as possible modifications to the interactions of the observed 125 GeV Higgs boson. </p> <p>Physicists use two complementary strategies to search for hints of the MSSM: looking directly for new particles, or indirectly through precise measurements of the Higgs boson’s properties. In another new analysis released by the ATLAS Collaboration, researchers followed the latter strategy, using the <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-027/">latest combination</a> of Higgs couplings measurements in all accessible decay channels to set constraints on MSSM parameters. They explored several MSSM benchmark scenarios, all of which assumed the 125 GeV Higgs boson to be the lightest scalar <em>h</em>. </p> <p>An example is shown in Figure 2, in which some of the new particles predicted in the model are relatively light. It shows that not only are large ranges of parameter space excluded, but that these exclusions also nicely complement those from previously-performed direct searches. </p> <h3><strong>So far, the Standard Model wins</strong></h3> <p>ATLAS’ new results set constraints on the possible nature of new physics under the EFT framework and exclude large swaths of parameter space in MSSM scenarios. Their success is but the first step in the new combined-measurement search strategy. By expanding the scope of future measurements to include more analyses – including those involving vector bosons and top quarks – and adding more data, physicists plan to give the Standard Model an even tougher challenge.</p> <hr class="divider"> <h3>Links</h3> <div class="span1of2"> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-053/">Interpretations of the combined measurement of Higgs boson production and decay</a> (ATLAS-CONF-2020-053)</li> <li>Higgs 2020 conference talk by Brian Moser: <a href="https://indico.cern.ch/event/900384/contributions/4063544/">Higgs EFT measurements in ATLAS</a></li> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-027/">A combination of measurements of Higgs boson production and decay using up to 139 fb<sup>−1</sup> of proton–proton collision data at 13 TeV collected with the ATLAS experiment</a> (ATLAS-CONF-2020-027)</li> <li><a href="https://e-publishing.cern.ch/index.php/CYRM/issue/view/32">Handbook of LHC Higgs Cross Sections: 4. Deciphering the Nature of the Higgs Sector</a> (CERN-2017-002-M)</li> <li>Benchmark theory papers for MSSM: <a href="https://arxiv.org/abs/1808.07542">MSSM Higgs Boson Searches at the LHC: Benchmark Scenarios for Run 2 and Beyond</a> (arXiv: 1808.07542) and <a href="https://arxiv.org/abs/1901.05933">MSSM Higgs Benchmark Scenarios for Run 2 and Beyond: the low tan β region</a> (arXiv: 1901.05933)+</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div> <div class="span1of2 last"> <p class="rtecenter"><iframe allow="encrypted-media" allowfullscreen="true" allowtransparency="true" frameborder="0" height="300" scrolling="no" src="https://www.facebook.com/plugins/video.php?height=300&amp;href=https%3A%2F%2Fwww.facebook.com%2FATLASexperiment%2Fvideos%2F372852944156945%2F&amp;show_text=false&amp;width=300" style="border:none;overflow:hidden" width="300"></iframe></p> </div><div style="clear: both; height: 0;"></div> </div><div style="clear: both; height: 0;"></div> </div> Wed, 28 Oct 2020 10:09:00 +0000 Steven Goldfarb 33197 at https://atlas.cern Leptons at a distance: a new search for long-lived particles https://atlas.cern/updates/briefing/leptons-at-distance <span>Leptons at a distance: a new search for long-lived particles</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Wed, 10/07/2020 - 11:33</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-subtitle field--type-text field--label-hidden field--item">ATLAS result sets new constraints on supersymmetric partners of electrons, muons and taus</div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/susy" hreflang="en">SUSY</a></div> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/long-lived-particles" hreflang="en">long-lived particles</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-040-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-040-1/file?size=large" /></a><figcaption>Figure 1: The efficiency of reconstructing a lepton from the decay of a long-lived particle, measured in simulated events, shown as a function of the distance between the lepton track and the collision point (d0). The solid blue circles show the efficiency using standard ATLAS reconstruction techniques. The solid purple squares indicate the efficiency using additional tracking for displaced particles and special identification criteria developed for this search. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Despite its decades of predictive success, there are important phenomena left unexplained by the Standard Model of particle physics. Additional theories must exist that can fully describe our Universe, even though definitive signatures of particles beyond the Standard Model have yet to turn up. </p> <p>ATLAS researchers are broadening their extensive search programme to look for more unusual signatures of unknown physics, such as long-lived particles. These new particles would have lifetimes of 0.01 to 10 ns; for comparison, the Higgs boson has a lifetime of 10<sup>–13</sup> ns. A theory that naturally motivates long-lived particles is <a href="https://atlas.cern/updates/atlas-feature/supersymmetry">supersymmetry</a> (SUSY). SUSY predicts that there are “superpartner” particles corresponding to the particles of the Standard Model with different spin properties. </p> <p>A <a href="https://cds.cern.ch/record/2740685">new search</a> from the ATLAS Collaboration – released this week for the 5th International Conference on Particle Physics and Astrophysics (<a href="https://indico.particle.mephi.ru/event/35/">ICPPA-2020</a>), held virtually from Moscow – looks for the superpartners of the electron, muon and tau lepton, called “sleptons” (“selectron”, “smuon”, and “stau”, respectively). The search considers scenarios where sleptons would be produced in pairs and couple weakly to their decay products and so become long-lived. In this model, each long-lived slepton would travel some distance (depending on their average lifetime) through the detector before decaying to a Standard Model lepton and a light undetectable particle. Physicists would thus observe two leptons that seem to come from different locations than where the proton–proton collision occurred. </p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">Previous best limits on these long-lived particles came from the LHC's predecessor: the Large Electron–Positron Collider (LEP). This new result is the first to make a statement on this model using LHC data.</h3> <hr class="divider"> <div class="narrow"> <figure class="right mobile-float img-50"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-040-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-040-2/file?size=large" /></a><figcaption>Figure 2: Upper limits set by the analysis on the lifetime of possible sleptons as a function of the slepton mass. The solid lines indicate the observed limit, the dotted lines show the limit expected in the case of no statistical fluctuations, and the coloured regions are excluded by the analysis result. The excluded area is smaller for staus than for selectrons and smuons because it depends on the produced Standard Model taus decaying to electrons or muons. The dependence of the limits on the slepton mass stems mostly from the slepton-pair production cross section that strongly decreases with mass. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>This unique signature presented a challenge for physicists. Although many theories predict particles that could travel in the ATLAS detector for some time before decaying, typical data reconstruction and analysis is oriented towards new particles that would decay instantaneously, the way heavy Standard Model particles do. ATLAS physicists thus had to develop new methods of identifying particles in order to increase the likelihood of reconstructing these “displaced” leptons. Figure 1 shows the large improvements in lepton reconstruction efficiency resulting from this work as a function of the lepton’s distance from the collision (d0). Only displaced electrons and muons were studied in this analysis, but the results could be applied to taus as well, since taus decay promptly into an electron or a muon in around one third of cases.</p> <p>Because the particles created by the decay of a long-lived particle would appear away from the collision, unusual background sources can arise: photons mis-identified as electrons, muons that are mis-measured, and poorly measured cosmic-ray muons. Cosmic-ray muons come from high-energy particles colliding with our atmosphere and can traverse the ATLAS detector. Since they do not necessarily pass through the detector near the collision point, they can appear as if originating from a long-lived particle decay. ATLAS physicists have developed techniques not only for reducing these sources’ contributions but also for estimating how much each contributes to the search.</p> <p>The analysis did not find any collision events with displaced leptons that passed the selection requirements, a result that is consistent with the low expected background abundance. Using these results, physicists set limits on the slepton mass and lifetime, as shown in Figure 2. For the slepton lifetime that this search is most sensitive to (around 0.1 nanoseconds) ATLAS was able to exclude selectrons and smuons up to a mass of around 700 GeV, and staus up to around 350 GeV. The previous best limits on these long-lived particles were around 90 GeV and came from the experiments on the Large Electron–Positron Collider (LEP), CERN’s predecessor to the LHC. This new result is the first to make a statement on this model using LHC data.</p> <hr class="divider"> <h3>Links</h3> <ul> <li><a href="https://cds.cern.ch/record/2740685">Search for displaced leptons in 13 TeV proton-proton collisions with the ATLAS detector</a> (ATLAS-CONF-2020-051)</li> <li>ICPPA-2020 talk by Lesya Horyn: <a href="https://indico.particle.mephi.ru/event/35/contributions/2386/">Searches for New Long-lived Particles with the ATLAS detector</a></li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Wed, 07 Oct 2020 09:33:00 +0000 Steven Goldfarb 24674 at https://atlas.cern Z bosons zoom through quark–gluon plasma as jets quench https://atlas.cern/updates/briefing/qgp-z-bosons-jet <span>Z bosons zoom through quark–gluon plasma as jets quench</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Tue, 08/25/2020 - 19:42</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-subtitle field--type-text field--label-hidden field--item">With new data from the LHC, ATLAS physicists have measured jet-quenching phenomena in the quark–gluon plasma with help of Z bosons.</div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/heavy-ion" hreflang="en">Heavy Ion</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-033-1" title="View on CDS"><img alt="Event Displays,Physics,Heavy Ion Collisions,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-033-1/file?size=large" /></a><figcaption>Figure 1: ATLAS event display showing the Z+jet process occurring in a lead–lead collision. In this event, the Z boson is identified by its decay to two muons (red lines). The jet can be seen as a small, collimated set of blue towers, surrounded by the transparent green cone in the lower image. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>When beams of lead ions collide head-on in the LHC, the matter comprising the nuclei melts away and forms a high-temperature quark–gluon plasma (QGP) – an extended region of interacting quarks and gluons. As high-momentum jets of particles attempt to traverse this region, their energy and radiative properties change through interactions with the QGP medium. This phenomenon is known as <em>jet quenching</em>. Its study can help physicists understand the properties of the QGP and give new insight into the theory of the strong nuclear force (quantum chromodynamics).</p> <p>The ATLAS Collaboration has performed extensive studies of how jets are quenched in the QGP. The emerging picture is that the quarks and gluons lose energy in the medium, fragmenting into fewer high-momentum particles. The remaining energy is redistributed by the QGP and appears as low-momentum, “thermal” particles over a broad area around the jet. For example, studies have shown that the total momentum in a jet is <a href="https://www.sciencedirect.com/science/article/pii/S037026931830995X?via%3Dihub">depleted relative to expectations from proton–proton collisions</a> and that the <a href="https://journals.aps.org/prc/abstract/10.1103/PhysRevC.98.024908">distribution of particles inside the jet is modified</a>.</p> <p>However, one challenge in interpreting these measurements is that they are made after the quenching process – so it is impossible to tell whether a given jet has traversed the medium or merely glanced at it. This can be overcome by studying events where the jet is produced as a partner to a high-momentum photon, with the two moving in opposite directions in the detector. Since photons do not interact via the strong nuclear force they pass through the QGP medium without being affected. Thus, photons serve as a “tag” or a control experiment for the jet’s momentum before quenching. ATLAS physicists have previously used this key feature to measure <a href="https://www.sciencedirect.com/science/article/pii/S037026931830950X">jet quenching</a> and <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.042001">jet structure modification</a> in photon–jet events using lead–lead data recorded in 2015.</p> <p><a href="//arxiv.org/abs/2008.09811"><u>In new results released today</u></a> based on the large lead–lead collision dataset accumulated in 2018, ATLAS researchers applied this same strategy to measure jet quenching tagged with another particle: the Z boson. Similar to photon-tagged events, a jet can be produced alongside a Z boson which decays into particles (two electrons or two muons) that do not interact with the QGP medium. An example of such a collision event can be seen in Figure 1, where a Z boson decays into two muons (red lines). </p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">Z bosons can be measured in the low momentum range, where the effects of jet quenching are expected to be large. This region is particularly interesting to explore, despite being experimentally difficult to access.</h3> <hr class="divider"> <div class="narrow"> <figure class="mobile-float img-50 right"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-034-1" title="View on CDS"><img alt="Physics Briefings,Outreach &amp; Education,Updates,ATLAS,Physics,Plot,Heavy Ion Collisions" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-034-1/file?size=large" /></a><figcaption>Figure 2: Ratio of the yield of charged particles opposite in angle to a Z boson between lead–lead collisions and proton-proton collisions. The charged particles are the result of jet fragmentation. As a result of the jet-quenching process, the ratio is below one at high transverse momentum (pTch), and above one at low transverse momentum. The data (red) are compared to theoretical predictions (purple, blue, green yellow). (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Unlike photons, Z bosons can be measured in a low momentum range, where experiments have difficulty triggering on photons and distinguishing them from various background particles. At these low scales, the jet momentum matches the QGP temperature scale more closely and quenching effects are expected to be large. This region is thus particularly interesting to explore, despite being experimentally difficult to access.</p> <p>The new ATLAS result measures the production of charged particles opposite a Z boson. The measurement compares lead–lead collisions and proton–proton collisions, using the leptonic Z boson “tag” to select a similar population of jets arising predominantly from high-momentum quarks. Physicists looked at the charged particle yield for each tagged Z-boson event, and measured the ratio of this quantity between head-on (“central”) lead–lead and proton–proton events (as shown in Figure 2). At large transverse momentum (&gt; 3 GeV), there are significantly fewer charged particles in lead–lead collisions, consistent with the picture of energy loss and softer fragmentation in the QGP. At small transverse momentum (&lt; 3 GeV), there are significantly more charged particles, reflecting the thermalization of the lost energy by the medium.</p> <p>Researchers then compared the results to a variety of state-of-the-art theoretical calculations, which describe the jet-quenching process according to different models. These calculations all indicate a suppression of high-momentum particles, with a corresponding enhancement of low-momentum particles, but each prediction differs quantitatively from the rest. These comparisons highlight the value of new experimental data to constrain theory in this particular area. The upcoming Run 3 of the LHC should bring many more Z boson events in lead–lead collisions – opening further avenues for these discriminating measurements.</p> <hr class="divider"> <h3>Links</h3> <ul> <li><a href="https://arxiv.org/abs/2008.09811">Medium-induced modification of Z-tagged charged particle yields in Pb+Pb collisions at 5.02 TeV with the ATLAS detector</a> (submitted to Phys. Rev. Lett, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2019-05/">see figures</a>)</li> <li><a href="https://www.sciencedirect.com/science/article/pii/S037026931830995X?via%3Dihub">Measurement of the nuclear modification factor for inclusive jets in lead–lead at 5.02 TeV with the ATLAS detector</a> (Phys. Lett. B 790 (2019) 108, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2017-10/">see figures</a>)</li> <li><a href="https://journals.aps.org/prc/abstract/10.1103/PhysRevC.98.024908">Measurement of jet fragmentation in lead–lead and proton–proton collisions at 5.02 TeV with the ATLAS detector</a> (Phys. Rev. C 98 (2018) 024908, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2017-04/">see figures</a>)</li> <li><a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.042001">Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from proton–proton and lead–lead Collisions in ATLAS</a> (Phys. Rev. Lett. 123 (2019) 042001, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2017-08/">see figures</a>)</li> <li><a href="https://www.sciencedirect.com/science/article/pii/S037026931830950X">Measurement of photon-jet transverse momentum correlations in 5.02 TeV lead–lead and proton–proton collisions with ATLAS</a> (Phys. Lett. B 789 (2019) 167, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2018-06/">see figures</a>)</li> <li><a href="https://atlas.cern/updates/physics-briefing/photon-tagged-jet-quenching-quark-gluon-plasma">Photon-tagged jet quenching in the quark-gluon plasma</a>, <em>Physics Briefing</em>, October 2017 </li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Tue, 25 Aug 2020 17:42:00 +0000 Steven Goldfarb 6681 at https://atlas.cern ATLAS observes W-boson pair production from light colliding with light https://atlas.cern/updates/briefing/observation-w-pair-from-light <span>ATLAS observes W-boson pair production from light colliding with light</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Wed, 08/05/2020 - 09:46</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/w-boson-0" hreflang="en">W boson</a></div> <div class="field--item"><a href="/tags/ichep-0" hreflang="en">ICHEP</a></div> <div class="field--item"><a href="/tags/ichep2020" hreflang="en">ICHEP2020</a></div> </div> <div class="field field--name-field-image-caption field--type-string-long field--label-hidden field--item">Figure 1: 2018 ATLAS event display consistent with the production of a pair of W bosons from two photons, where the W bosons decay into a muon and an electron (visible in the detector) and neutrinos (not detected). The muon path (red line) and electron path (yellow line) are shown. The electron deposits its energy in the electromagnetic calorimeter (yellow blocks). The many particles reconstructed in the Inner Detector are shown in orange. Top left corner shows that these particles do not originate from the same interaction and are thus attributed to additional proton–proton interactions. (Image: ATLAS Collaboration/CERN)</div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-031-1" title="View on CDS"><img alt="Event Displays,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-031-1/file?size=large" /></a><figcaption>Figure 1: A 2018 ATLAS event display consistent with the production of a pair of W bosons from two photons, where the W bosons decay into a muon and an electron (visible in the detector) and neutrinos (not detected). The muon path (red line) and electron path (yellow line) are shown. The electron deposits its energy in the electromagnetic calorimeter (yellow blocks). The many particles reconstructed in the Inner Detector are shown in orange. Top left corner shows that these particles do not originate from the same interaction and are thus attributed to additional proton–proton interactions. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p><strong>The ATLAS Collaboration announces the <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-038/">first observation</a> of two W bosons produced from the scattering of two photons — particles of light — at the International Conference on High-Energy Physics (<a href="http://ichep2020.org/">ICHEP 2020</a>).</strong></p> <figure class="right mobile-float img-40"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-027-3" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-027-3/file?size=large" /></a><figcaption>Figure 2: Feynman diagram depicting the production of a pair of W bosons from two photons in a four force-carrier interaction. The photons are scattered off of two protons which in the process lose energy but remain intact. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>In everyday life, two crossing light beams follow the rules of classical electrodynamics and do not deflect, absorb or disrupt one another. However, at the high energies seen in LHC collisions, effects of <em>quantum electrodynamics</em> become important. For a short moment, photons radiated off the incoming proton beams can scatter and transform into a particle–antiparticle pair which appears as light-by-light interactions in the detector. This process was <a href="https://atlas.cern/updates/physics-briefing/atlas-observes-light-scattering-light">first observed</a> by the ATLAS Collaboration in 2019. Indeed, the Standard Model describes quantum electrodynamics as part of electroweak theory, which not only predicts that force-carrying particles – the W bosons, Z boson and photon – interact with ordinary matter, but also among themselves. </p> <p>The <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-038/">newly observed process</a> proceeds via a very rare type of phenomenon where two photons collide to directly produce two W bosons of opposite electric charge via a four force-carrier interaction, among others (see Figure 2). Although the <a href="https://arxiv.org/abs/1607.03745">ATLAS</a> and <a href="https://arxiv.org/abs/1604.04464">CMS</a> Collaborations saw first evidence of this process in data recorded during Run 1 of the LHC (2011–2012), its observation required the substantially larger dataset taken during Run 2 (2015–2018). </p> <p>This rare process occurs as bunches of high-energy protons skim past each other in “ultra-peripheral collisions”, if only their surrounding electromagnetic fields interact. Quasi-real photons from these fields scatter off one another to produce a pair of W bosons and leave a distinct signature in the ATLAS experiment. As the skimming protons stay intact, the only detectable particles produced in the interaction are the visible decay products of the W bosons – namely, for this measurement, an electron and a muon with opposite electric charge. </p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The ATLAS Collaboration has observed the rare interaction of two W bosons with two photons with statistical significance of 8.4 standard deviations.</h3> <hr class="divider"> <div class="narrow"> <figure class="left mobile-float img-50"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-027-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-027-2/file?size=large" /></a><figcaption>Figure 3: The distribution of the number of particles reconstructed in the ATLAS inner detector, in addition to the electron and the muon. The background process with the largest contribution is the W-boson-pair production from proton constituents; its simulation (blue) describes the observed data (black points) very well. The photon-induced W-boson pair production accumulates at low particle multiplicities (white area). (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>ATLAS physicists had to overcome several unique challenges to observe this process, starting with separating the signal from background. LHC protons can break up into their constituents and fragment into several detectable particles at very low energy. In particular, W-boson pairs can be produced from the proton’s constituents. This background process is hundreds of times more likely to occur than the photon–photon production of W-boson pairs, and can mimic its signature. To enhance the signal over such a background, physicists only selected collisions where no other charged particles are measured in the vicinity of the electron and the muon, as reconstructed in the <a href="http://atlas.cern/updates/experiment-briefing/inner-detector-alignment">ATLAS Inner Detector</a>.</p> <p>Further, a typical collision event contains particles from 20 to 60 additional proton–proton interactions occurring simultaneously as bunches of proton cross in ATLAS. These additional particles can prevent the identification of signal events if they are produced in close proximity to the photon–photon interaction (see Figure 1). </p> <p>Physicists developed novel experimental techniques to precisely determine the contributions of these effects. Simulated events can be used to estimate the expected backgrounds, but detailed tuning on the data is needed to ensure that they provide a faithful description. Physicists performed auxiliary measurements using data consistent with resonant Z boson production, a well-understood process produced with high frequency and purity at the LHC. This dataset was used to count particles from both the additional proton-proton interactions and the proton fragmentation, and the findings allowed ATLAS physicists to tune the simulation of such events. The accurate description of these background processes made the observation of this rare phenomenon possible (see Figure 3, where the photon–photon signal interactions accumulate at low particle multiplicity).</p> <p>A total of 307 events matching the selection requirements were found in the analysed dataset, of which 174 were attributed to be from the photon–photon production of W-boson pairs and the remaining events to various background processes. Such a yield corresponds to a statistical significance of 8.4 standard deviations, which is well above the established 5 standard deviations criterion for the unambiguous observation of a process. The cross section is measured to be 3.13 ± 0.42 fb. This means that only one or two such interactions occurred in the 30 trillion proton–proton interactions of a typical day of data-taking in 2018.</p> <p>The four force-carrier interaction is an integral part of electroweak theory and, at the same time, can be sensitive to modifications of the Standard Model by unaccounted-for new physics. The experimental techniques presented by the ATLAS Collaboration will enable future measurements that can probe such modifications and test the electroweak theory in a novel way.</p> <hr class="divider"> <h3>Links</h3> <ul> <li><strong><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-038/">Observation of photon-induced W<sup>+</sup> W<sup>– </sup>production in proton–proton collisions at 13 TeV using the ATLAS detector</a> (ATLAS-CONF-2020-038)</strong></li> <li><a href="https://home.cern/news/news/physics/rare-phenomenon-observed-atlas-features-lhc-high-energy-photon-collider">Rare phenomenon observed by ATLAS features the LHC as a high-energy photon collider</a>, <em>CERN Press Update</em>, 5 August 2020</li> <li><a href="https://arxiv.org/abs/1607.03745">Measurement of exclusive γγ→W<sup>+</sup>W<sup>−</sup> production and search for exclusive Higgs boson production in proton–proton collisions at 8 TeV using the ATLAS detector</a> (arXiv: 1607.03745)</li> <li>CMS Collaboration: <a href="https://arxiv.org/abs/1604.04464">Evidence for exclusive gamma gamma to W<sup>+</sup>W<sup>−</sup> production and constraints on anomalous quartic gauge couplings in proton–proton collisions at 7 and 8 TeV</a> (arXiv: 1604.04464)</li> <li>CMS Collaboration: <a href="https://arxiv.org/abs/1305.5596">Study of exclusive two-photon production of W<sup>+</sup>W<sup>−</sup> in proton–proton collisions at 7 TeV and constraints on anomalous quartic gauge couplings</a> (arXiv: 1305.5596)</li> <li><a href="https://atlas.cern/updates/physics-briefing/atlas-observes-light-scattering-light">ATLAS observes light scattering off light</a>, <em>Physics Briefing</em>, March 2019</li> <li><a href="https://atlas.cern/updates/physics-briefing/evidence-three-massive-vector-boson-production">ATLAS finds evidence of three massive vector boson production</a>, <em>Physics Briefing</em>, March 2019</li> <li><a href="https://atlas.cern/updates/physics-briefing/milestone-electroweak-symmetry-breaking">New milestone reached in the study of electroweak symmetry breaking</a>, <em>Physics Briefing</em>, July 2019</li> <li><a href="https://atlas.cern/updates/physics-briefing/weak-lightsabers">Quarks observed to interact via minuscule “weak lightsabers”</a>, <em>Physics Briefing</em>, June 2018</li> <li><a href="https://atlas.cern/updates/press-statement/atlas-sees-first-direct-evidence-light-light-scattering-high-energy">ATLAS sees first direct evidence of light-by-light scattering at high energy</a>, <em>Press Statement</em>, April 2017</li> <li><a href="https://atlas.cern/updates/physics-briefing/atlas-finds-evidence-rare-electroweak-w-w-production">ATLAS finds evidence for the rare electroweak W±W± production</a>, <em>Physics Briefing</em>, September 2014</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Wed, 05 Aug 2020 07:46:00 +0000 Steven Goldfarb 6679 at https://atlas.cern New ATLAS result marks milestone in the test of Standard Model properties https://atlas.cern/updates/briefing/test-lepton-flavour-violation <span>New ATLAS result marks milestone in the test of Standard Model properties</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Mon, 08/03/2020 - 10:41</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/lepton-flavour-violation" hreflang="en">lepton flavour violation</a></div> <div class="field--item"><a href="/tags/ichep-0" hreflang="en">ICHEP</a></div> <div class="field--item"><a href="/tags/ichep2020" hreflang="en">ICHEP2020</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-028-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-028-1/file?size=large" /></a><figcaption>Figure 1: Diagrams of a lepton-flavour-violating Z-boson decay (left) and of the two main backgrounds to the search: a lepton-flavour-conserving Z-boson decay into a pair of tau leptons (middle) and a W-boson decay with leptons (right). The green arrows represent electrons or muons (l), the blue triangles are the visible component of hadronically-decaying tau leptons (τ had-vis) or the hadronisation of a quark or a gluon, and the dashed blue lines represent undetected neutrinos. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>The ATLAS Collaboration has released a new study into a key building block of matter: <em>leptons</em>. This type of particle comes in three different families (flavours) and, according to the Standard Model, should follow strict rules. For instance, except for their mass, leptons of different flavours have identical properties – a feature known as lepton flavour universality. This was recently corroborated by a <a href="https://atlas.cern/updates/physics-briefing/addressing-long-standing-tension-standard-model">key measurement of the W-boson decay rates into leptons by the ATLAS Collaboration</a>. </p> <p>Yet the Standard Model has known shortcomings. For example, it predicted that leptons could only interact with other leptons of the same flavour. Experiments observed neutrinos (neutral leptons) violate this hypothesis, transforming from one flavour into another in processes known as neutrino oscillation. This led physicists to realise that neutrinos were not, in fact, massless, as originally assumed in the Standard Model. Such discoveries show that the fundamental structure of Nature is more complex than one had thought. </p> <p>Could the violation of lepton flavour seen in neutral leptons also occur among charged leptons (with dramatic consequences)? In a new result, the ATLAS Collaboration searched for lepton-flavour-violating decays of the Z boson, where the Z boson decays into two charged leptons of different flavours. Such events are predicted from neutrino oscillation to be so rare – accounting for just one in 10<sup>54</sup> Z-boson decays – that they should be undetectable. If they were to be observed at ATLAS, it would be an unequivocal sign of new physics beyond the Standard Model.</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">This new ATLAS result is an important step forward from the legacy of LEP experiments. Such searches for highly suppressed processes can reveal hints for new particles or interactions.</h3> <hr class="divider"> <div class="narrow"> <p>Physicists examined ATLAS data collected over two runs of the Large Hadron Collider (LHC, 2012–2018) to set strong constraints on lepton-flavour-violating decays involving a tau lepton (τ) and an electron (e) or a muon (μ). While there have been several low-energy experiments that specialise in lepton-flavour-violating searches, they cannot easily probe transitions involving Z bosons. This is an area where high-energy accelerators, such as CERN’s previous <a href="https://home.cern/science/accelerators/large-electron-positron-collider">Large Electron Positron (LEP) collider</a> (1989–2000) and now the LHC, play a special role. </p> <figure class="right mobile-float img-50"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-028-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-028-2/file?size=large" /></a><figcaption>Figure 2: Distribution of the neural-network output of some of the Z-boson-decay candidates analysed in the search. In the upper panel, data (black dots) are compared to the stacked expected contributions from background processes, mainly Z-boson decays to tau-lepton pairs (dark blue) and W-boson decays (yellow). The expected contribution from signal events with a decay rate of five in ten thousand is shown by the red dashed line. The lower panel shows the ratio of the data to the background prediction. No significant excess of events is seen in the data. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>The ATLAS result marks a new milestone in a legacy of precision measurements established by LEP. Searching for rare Z-boson decays is a great challenge for LHC experiments. Whereas LEP produced an abundance of Z bosons in a relatively clean environment, only a small fraction of LHC collisions produce a Z boson – and always along with several background collisions. However, the LHC has one major advantage: it can produce Z bosons at a much faster rate! More than 20 years on, ATLAS’ result now supersedes those of the LEP experiments (<a href="https://link.springer.com/article/10.1007/BF01553981">OPAL</a>, <a href="https://link.springer.com/article/10.1007%2Fs002880050313">DELPHI</a>). </p> <p>To analyse the enormous LHC Run 2 dataset – with its 8 billion Z bosons – ATLAS physicists developed a state-of-the-art machine-learning method using <em>deep neural networks</em>. The neural networks were trained to identify the kinematic properties of the signal process, where a Z boson decays into an electron or a muon and a tau lepton, which itself is unstable and decays (Figure 1). They also had to differentiate the signal process from mainly two others that produce the same particles: the lepton-flavour-conserving Z-boson decay into a pair of tau leptons, where one tau lepton decays into an electron or a muon (plus undetected neutrinos), and the W-boson decay into leptons, produced together with an additional jet of particles. </p> <p>The output distributions of the neural networks for the selected candidate event were studied to determine the presence of lepton-flavour-violating Z-boson decays (Figure 2 for the tau lepton and muon case). The classification power of the neural networks – combined with the unprecedented number of Z-boson decays studied – allowed ATLAS to set constraints on the maximum rate at which lepton-flavour-violating Z-boson decays involving a tau lepton can occur. The result excludes, at 95% confidence level, Z-boson decay rates greater than 8.1x10<sup>-6</sup> (Z→τe) and 9.5x10<sup>-6</sup> (Z→τμ). </p> <p>The new ATLAS result provides experimental guidance towards new theories that could explain the shortcomings of the Standard Model. The precision of the result is largely limited by the number of analysed Z-boson decays. Therefore, ATLAS is looking forward to improved sensitivity that is to be expected from the additional data of future LHC runs.</p> <hr class="divider"> <h3>Links</h3> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-035/">Lepton Flavour Violation at the LHC: a search for Z→eτ and Z→μτ decays with the ATLAS detector</a> (ATLAS-CONF-2020-035)</li> <li>ICHEP2020 presentation by Karl Jakobs: <a href="https://indico.cern.ch/event/868940/contributions/3905674/">ATLAS Highlights</a></li> <li>OPAL Collaboration, <a href="https://link.springer.com/article/10.1007/BF01553981">A search for lepton flavor violating Z0 decays</a>, Z. Phys. C67 (1995) 555 </li> <li>DELPHI Collaboration, <a href="https://link.springer.com/article/10.1007%2Fs002880050313">Search for lepton flavor number violating Z0 decays</a>, Z. Phys. C73 (1997) 243</li> <li><a href="http://atlas.cern/updates/physics-briefing/addressing-long-standing-tension-standard-model">New ATLAS result addresses long-standing tension in the Standard Model</a>, <em>Physics Briefing</em>, 28 May 2020</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Mon, 03 Aug 2020 08:41:00 +0000 Steven Goldfarb 6677 at https://atlas.cern New measurements of the Higgs boson find strength in unity https://atlas.cern/updates/briefing/higgs-boson-finds-strength-unity <span>New measurements of the Higgs boson find strength in unity</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Fri, 07/31/2020 - 10:38</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-subtitle field--type-text field--label-hidden field--item">ATLAS reports an important boost in the precision of combined measurements of Higgs-boson couplings, as analyses of the full Run-2 dataset proceed.</div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/higgs-boson" hreflang="en">Higgs boson</a></div> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/ichep2020" hreflang="en">ICHEP2020</a></div> <div class="field--item"><a href="/tags/ichep-0" hreflang="en">ICHEP</a></div> </div> <div class="field field--name-field-image-caption field--type-string-long field--label-hidden field--item">Figure 1: Event display of a Higgs-boson candidate produced in association with a Z boson (ZH production), with the Higgs boson decaying to four leptons (H → ZZ*→ 2e2μ), and the Z boson to a pair of muons (Z→μμ). The Higgs boson is reconstructed from the two electrons (two green tracks and bars representing energy deposited in the calorimeter) and two muons (two red tracks on the left passing through the blue muon chambers). The associated Z boson recoils against the Higgs boson to produce two additional muons (two red tracks on the right). (Image: ATLAS Collaboration/CERN)</div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p>The <a href="http://atlas.cern/updates/atlas-feature/higgs-boson">Higgs boson</a>, first predicted in the 1960s and discovered by the ATLAS and CMS experiments in 2012, is a unique elementary particle arising from the mass-generating Higgs mechanism of the Standard Model. It thus has a peculiar affinity to mass: the larger the mass of an elementary particle, the stronger its interaction (or <em>coupling</em>) with the Higgs boson. Any deviation from this pattern would reveal new physics. </p> <p>Physicists can study Higgs-boson couplings in several ways: by measuring the rates of different Higgs boson production mechanisms and decays, and also by studying the particle’s kinematic properties. The ATLAS Collaboration has just presented precise new measurements of these key quantities. Several of these measurements were updated to use the full LHC Run 2 dataset (2015–2018), to provide the best precision to date. </p> <p>When combined, ATLAS’ new measurements give detailed insight into this one-of-a-kind particle. They significantly outperform previous measurements, with the overall production rate of the Higgs boson found to be in good agreement with the Standard Model, within a measurement precision of 5% and about 4% uncertainty in the Standard Model prediction. </p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The latest ATLAS results significantly outperform previous measurements, giving physicists new insight into this one-of-a-kind particle.</h3> <hr class="divider"> <div class="narrow"> <figure class="right mobile-float img-60"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-023-3" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-023-3/file?size=large" /></a><figcaption>Figure 2: Cross sections for ggF, VBF, WH, ZH and ttH+tH normalized to their Standard Model predictions, measured assuming Standard Model values for the decay branching fractions. The black error bars, blue boxes and yellow boxes show the total, systematic, and statistical uncertainties in the measurements, respectively. The gray bands indicate the theory uncertainties in the Standard Model cross-section predictions. The compatibility level between the measurement and the Standard Model prediction is 86%. (Image: ATLAS/CERN)</figcaption></figure> <h3><strong>Channel surfing with Higgs boson decays</strong></h3> <p>ATLAS physicists began by measuring all of the main decay “channels” of the Higgs boson: into a pair of photons, W or Z bosons, tau leptons, bottom quarks – and even muons. Though the coupling to muons is difficult to probe, ATLAS physicists recently reported <a href="https://atlas.cern/updates/physics-briefing/new-search-rare-higgs-decays-muons">a first hint of the Higgs boson decay to muons</a>. ATLAS researchers also searched for Higgs bosons decaying to “invisible” particles, leaving only missing transverse energy in the detector – a possible portent of dark matter, for example. Their new result sets the <a href="https://atlas.cern/updates/physics-briefing/probing-dark-matter-higgs-boson">strongest limits</a> yet on this process, establishing that less than 13% of Higgs boson decays could be into “invisible” particles. </p> <p>These measurements could then be broken down into the major production modes of the Higgs boson: gluon fusion (ggF), vector-boson fusion (VBF), the associated production with a W or Z boson (WH, ZH), and the associated production with top quarks (ttH, tH), as shown in Figure 2. All of these are now observed and precisely measured, with the experimental sensitivity of some modes nearing the precision of state-of-the-art theory predictions. ATLAS has furthermore established for the first time the separate observation of the associated production of the Higgs boson with, respectively, a W boson and a Z boson. </p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">Across a wide range of masses, ATLAS physicists found that the strength of the Higgs boson coupling increases with the mass of the elementary particle.</h3> <hr class="divider"> <div class="narrow"> <p>Further, the kinematic properties of the Higgs boson were assessed with unprecedented precision. Physicists introduced finer partitions of the various production modes – studying, for example, the Higgs boson transverse momentum or the number of jets in an event – to uncover potential hints of new physics. For the first time, ATLAS has also measured the differential distribution of the Higgs boson transverse momentum in ttH production, shedding new light on the boson’s interaction with the top quark.</p> <p>With these measurements in hand, physicists were able to decipher the Higgs-boson couplings to other elementary particles. As shown in Figure 3, the strength of the coupling increases with the mass of the elementary particle, in good agreement with the Standard Model. This holds true across a wide range in masses, from the top quark (the heaviest particle in the Standard Model) down to the muon (1600 times lighter than the top quark). </p> <div class="span1of2"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-023-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-023-1/file?size=large" /></a><figcaption>Figure 3: The coupling-strength for fermions (t, b, τ, μ) and weak gauge bosons (W, Z) on the y-axis vs their mass on the x-axis. The Standard Model prediction is also shown (dotted line). The lower inset shows the ratios of the values to their Standard Model predictions. The level of compatibility between the combined measurement and the Standard Model prediction is 84%. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div> <div class="span1of2 last"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-023-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-023-2/file?size=large" /></a><figcaption>Figure 4: The measured coupling to photons on the y-axis vs the coupling to gluons on the x-axis. The best-fit value for the two measurements is shown by a cross and the Standard Model hypothesis by a star. Ellipses show the 68% and 95% confidence-level contours from a combined fit. The compatibility level between the combined measurement and the Standard Model prediction is 51%, well within the one standard deviation (68%) level. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div><div style="clear: both; height: 0;"></div> <h3><strong>Exploration through combination</strong></h3> <p>ATLAS physicists paid particular attention to processes such as gluon-fusion production of the Higgs boson and Higgs-boson decays to a pair of photons. Both the gluon and photon are massless, and thus cannot directly interact with the Higgs boson. These processes are therefore mediated by other massive particles via <em>loop interactions</em>, which could be hideouts for new particles. </p> <p>Though experiments cannot directly see these loop interactions, there are still ways to infer their content. The presence of new particles would change the rate for ggF production or the Higgs boson decaying into photons. In Figure 4, the measured gluon and the photon couplings are compared to theoretical predictions. A deviation of the measured values from unity, if established, would be a smoking gun for new physics lurking in loop interactions. Instead, ATLAS physicists observed a good agreement with the Standard Model, with measured uncertainties on the measured gluon and photon couplings as low as 5%, and an overall agreement with expectations at the 51% confidence level. </p> <p>Finally, by combining together the various Higgs-boson decay measurements and including these loop interactions, ATLAS physicists set another limit on new physics. Showing the value of combined studies, this result sets a new limit of 9% for Higgs boson decays to “invisible” particles – an improvement from the 13% of the measurement quoted above.</p> <h3><strong>The Standard Model remains unperturbed</strong></h3> <p>Thanks to the excellent performance of the LHC and the ATLAS detector during Run 2, several ATLAS results have been combined to probe the couplings of the Higgs boson at unprecedented levels. Though the Standard Model remains unperturbed, the exploration is just beginning! Some important but difficult analysis channels are still to use the full Run-2 dataset – offering additional insight into the Higgs boson’s secrets.</p> <hr class="divider"> <h3><strong>Links</strong></h3> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-027/">A combination of measurements of Higgs boson production and decay using up to 139 fb<sup>−1</sup> of proton–proton collision data at 13 TeV collected with the ATLAS experiment</a> (ATLAS-CONF-2020-027)</li> <li>ICHEP2020 presentation by Matt Klein: <a href="https://indico.cern.ch/event/868940/contributions/3813475/">Combined Higgs boson measurements at the ATLAS experiment</a> </li> <li><a href="https://arxiv.org/abs/2007.07830">A search for the dimuon decay of the Standard Model Higgs boson with the ATLAS detector</a> (arXiv: 2007.07830, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2019-14/">see figures</a>)</li> <li><a href="https://atlas.cern/updates/physics-briefing/new-search-rare-higgs-decays-muons">ATLAS one step closer in the search for rare Higgs boson decays to muons</a>, <em>Physics Briefing</em>, July 2020</li> <li><a href="https://atlas.cern/updates/physics-briefing/probing-dark-matter-higgs-boson">Probing dark matter with the Higgs boson</a>, <em>Physics Briefing</em>, April 2020</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Fri, 31 Jul 2020 08:38:00 +0000 Steven Goldfarb 6676 at https://atlas.cern Looking forward: ATLAS measures proton scattering when light turns into matter https://atlas.cern/updates/briefing/looking-forward-light-matter <span>Looking forward: ATLAS measures proton scattering when light turns into matter</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Thu, 07/30/2020 - 14:47</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/ichep2020" hreflang="en">ICHEP2020</a></div> <div class="field--item"><a href="/tags/ichep-0" hreflang="en">ICHEP</a></div> <div class="field--item"><a href="/tags/forward-detectors" hreflang="en">forward detectors</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p><strong>Today, at the International Conference for High Energy Physics (<a href="http://ichep2020.org/">ICHEP 2020</a>), the ATLAS Collaboration <a href="http://indico.cern.ch/event/868940/contributions/3816380/">announced first results</a> using the ATLAS Forward Proton (AFP) spectrometer (Figure 1). With this instrument, physicists directly observed and measured the long sought-after prediction of proton scattering when particles of light turn into matter. </strong></p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-024-3" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-024-3/file?size=large" /></a><figcaption>Figure 1: Schematic diagram of ATLAS Forward Proton (AFP) spectrometer relative to the main ATLAS detector (not to scale). After the incident proton beams intersect, the leptons are detected by the main ATLAS detector and the scattered proton is detected by AFP. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>In 1928, theoretical physicist Paul Dirac predicted the existence of the positron, the positively-charged antimatter partner to the electron. When brought together, this matter–antimatter pair annihilates into two particles of light (photons). Remarkably, quantum mechanics predicts that the reverse can also occur. Two photons with sufficient energy can turn into a matter–antimatter pair, as shown in Figure 2. </p> <figure class="right mobile-float img-40"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-024-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-024-2/file?size=large" /></a><figcaption>Figure 2: Diagram of a pair of photons (γ) turning into a pair of leptons (electrons or muons) (ℓ). The scattered protons (p) can remain intact in such interactions, but deflected from their paths along the beam so that they can be measured in a proton spectrometer. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>To observe this phenomenon, physicists can use the LHC, a proton–proton collider, as a photon–photon collider. Usually, particles are created by protons colliding head-on which break apart. However, if two protons pass very close to each other, they can scatter via the electromagnetic force to produce photons that turn into a matter–antimatter pair. The two protons remain intact, continuing their path in the LHC beam pipe, which the AFP spectrometer can detect. Observing these intact scattered protons is a hallmark of a photon–photon collision.</p> <p>The AFP spectrometer is unique in many ways. <a href="https://atlas.cern/updates/atlas-news/atlas-starting-line">Installed in 2017</a>, it is one of the newest additions to the ATLAS experiment. It sits either side of the main ATLAS cavern, just over 200 metres downstream from the collision point as shown in Figure 1. Its detectors are based on silicon technology, which reach directly into the LHC beam pipe to only two millimetres from the proton beam itself. If a scattered proton emits a photon and loses a few percentage points of energy, the LHC magnets deflect the proton into the AFP spectrometer. These scattered protons are among the highest-energy particles measured at the LHC. </p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The new ATLAS Collaboration observation of this striking phenomenon has a statistical significance over 9 standard deviations.</h3> <hr class="divider"> <div class="narrow"> <p>Physicists studied data recorded by the AFP spectrometer throughout 2017 to establish direct evidence of these scattered protons when matter – electron–positron or muon–antimuon pairs – are created from the interaction of two photons. This was achieved by comparing the proton energy loss measured by the AFP spectrometer from the proton deflection angle to the produced matter–antimatter pair recorded in the central ATLAS experiment, as shown in Figure 3. If the scattered proton arose while photons turned into matter, the measurements from both locations are predicted to be equal (within the measurement precision). </p> <p>The ATLAS Collaboration has observed this striking phenomenon, recording 180 events that have an intact proton detected by the AFP spectrometer and a matching electron–positron or muon–antimuon pair measured in the main ATLAS detector. The expected background from accidentally matching forward protons amounts to about 20 events. The statistical significance of this result thus exceeds 9 standard deviations for each electron and muon channels.</p> <p>This landmark measurement using the AFP spectrometer provides valuable information about how often the protons stay intact, which is challenging to calculate from theory. These measurements are important tests of how light interacts with matter at the highest laboratory energies. Certain theories predict such interactions are modified by new particles that could explain the mysterious dark matter in our universe. With more data, physicists can use the AFP to search for these phenomena in new ways. </p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-024-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-024-1/file?size=large" /></a><figcaption>Figure 3: The fractional proton energy loss measured by the AFP spectrometer (ξAFP) is compared to that measured from the electron or muon pairs in the central ATLAS detector (ξll). A signal peak is observed when these two quantities are approximately equal, indicating that the scattered proton emitted a photon that produced the lepton pair. The labels A and C denote opposite sides of the collision point along the beam line. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <hr class="divider"> <h3>Links</h3> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-041/">Observation and measurement of forward proton scattering in association with lepton pairs produced via the photon fusion mechanism at ATLAS</a> (ATLAS-CONF-2020-041)</li> <li>ICHEP talk by Mateusz Dyndal: <a href="https://indico.cern.ch/event/868940/contributions/3816380/">Measurements of photon-photon fusion at ATLAS</a> </li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Thu, 30 Jul 2020 12:47:00 +0000 Steven Goldfarb 6675 at https://atlas.cern ATLAS probes interactions between heavyweights of the Standard Model https://atlas.cern/updates/briefing/probes-heavyweights-standard-model <span>ATLAS probes interactions between heavyweights of the Standard Model</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Thu, 07/30/2020 - 10:47</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/top-quark" hreflang="en">top quark</a></div> <div class="field--item"><a href="/tags/higgs-boson" hreflang="en">Higgs boson</a></div> <div class="field--item"><a href="/tags/ichep2020" hreflang="en">ICHEP2020</a></div> <div class="field--item"><a href="/tags/ichep-0" hreflang="en">ICHEP</a></div> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-026-1" title="View on CDS"><img alt="Proton Collisions,Event Displays,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-026-1/file?size=large" /></a><figcaption>Figure 1: A spectacular candidate ttZ production event display with four leptons. Three reconstructed muons are represented by the red lines. Two of those were identified as having originated from the decay of a Z boson. Two reconstructed jets were identified as originating from b-quarks. A reconstructed electron is represented by the green track together with calorimeter energy deposits. The missing transverse momentum, originating from neutrinos from a semileptonic top decay escaping the detector, is represented by the dotted white line. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>In the contest for the heaviest known elementary particle, the top quark and Z boson rank first and third, respectively. When a proton–proton collision produces a top-quark pair together with a Z boson – a process known as ttZ production – their total mass can reach an impressive 440 GeV! The discovery of this highly energetic process thus required the record collision energy and rate of the LHC; no previous collider could come close. </p> <p>The complexity of ttZ production has made it a wonderful benchmark test for the Standard Model. It allows physicists to directly probe the electroweak interaction between the Z boson and the top quark, sharing complementarity of information with <a href="https://atlas.cern/updates/physics-briefing/observation-tth-production">the related top-pair production with the Higgs boson</a>. The Standard Model predicts the strength of this interaction and thus how often ttZ production should occur in the LHC, and it predicts the energies and relative emission directions of the top quarks and the Z boson. As new physics phenomena may modify some (or all) of these properties, measurements of ttZ production could give profound new insight to the current understanding of particle physics. </p> <p>Physicists have come a long way since the <a href="https://link.springer.com/article/10.1140/epjc/s10052-014-3060-7">process’</a> <a href="https://link.springer.com/article/10.1007/JHEP11(2015)172">discovery</a> in 2014. The outstanding performance of the LHC and the ATLAS detector during Run 2 (2015-2018) provided a large dataset of ttZ events, from which researchers could explore the process’ dynamics in great detail. The results of their efforts were <a href="http://indico.cern.ch/event/868940/contributions/3816382/">presented today</a> at the International Conference on High-Energy Physics (ICHEP 2020).</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The high mass and electroweak interactions occurring in ttZ production make it very rare, even at the LHC. ATLAS physicists studied the full dataset collected during Run 2 of the LHC for a precise measurement of its cross section. </h3> <hr class="divider"> <div class="narrow"> <p>The <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-028/">new ATLAS result</a> focuses on the cleanest signatures of this process, studying events with two leptons (electrons or muons) from the Z-boson decay and either one or two additional electrons or muons from the top-quark pair decay. To complete their event selection, physicists also looked for the presence of additional jets stemming from b-quarks created in a top-quark decay.</p> <p>The resulting sample of events contained little contamination from other processes mimicking ttZ production. ATLAS physicists could thus take a precise new measurement of the ttZ production <a href="http://atlas.cern/glossary/cross-section">cross section</a>, found to be σ<sub>tt̄Z</sub> =1.05 ± 0.05(stat.) ± 0.09(syst.) pb, which is in agreement with Standard Model prediction. </p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-025-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-025-2/file?size=large" /></a><figcaption>Figure 2: Kinematic properties of the Z boson in ttZ events: (left) momentum transverse to the LHC proton beam and (right) rapidity, which is a variable related with the direction of emission with respect to the beam. The measurement results will serve to improve theoretical predictions, some of which are also shown in the plots. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Further, the abundance of ttZ events allowed physicists to study the kinematic properties of this process. The probability of producing ttZ events was examined as a function of ten different kinematic variables, such as the energy and direction of the Z boson. These differential cross-section measurements were compared to the currently most precise theoretical predictions, and were found to be well reproduced (see Figure 2). </p> <p>The Standard Model continues to accurately predict these results: another celebration of the most predictive theory ever constructed by humanity. Although this latest analysis does not hint towards new phenomena, the next 15 years will see the LHC provide 20 times the number of collisions seen thus far. Such a yield will significantly increase ATLAS’ statistical precision and sensitivity. Will the Standard Model survive these future tests as well?</p> <hr class="divider"> <p>Links</p> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-028/">Measurement of the fiducial and differential cross-section of a top quark pair in association with a Z boson at 13 TeV with the ATLAS detector</a> (ATLAS-CONF-2020-028)</li> <li>ICHEP 2020 talk by Knut Zoch: <a href="https://indico.cern.ch/event/868940/contributions/3816382/">Using associated production of top quarks to neutral bosons to probe standard model couplings and search for new physics</a></li> <li><a href="https://link.springer.com/article/10.1007/JHEP11(2015)172">Measurement of the ttW and ttZ production cross sections in proton–proton collisions at 8 TeV with the ATLAS detector </a>(JHEP 11 (2015) 172, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2013-05/">see figures</a>)</li> <li>CMS Collaboration, <a href="https://link.springer.com/article/10.1140/epjc/s10052-014-3060-7">Measurement of top quark-antiquark pair production in association with a W or Z boson in proton–proton collisions at 8 TeV</a> (Eur. Phys. J. C 74 (2014) 3060)</li> <li><a href="http://atlas.cern/updates/physics-briefing/new-evidence-top-quark-pairs-produced-w-or-z-bosons">New evidence for top quark pairs produced with W or Z bosons</a>, <em>Physics Briefing</em>, July 2014</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Thu, 30 Jul 2020 08:47:00 +0000 Steven Goldfarb 6674 at https://atlas.cern Jetting into the dark side: a precision search for dark matter https://atlas.cern/updates/briefing/precision-search-dark-matter <span>Jetting into the dark side: a precision search for dark matter</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Mon, 07/27/2020 - 19:13</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/ichep2020" hreflang="en">ICHEP2020</a></div> <div class="field--item"><a href="/tags/ichep-0" hreflang="en">ICHEP</a></div> <div class="field--item"><a href="/tags/dark-matter" hreflang="en">dark matter</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-020-1" title="View on CDS"><img alt="Proton Collisions,Event Displays,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-020-1/file?size=large" /></a><figcaption>Figure 1: A monojet event recorded by the ATLAS experiment in 2017, with a single jet of 1.9 TeV transverse momentum recoiling against corresponding missing transverse momentum (MET). The green and yellow bars show the energy deposits in the electromagnetic and hadronic calorimeters, respectively. The MET is shown as the red dashed line on the opposite side of the detector. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>The nature of <a href="http://atlas.cern/updates/atlas-feature/dark-matter">dark matter</a> remains one of the great unsolved puzzles of fundamental physics. Unexplained by the Standard Model, dark matter has led scientists to probe new physics models to understand its existence. Many such theoretical scenarios postulate that dark matter particles could be produced in the intense high-energy proton–proton collisions of the LHC. While the dark matter would escape the ATLAS detector unseen, it could occasionally be accompanied by a visible jet of particles radiated from the interaction point, thus providing a detectable signal.<br /> <br /> The ATLAS Collaboration set out to find just that. Today, at the International Conference in High-Energy Physics (<a href="http://ichep2020.org/">ICHEP 2020</a>), ATLAS presented a <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-048/">new search</a> for novel phenomena in collision events with jets and high missing transverse momentum (MET). The search was designed to uncover events that could indicate the existence of physics processes that lie outside the Standard Model and, in doing so, open a window to the cosmos.<br /> <br /> To identify such events, physicists exploited the principle of momentum conservation in the transverse detector plane – that is, perpendicular to the beam direction – looking for visible jets recoiling from something invisible. As events with jets are common at the LHC, physicists further refined their parameters: the events had to have at least one highly energetic jet and significant MET, generated by the momentum imbalance of the “invisible” particles. This is known as a monojet event – a spectacular example of which can be seen in Figure 1, a 2017 event display featuring the highest-momentum (1.9 TeV) monojet recorded so far by ATLAS.<br /> <br /> A plethora of exotic phenomena, not directly detectable by collider experiments, could also have yielded this characteristic monojet signature. ATLAS physicists thus set out to make their study inclusive of several new physics models, including those featuring supersymmetry, dark energy, large extra spatial dimensions, or axion-like particles.</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">Today's new result sets the most stringent limit on dark matter of any collider experiment so far – a milestone for the ATLAS Collaboration's search programme.</h3> <hr class="divider"> <div class="narrow"> <figure class="right mobile-float img-50"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-021-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-021-1/file?size=large" /></a><figcaption>Figure 2: Missing transverse momentum distribution after the monojet selection in data and in the Standard Model predictions. The different background processes are shown with colours. The expected distributions of dark energy, supersymmetric and weakly-interacting massive particle scenarios are illustrated with dashed lines. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Evidence of new phenomena would be seen in an excess of collision events with large MET when compared to the Standard Model expectation. Accurately predicting the different background contributions was a key challenge, as several abundant Standard Model processes could exactly mimic the signal topology – such as the production of a jet plus a Z boson, which then decays to two neutrinos that also leave ATLAS without being directly detected.</p> <p>Physicists used a combination of data-driven techniques and high-precision theoretical calculations to estimate the Standard Model background. The total background uncertainty in the signal region ranges from about 1% to 4% in the range of MET between 200 GeV and 1.2 TeV. The shape of the MET spectrum was used to enhance the discrimination power between signals and backgrounds, thus increasing the discovery potential. Figure 2 shows a comparison of the MET spectrum observed in the entire dataset collected from the ATLAS experiment during Run 2 (2015–2018), and the Standard Model expectation.<br /> <br /> As no significant excess was observed, physicists used the level of agreement between data and the prediction to set limits on the parameters of new physics models. In the context of weakly-interacting massive particles (a popular dark matter candidate), ATLAS physicists were able to exclude dark matter particle masses up to about 500 GeV and interaction axial-vector mediators up to 2 TeV, both at the 95% confidence level. These results provide the most stringent dark matter limits in collider experiments so far, and a milestone of the ATLAS search programme.</p> <hr class="divider"> <h3>Links</h3> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-048/">Search for new physics in events with jets and missing transverse momentum in proton–proton collisions at 13 TeV with the ATLAS detector</a> (ATLAS-CONF-2020-048)</li> <li>ICHEP 2020 presentation by Ben Carlson: <a href="https://indico.cern.ch/event/868940/contributions/3814836/">Dark Matter searches with the ATLAS detector</a></li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Mon, 27 Jul 2020 17:13:00 +0000 Steven Goldfarb 6673 at https://atlas.cern ATLAS one step closer in the search for rare Higgs boson decays to muons https://atlas.cern/updates/briefing/new-search-rare-higgs-decays-muons <span>ATLAS one step closer in the search for rare Higgs boson decays to muons</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Thu, 07/23/2020 - 18:51</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/higgs-boson" hreflang="en">Higgs boson</a></div> <div class="field--item"><a href="/tags/higgs-group" hreflang="en">Higgs group</a></div> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-029-1" title="View on CDS"><img alt="Higgs Candidates,Proton Collisions,Event Displays,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-029-1/file?size=large" /></a><figcaption>Figure 1: A Run 2 ATLAS event containing two muons (red) with mass compatible with that of the Higgs boson, and two forward jets (yellow cones). (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>"Who ordered that?" commented physicist Isidor Isaac Rabi when the muon was discovered in 1936. In the 80 years since, scientists have learnt a lot about the muon’s role in our Universe and have studied its properties with extreme precision. Muons have even been used via decays of intermediate weak bosons in the detection of new particles, such as the Higgs boson – now the centerpiece of its own extremely rich field of research.</p> <p>In the Standard Model, elementary particles acquire mass through interaction with the Higgs field: the stronger the interaction, the larger the mass of the particle. So far, physicists have collected conclusive evidence of the Higgs boson interacting with bosons and the heaviest elementary fermions belonging to the third fermion generation (tau-lepton as well as top and bottom quarks). Yet to date, there is no indication whether the Higgs boson interacts with the next lighter fermions, muon or charm quark, belonging to the second fermion generation.</p> <p>The ATLAS Collaboration has released a <a href="http://arxiv.org/abs/2007.07830">new paper on the search for the Higgs-boson decay to a pair of muons</a>. The new study uses the entire dataset collected by the ATLAS experiment during Run 2 of the LHC (2015–2018) to give a first hint of this elusive process.</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The ATLAS Collaboration studied the entire dataset collected during Run 2 of the LHC (2015–2018) to give a first hint of the elusive Higgs-boson decay to muon pairs.</h3> <hr class="divider"> <div class="narrow"> <h3><strong>The wheat from the chaff</strong></h3> <p>Despite a simple experimental signature, spotting this rare decay continues to be a challenge. This is due to the low probability of the Higgs boson decaying to muons (predicted to be just 0.02%) and the large number of events from similar Standard Model background processes that can dominate the search. Only 0.2% of selected muon-pair events with masses between 120 and 130 GeV from proton–proton collisions are expected to come from a Higgs-boson decay.</p> <p>Fortunately, a signal can be distinguished from background processes by looking at the shape of the mass distribution of the precisely measured muon pairs. Higgs-boson events will cluster around the Higgs-boson mass of 125 GeV, producing a narrow peak that can be distinguished from the smoothly-falling distribution of background events. By fitting the invariant-mass spectrum, ATLAS physicists are able to directly constrain the background and extract a possible signal.</p> <figure class="right mobile-float img-60"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-032-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-032-1/file?size=large" /></a><figcaption>Figure 2: The invariant-mass spectrum of the reconstructed muon-pairs in ATLAS data. Events are weighted according to the expected signal-to-background ratio of their category. In the top panel, the signal-plus-background fit is visible in blue, while in the lower panel the fitted signal (in red) is compared to the difference between the data and the background model. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <h3><strong><em>Divide et impera</em></strong></h3> <p>To further increase the sensitivity of their analysis, ATLAS physicists divided their events into 20 mutually-exclusive “categories”. These categories focussed on the features of an event – such as the number and properties of its additional jets or leptons – to target specific production modes of the Higgs boson, including the scattering of two gluons or two weak bosons, and the associated production with a weak W or Z boson or a top-quark pair. Inside these categories, events were further split using dedicated multivariate discriminants (<em>Boosted Decision Trees</em>). As a result of this complex division, ATLAS physicists could separate out the few Higgs-boson-like events from the more common, but less Higgs-boson-like, events.</p> <p>In addition, ATLAS physicists developed a robust (and ambitious) background-modelling strategy using a variety of simulation techniques to create more than 10 billion simulated events. Detailed ATLAS detector simulations (totalling about five times the Run-2 dataset) were complemented by dedicated fast simulation samples (more than 100 times the dataset). The fast simulation samples were crucial to ensure that the overwhelming backgrounds could not mimic a false signal, while maximising the analysis sensitivity to a real signal.</p> <h3><strong>Let the die be cast</strong></h3> <p>The new ATLAS result gives a first hint of the Higgs boson decaying to a muon pair; the significance of the observed signal amounts to 2.0 standard deviations and the ratio of the observed signal yield to the one expected in the Standard Model is 1.2 ± 0.6. The data, together with the signal-plus-background fit, are shown in Figure 2, where data events are weighted to reflect the signal-to-background ratio of their respective categories. More data to be collected in Run 3 (2022–2024) and during the operation of the High-Luminosity LHC will help close in on this first hint. </p> <hr class="divider"> <h3>Links</h3> <ul> <li><a href="https://arxiv.org/abs/2007.07830">A search for the dimuon decay of the Standard Model Higgs boson with the ATLAS detector</a> (arXiv: 2007.07830, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2019-14/">see figures</a>)</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a></li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Thu, 23 Jul 2020 16:51:00 +0000 Steven Goldfarb 6672 at https://atlas.cern Keeping the ATLAS Inner Detector in perfect alignment https://atlas.cern/updates/experiment-briefing/inner-detector-alignment <span>Keeping the ATLAS Inner Detector in perfect alignment </span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Thu, 07/16/2020 - 12:46</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/experiment-briefing" hreflang="en">Experiment Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/inner-detector" hreflang="en">inner detector</a></div> <div class="field--item"><a href="/tags/run-2" hreflang="en">Run 2</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p><strong>How do you track a particle’s trajectory when your detector keeps moving? What if you find slight biases in your detector’s measurements? These were the challenges faced by the ATLAS Inner Detector during Run 2 of the LHC (2015–2018). Located at the heart of the experiment, the Inner Detector provides efficient and precise measurements of charged-particle tracks. In a <a href="https://arxiv.org/abs/2007.07624">new paper released today</a>, physicists describe the complex solutions they developed to align the Inner Detector, ensuring the continued accuracy of the experiment.</strong></p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-1" title="View on CDS"><img alt="Inner Detector,Technology,Physics,Plots or Distributions,Detectors,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-1/file?size=large" /></a><figcaption>Figure 1: The structure of the ATLAS Inner Tracking Detector made of highly granular silicon pixels, silicon strips and straw tubes (left). A schematic representation of a charged particle crossing sub-detector planes (right). Red stars indicate every detector measurement (hit). The fitted track is shown with a black arrow. Green dots indicate the intersection of the track with each surface. The blue lines represent the track-to-hit residual of each layer. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>The ATLAS Inner Tracking Detector is composed of three sub-detectors using different technologies: highly granular silicon pixels closest to the proton beams, followed by silicon strips and straw tubes. As charged particles traverse the detector, they leave behind a track of small energy deposits (or hits) in each sub-detector, allowing physicists to reconstruct the particle’s trajectory. Standing over 2 m tall and 6 m long, the ATLAS Inner Detector is able to measure the position of charged particles that pass through it to better than a 100th of a millimetre. To reach that precision, the detector must be aligned to equal or better accuracy. </p> <p>Crucial to an accurate reconstruction is a detailed knowledge of the Inner Detector’s geometry – that is, the location and orientation of every individual sub-detector element. Detectors are far from stationary; during high-intensity LHC collisions, detector elements can shift due to fluctuations in temperature or changes in the magnetic field strength. To track these movements, ATLAS physicists regularly align the Inner Detector to determine the actual geometry of every active sub-detector element and to follow changes with time. This alignment has to be done through indirect methods, as the detector is inaccessible while the LHC is in operation.</p> <p>As shown in Figure 1, there is a small difference between where a hit is recorded (red star) and the intersection of a reconstructed particle’s track with the detector plane (green dot). This is known as the <em>track-to-hit residual</em>, shown in the figure with blue lines. For a collection of tracks crossing an individual sub-detector, the measured residuals will be distributed randomly. If the position of a sub-detector is correctly known, the residuals will balance each other out. On the contrary, if the assumed position is wrong, physicists will notice a systematic bias in the residuals in one particular direction, indicating the misalignment of a sub-detector.</p> <p>This is the basic idea behind ATLAS’ alignment algorithm. Physicists shift and rotate the assumed position of every detector element until no systematic biases in the track-to-hit residuals are observed. This is done iteratively, following the hierarchical structure of the Inner Detector. First, physicists align the large physical structures that capture the collective movements of the Inner Detector. Then they move on to aligning the individual sub-detector elements.</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">Detectors are far from stationary; during high-intensity LHC collisions, detector elements can shift due to fluctuations in temperature or changes in the magnetic field strength. </h3> <hr class="divider"> <div class="narrow"> <h3><strong>What do you do if your detector moves?</strong></h3> <p>During Run 2 of the LHC, ATLAS researchers found that parts of the Inner Detector showed signs of short-timescale movements while recording data. A good example of this was the vertical displacement of the Pixel sub-detector, as shown in Figure 2 for an average fill of the LHC. What could be causing this movement?</p> <figure class="right mobile-float img-60"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-6" title="View on CDS"><img alt="Inner Detector,Technology,Physics,Plots or Distributions,Detectors,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-6/file?size=large" /></a><figcaption>Figure 2: The vertical position relative to its position at the start of the LHC fill of the Pixel sub-detector vs. the time since the start of an LHC fill. The blue dashed line shows the fill averaged Pixel vertical position. The instantaneous luminosity of the LHC fill is shown in green. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>The high frequency of LHC collisions saw the Inner Detector on-module readout chips saving data up to 100,000 times per second. This required a significant amount of electrical power, which in turn caused a rise in temperature at the centre of the ATLAS detector. For the Pixel sub-detector, this excess of heat would bring the sub-detector’s cooling liquid to a boil, causing a rapid change in the coolant’s mass within the detector. The sub-detector would experience significant displacements over the first hour of data-taking, until thermal equilibrium between the sub-detector and the cooling system was reached. </p> <p>In time, as the intensity of the collisions decreased during a proton–proton fill of the LHC, so would the Pixel’s heat dissipation needs. The Pixel’s coolant would gradually return to its liquid form, increasing the sub-detector’s overall mass and inducing it to drift slowly in the opposite direction. This correlation between the Pixel sub-detector’s vertical position and the instantaneous luminosity (or collision rate) of the LHC is clearly visible in Figure 2.</p> <p>Clearly the average position across an LHC fill does not accurately describe the sub-detector’s position! To solve this issue, ATLAS physicists had to develop a new, automated alignment scheme for the Inner Detector. This dynamic alignment would update throughout each LHC fill, calibrating the data recorded by ATLAS accordingly. New alignment constants were derived every 20 minutes during the first hour of data-taking and every 100 minutes for the rest of the fill. Figure 3 shows the evolution of the vertical positions of the innermost layer of the Pixel sub-detector, the so-called Insertable B-Layer (IBL), and the average of the other Pixel layers over a period of two months.</p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-5" title="View on CDS"><img alt="Inner Detector,Technology,Physics,Plots or Distributions,Detectors,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-5/file?size=large" /></a><figcaption>Figure 3: Vertical corrections to the position of the innermost layer Pixel, IBL, (in blue) and the remaining Pixel layers (in red) for LHC fills recorded in June and July, 2016. Unlike its surrounding layers, the IBL’s vertical position was stable during data-taking. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">ATLAS developed a dynamic new alignment scheme for the Inner Detector. It would update throughout each LHC fill and calibrate the recorded data ATLAS accordingly. </h3> <hr class="divider"> <div class="narrow"> <h3><strong>How do you ensure your tracking is not biased?</strong></h3> <p>Surrounding the ATLAS Inner Detector is a strong magnetic field, which bends the paths of charged particles allowing physicists to measure their momentum and charge. Ideally, a charged particle moving through an homogeneous magnetic field should follow a spiral path (helix). However, small misalignments in the Inner Detector can alter the spiral shape of the reconstructed particle’s track. While these are usually corrected for by ATLAS’ alignment procedure, there are some deformations to which the alignment algorithm is not sensitive that can bias the track’s parameters while maintaining its spiral shape.</p> <p>The two most common deformations that bias the curvature (momentum) of a particle’s track are shown in Figure 4. A rotation of the detector layers (left) can cause a <strong>sagitta bias</strong>, which has opposite effects on positively and negatively charged particles. The sagitta of a circular arc is the distance from the center of the arc to the center of a line joining the two ends of the arc. The smaller the sagitta, the larger is a particle’s momentum. A radial expansion of the detector layers (right) can cause <strong>length-scale bias</strong>, affecting positive and negative particles equally. ATLAS physicists had to develop new ways to correct for these biases, independent of the alignment procedure.</p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-4" title="View on CDS"><img alt="Inner Detector,Technology,Physics,Plots or Distributions,Detectors,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-4/file?size=large" /></a><figcaption>Figure 4: Visualisation of a possible sagitta bias (left) due to a rotation of detector layers and length-scale bias (right) due to a radial expansion of detector layers. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Physicists turned to their data in search of a solution. Sagitta biases, for example, could be measured using any known particle that decays into a pair of stable particles. At ATLAS, this was best seen in the decay of a Z boson into a pair of oppositely-charged muons. The positive and negative muons from these decays would emerge in roughly opposite directions with, on average, the same energy. If physicists found a systematic difference in the measured momentum of the oppositely-charged muons, it would imply the presence of a sagitta bias in that region of the detector. This bias could then be measured and corrected for. The measured sagitta distortions can be seen in Figure 5, also examining their change during Run 2 of the LHC. They remained consistent, indicating a stable detector geometry throughout operation.</p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-3" title="View on CDS"><img alt="Inner Detector,Technology,Physics,Plots or Distributions,Detectors,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-3/file?size=large" /></a><figcaption>Figure 5: Left: The measured sagitta distortions (δ) depending on the track direction (φ and η) using the data-correction technique. The central region (|η| &lt; ~1.2) of the Inner Detector is largely free of sagitta bias, while the outer regions exhibit some areas of small residual sagitta bias. Right: φ-averaged sagitta distortions (δ) across the Inner Detector (η). (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>To correct for length-scale biases, ATLAS physicists studied the mass measurements of well-known particles, such as the J/ψ meson or the Z boson. Both of these particles decay to pairs of oppositely-charged muons, whose measured energy and direction would be impacted by length-scale biases.</p> <p>Physicists developed a new method to differentiate between radial biases – which only affect the transverse component of the track – and scale biases – which affect longitudinal and transverse components in the same way. As seen in Figure 6, a radial bias would show up as a tilt of the reconstructed mass, while an offset would indicate the presence of a scale bias. ATLAS physicists found the existence of a scale bias (ε<sub>s</sub>= -0.9x10<sup>-3</sup>) but no significant radial bias (ε<sub>r</sub>). </p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-2" title="View on CDS"><img alt="Inner Detector,Technology,Physics,Plots or Distributions,Detectors,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-018-2/file?size=large" /></a><figcaption>Figure 6: The reconstructed mass of the J/ψ (left) and the Z (right) as a function of the energy and velocity of the muons (sin2α). (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p><strong>Today’s new paper shows but a fraction of the work and dedication that goes into the operation of an experiment as complex as ATLAS. Physicists are now preparing the Inner Detector for its next challenge: Run 3 of the LHC which is scheduled to start in early 2022.</strong></p> <hr class="divider"> <h3><strong>Links</strong></h3> <ul> <li><a href="https://arxiv.org/abs/2007.07624">Alignment of the ATLAS Inner Detector in Run-2</a> (arXiv: 2007.07624)</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Thu, 16 Jul 2020 10:46:00 +0000 Steven Goldfarb 6671 at https://atlas.cern New ATLAS result addresses long-standing tension in the Standard Model https://atlas.cern/updates/briefing/addressing-long-standing-tension-standard-model <span>New ATLAS result addresses long-standing tension in the Standard Model</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Thu, 05/28/2020 - 11:01</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/standard-model-0" hreflang="en">Standard Model</a></div> <div class="field--item"><a href="/tags/lhcp-0" hreflang="en">LHCP</a></div> <div class="field--item"><a href="/tags/lhcp-2020-0" hreflang="en">LHCP 2020</a></div> <div class="field--item"><a href="/tags/top-quark" hreflang="en">top quark</a></div> <div class="field--item"><a href="/tags/topq-group" hreflang="en">TOPQ group</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p>Perhaps the best-known particle in the lepton family is the electron: a key building block of matter and central to our understanding of electricity. But the electron is not an only child. Electrons are accompanied by heavier siblings, the muon and tau-lepton, to give three lepton <em>flavours</em>. According to the Standard Model of particle physics, the only difference between these siblings should be their mass: the muon is heavier than the electron, and the tau-lepton is heavier than the muon. It is a remarkable feature of the Standard Model that each flavour is equally likely to interact with a W boson; this is known as <em>lepton flavour universality</em>. </p> <p>This week, at the Large Hadron Collider Physics (<a href="http://www.lhcp2020.fr/">LHCP 2020</a>) conference, the ATLAS Collaboration presented a <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-014/">precise measurement of lepton flavour universality</a> using a brand-new technique. Physicists examined collision events where pairs of top quarks decay to pairs of W bosons, and subsequently into leptons. They then measured the relative probability that this lepton is a muon or a tau-lepton – a ratio known as R(τ/μ). According to the Standard Model, R(τ/μ) should be unity, as the interaction strength with a W boson should be the same for a tau-lepton and a muon. But there has been long-standing tension with this prediction, ever since experiments at the Large Electron-Positron (LEP) collider in the 1990s <a href="https://arxiv.org/abs/1302.3415">measured</a> R(τ/μ) to be 1.070 ± 0.026, deviating from the Standard Model expectation by 2.7 standard deviations. This has motivated a new measurement with higher precision. </p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The new ATLAS measurement of lepton flavour universality is in agreement with the Standard Model expectation, suggesting that the previous discrepancy measured by the Large Electron-Positron collider may be due to a fluctuation.</h3> <hr class="divider"> <div class="narrow"> <figure class="right mobile-float img-50"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-012-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-012-2/file?size=large" /></a><figcaption>Figure 1: An example |d0μ| distribution (in the eμ-channel for the highest pTμ bin) after the fit to data has been performed. This shows the separation power between prompt μ and (τ→μ) contributions. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>LEP collided electrons and their anti-particles (positrons), which provided a very clean environment for precision measurements like this one. Yet it produced a relatively low number of W bosons, limiting the measurement’s statistical precision. The LHC collides protons, which are strongly interacting composite particles that create more complicated collisions. This gives larger backgrounds to such measurements, such that it had not been thought possible to reach the same level of precision as LEP. The LHC – often referred to as a top-quark factory – produces over 100 million collisions containing top-quark pairs, thus providing a huge dataset from which to measure R(τ/μ). </p> <p>One of the complexities of this analysis comes in identifying the origin of the muons used to measure R(τ/μ). They are produced either from the W boson directly decaying to a muon (prompt μ), or from the W boson decaying to a tau-lepton and the tau-lepton subsequently decaying to a muon (τ→μ) plus two invisible neutrinos. But how can physicists tell which is which?</p> <p>The tau-lepton has a significant lifetime – a tau-lepton with 40 GeV momentum flies about 2 millimetres in the detector before decaying into multiple particles, including a muon. This leads to a slightly different signature in the detector compared to the muons coming directly from W boson decays (with no measurable flight length). Fortunately, the ATLAS detector is excellent at reconstructing muons as they pass through the experiment. Physicists are able to determine the origins of the muons by examining how close the muon track is to the beam line (|d<sub>0</sub><sup>μ</sup>|) and its transverse momentum (p<sub>T</sub><sup>μ</sup>), as shown in Figure 1. They can then extract R(τ/μ) and, in doing so, cancel out several systematic uncertainties which appear in the numerator and denominator. This is a key feature of the analysis and is what enables the high precision of the measurement.</p> <p>The new ATLAS measurement gives a value of R(τ/μ) = 0.992 ± 0.013. This is the most precise measurement of this ratio to date, with an uncertainty half the size of that from the combination of LEP results. Figure 2 compares this result to the previous LEP measurement. The ATLAS measurement is in agreement with the Standard Model expectation and suggests that the previous LEP discrepancy may be due to a fluctuation. The Standard Model survives this latest test of lepton flavour universality!</p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-012-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-012-1/file?size=large" /></a><figcaption>Figure 2: A comparison of the ATLAS measurement to that from LEP. The vertical dashed line indicates equal branching ratios to different lepton flavours. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <hr class="divider"> <p>Links</p> <ul> <li><div><span style="font-size: 13px;"><a href="http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2018-29/">Test of the universality of τ and μ lepton couplings in W boson decays from ttbar events at 13 TeV with the ATLAS detector</a> </span>(<a href="https://arxiv.org/abs/2007.14040"><span style="font-size: 13px;">arXiv:2007.14040</span>)</a></div></li> <li><div>LHCP 2020 talk: <a href="https://indico.cern.ch/event/856696/contributions/3742185/attachments/2045772/3427421/Presentation.pdf">Measurements of top properties in ATLAS</a> by C. Young<br /> LHCP 2020 talk: <a href="https://indico.cern.ch/event/856696/contributions/3722366/attachments/2044837/3425478/RareTop_LHCP_ATLAS.pdf">Rare top production processes, rare top decays</a> by E. Shabalina</div></li> <li><a href="https://arxiv.org/abs/1302.3415">Electroweak Measurements in Electron-Positron Collisions at W-Boson-Pair Energies at LEP</a> (arXiv: 1302.3415)</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Thu, 28 May 2020 09:01:00 +0000 Steven Goldfarb 6667 at https://atlas.cern Fantastic decays and where to find them https://atlas.cern/updates/briefing/fantastic-decays-and-where-find-them <span>Fantastic decays and where to find them</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Wed, 05/27/2020 - 15:08</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-subtitle field--type-text field--label-hidden field--item">New searches for supersymmetry question common assumptions </div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/susy" hreflang="en">SUSY</a></div> <div class="field--item"><a href="/tags/supersymmetry" hreflang="en">supersymmetry</a></div> <div class="field--item"><a href="/tags/lhcp-0" hreflang="en">LHCP</a></div> <div class="field--item"><a href="/tags/lhcp-2020-0" hreflang="en">LHCP 2020</a></div> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p><a href="https://atlas.cern/updates/atlas-feature/broken-symmetry-searches-supersymmetry-lhc">Supersymmetry</a> (SUSY) offers an elegant solution to the limitations of the Standard Model, extending it to give each elementary particle a “superpartner” with different <a href="https://atlas.cern/glossary/spin">spin</a> properties. Yet SUSY also contains interactions that would cause phenomena not observed in nature, such as the decay of protons. This has traditionally been avoided by requiring the conservation of a property known as “<a href="https://atlas.cern/glossary/r-parity">R-parity</a>” (or “matter-parity”), which incorporates the baryon number, lepton number and spin. As a direct consequence, SUSY particles cannot decay only into Standard Model particles and each decay chain includes the lightest stable SUSY particle, usually assumed to be invisible to the ATLAS experiment.</p> <p>However, this may not be the only solution. ATLAS physicists are also considering SUSY models with R-parity violation (or “RPV”), which would allow the lightest SUSY particle to be observed decaying directly into Standard Model particles. These new models would forbid the violation of the baryon number or the lepton number, but not both — thus maintaining the proton’s stability.</p> <p>The ATLAS Collaboration has released two new searches for SUSY with RPV, studying the full LHC Run-2 dataset at 13 TeV proton–proton collision energy (2015–2018). The analyses looked for the production of pairs of SUSY particles, each decaying fully to observable Standard Model particles through different RPV interactions.</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">ATLAS physicists are also considering SUSY models with matter-parity violation, which would allow the lightest SUSY particle to be observed decaying directly into Standard Model particles. </h3> <hr class="divider"> <div class="narrow"> <h3><strong>Searching for new phenomena with many b-jets</strong></h3> <figure class="right mobile-float img-40"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-1/file?size=large" /></a><figcaption>Figure 1: A diagram of the pair production of two stop particles targeted by the search for multiple b-jets. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>ATLAS physicists considered RPV interactions that allow for baryon-number violation in their new search for pairs of top squarks (or “stops”), the supersymmetric partners of the well-known top quarks. If present in nature and not too heavy, the stop pair could be produced at the LHC at a high rate and would leave a unique signature in the ATLAS experiment. The stop pair would decay via RPV interactions to eight “jets” – collimated sprays of particles – with six originating from the decay of bottom quarks (“b-jets”) (see Figure 1). These b-jets inherit very special features from the bottom quark, such as presence of particles with measurably long lifetime, making it possible to identify them experimentally. In the Standard Model, events with this many b-jets are extremely rare, and thus a signal would easily stand out (see Figure 2).</p> <p>However, a very challenging aspect of this search is predicting the Standard Model background processes. If not properly identified, such processes could mimic a SUSY signal. For this <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-016/">new analysis</a>, ATLAS physicists employed a sophisticated "data-driven” method to tackle this problem, using data with fewer b-jets to predict the background with many b-jets. No signs of stops were found in this search. As shown in Figure 3, the results were used to limit the range of stop masses where SUSY particles could have escaped this search.</p> <div class="span1of2"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-2/file?size=large" /></a><figcaption>Figure 2: The expected background and observed data in the different jet (j) and b-jet (b) multiplicity bins. The signal model has a larger number of b-jets than the background. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div> <div class="span1of2 last"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-3" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-3/file?size=large" /></a><figcaption>Figure 3: Observed (expected) limits on the stop pair production are shown by the red line (black dashed line). The mass of stop is shown on the x-axis, while the mass of the intermediary chargino is shown on the y-axis. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div><div style="clear: both; height: 0;"></div> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">In one analysis, physicists searched for SUSY particles created via electroweak interactions. These would be very rarely produced at the LHC – and are only now accessible thanks to the enormous dataset recorded during Run 2.</h3> <hr class="divider"> <div class="narrow"> <h3><strong>Exploring decays with three leptons</strong></h3> <figure class="right mobile-float img-40"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-6" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-6/file?size=large" /></a><figcaption>Figure 4: A diagram of the pair production of a chargino and a neutralino targeted by the search for a trilepton resonance. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Supersymmetry may also be hiding in processes that violate lepton-number conservation. Another <a href="http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-009/">new ATLAS search</a> looked for the rare production of pairs of new SUSY particles – charginos and neutralinos – whose nearly identical masses allow both to decay through RPV interactions. These SUSY particles are produced through the electroweak interaction and are thus rare at the LHC – only now accessible thanks to the enormous dataset recorded during Run 2.</p> <p>ATLAS physicists searched for a chargino decaying to a lepton and a Z boson that subsequently decays to leptons, giving a signature with a spectacular three-lepton (trilepton) resonance (see Figure 4). The chargino is produced alongside a second chargino or neutralino that would also decay, leading to a number of different signatures in the ATLAS detector. In events where both decays could be fully reconstructed, physicists were able to use the lack of mass asymmetry of both SUSY particles as a powerful handle for identification.</p> <p>This was the first 13 TeV search for new resonances in a trilepton mass distribution. As shown in Figure 5, the trilepton mass distribution should peak sharply for a signal over the expected Standard Model distribution. No significant excess was found, and the results were used to place constraints on the masses of charginos and neutralinos.</p> <div class="span1of2"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-5" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-5/file?size=large" /></a><figcaption>Figure 5: The trilepton invariant-mass distribution in the fully-reconstructed signal region, which is seen to peak sharply for signal events of three example masses as opposed to the smooth distribution from Standard Model processes. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div> <div class="span1of2 last"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-4" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-010-4/file?size=large" /></a><figcaption>Figure 6: Observed and expected 95% exclusion limits on chargino–chargino and chargino–neutralino production for equal branching fractions to electrons, muons and tau-leptons. The grey hashed region shows masses and branching fractions to Z bosons that are excluded. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div><div style="clear: both; height: 0;"></div> <p>The strength of the exclusion depends on how often the SUSY particles decay into each lepton flavour (electron, muon and tau lepton) and into each boson type (W, Z or Higgs boson). The mass limits are most sensitive to the decay fraction to Z bosons, and are seen in Figure 6 to exclude masses up to 975 GeV for a 100% decay fraction to Z bosons and democratic decay fractions to each lepton flavour.</p> <hr class="divider"> <p>Links</p> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-016/">Search for phenomena beyond the Standard Model in events with large b-jet multiplicity using the ATLAS detector at the LHC</a> (ATLAS-CONF-2020-016)</li> <li><a href="http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-009/">Search for trilepton resonances from chargino and neutralino pair production in 13 TeV proton-proton collisions with the ATLAS detector</a> (ATLAS-CONF-2020-009)</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Wed, 27 May 2020 13:08:00 +0000 Steven Goldfarb 6666 at https://atlas.cern ATLAS finds evidence of spectacular four-top quark production https://atlas.cern/updates/briefing/evidence-four-top-quark-production <span>ATLAS finds evidence of spectacular four-top quark production</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Tue, 05/26/2020 - 16:20</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/top-quark" hreflang="en">top quark</a></div> <div class="field--item"><a href="/tags/lhcp-2020-0" hreflang="en">LHCP 2020</a></div> <div class="field--item"><a href="/tags/topq-group" hreflang="en">TOPQ group</a></div> <div class="field--item"><a href="/tags/lhcp-0" hreflang="en">LHCP</a></div> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> </div> <div class="field field--name-field-image-caption field--type-string-long field--label-hidden field--item">Figure 1: Event display of a candidate four-top-quark event, where two of the top quarks decay leptonically (one with a resulting muon (red) and one with an electron (green)), and two top quarks decay hadronically (green and yellow rectangles). The jets (b-tagged jets) are shown as yellow (blue) cones. (Image: ATLAS Collaboration/CERN)</div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p><strong>In a new result released today, the ATLAS Collaboration announced strong evidence of the production of four top quarks. This rare Standard Model process is expected to occur only once for every 70 thousand pairs of top quarks created at the LHC and has proven extremely difficult to measure.</strong></p> <p>The top quark is the most massive elementary particle in the Standard Model, clocking in at 173 GeV, which is equivalent to the mass of a gold atom. But contrary to gold, whose mass is mostly due to the nuclear binding force, the top quark gets all its mass from the interaction with the Higgs field. So when four top quarks are produced in a single event, they create the heaviest particle final state ever seen at the LHC, with almost 700 GeV in total. This is an ideal environment to search for new physics with yet unknown particles contributing to the process. Should they exist, physicists will see additional production of four top quarks above what is predicted by the Standard Model, further motivating a detailed study of the process.</p> <p>In this <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-013/">new search for four-top-quark production</a>, ATLAS physicists studied the full Run 2 dataset recorded between 2015 and 2018. When produced through proton–proton collisions at the LHC, this process leaves spectacular signatures in the ATLAS detector. The four top quarks produce four W bosons and four jets – collimated sprays of particles – originating from bottom quarks. The W bosons then, in turn, each decay into two jets or one charged lepton (electron, muon or tau leptons) and an invisible neutrino. As a final step, the tau leptons decay into a lighter lepton or a jet, with additional neutrinos.</p> <p>For this result, physicists chose to focus on collision events producing two leptons with the same charge or three leptons. Despite accounting for only 12% of all four-top-quark decays, these signatures are easier to distinguish from background processes in the ATLAS detector. Detecting a signal nevertheless required a detailed understanding of the remaining background processes and the use of sophisticated separation techniques.</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">When four top quarks are produced in a single event, they create the heaviest particle final state ever seen at the LHC, with almost 700 GeV in total. </h3> <hr class="divider"> <div class="narrow"> <p>ATLAS physicists trained a multivariate discriminant (<em>boosted decision tree</em>) using the distinct features of the signal, including the high numbers of jets, their quark-flavour origin (bottom quark or not), and the energies and angular distributions of the measured particles. The main background processes that resemble the signal stem from the production of a pair of top quarks in association with other particles, such as a W or Z boson, a Higgs boson, or another top quark. Some of these processes have themselves only recently been observed by the ATLAS and CMS Collaborations.</p> <div class="span1of2"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-011-3" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-011-3/file?size=large" /></a><figcaption>Figure 2: The boosted decision tree (BDT) score output for the signal region (SR). The data are shown in black; the simulated signal in red. The y-axis shows the number of events and is in the logarithmic-scale. The band includes the total uncertainty on the post profile-likelihood fit (post-fit) computation. The ratio of the data to the total post-fit computation is shown in the lower panel. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div> <div class="span1of2 last"> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-011-4" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-011-4/file?size=large" /></a><figcaption>Figure 3: Comparison between data and prediction for signal-region events with a BDT score greater than 0 for the distributions of the number of jets. The band includes the total uncertainty on the post profile-likelihood fit (post-fit) computation. The ratio of the data to the total post-fit computation is shown in the lower panel. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div><div style="clear: both; height: 0;"></div> <p>Each background process was individually evaluated, primarily through dedicated simulations which included information from the best available theoretical predictions. The most difficult background processes – the top-quark-pair production with a W boson and backgrounds with <em>fake leptons</em> – had to be determined using data from dedicated control regions. Fake leptons arise when the charge of a lepton is misidentified, or when leptons come from a different process, but are attributed to the signal. Both had to be well-understood and precisely evaluated in order to reduce the systematic uncertainty on the final result.</p> <p>ATLAS measured the cross section for the production of four top quarks to be 24 <sup>+7</sup><sub>–6</sub> fb, which is consistent with the Standard Model prediction (12 fb) at 1.7 standard deviations. The signal significance amounts to 4.3 standard deviations, for an expected significance of 2.4 standard deviations were the four-top-quark signal equal to the Standard Model prediction. The measurement provides strong evidence for this process.</p> <p>Additional data from the next LHC run – along with further developments of the analysis techniques employed – will improve the precision of this challenging measurement.</p> <hr class="divider"> <h3>Links</h3> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-013/">Evidence for tt̄tt̄ production in the multilepton final state in proton-proton collisions at 13 TeV with the ATLAS detector</a> (ATLAS-CONF-2020-013)</li> <li>LHCP 2020 talk: <a href="https://indico.cern.ch/event/856696/contributions/3722366/attachments/2044837/3425478/RareTop_LHCP_ATLAS.pdf">Rare top production processes, rare top decays</a> by E. Shabalina</li> <li>CMS Collaboration: <a href="https://link.springer.com/article/10.1140/epjc/s10052-019-7593-7">Search for production of four top quarks in final states with same-sign or multiple leptons in proton-proton collisions at 13 TeV</a> (Eur. Phys. J. C 80 , 2 (2020) 75, <a href="https://arxiv.org/abs/1908.06463">arXiv: 1908.06463</a>)</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Tue, 26 May 2020 14:20:00 +0000 Steven Goldfarb 6665 at https://atlas.cern ATLAS measures light scattering on light and constrains axion-like particles https://atlas.cern/updates/briefing/light-scattering-light-constrains-axion-particles <span>ATLAS measures light scattering on light and constrains axion-like particles</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Mon, 05/25/2020 - 17:40</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/hion-group-0" hreflang="en">HION group</a></div> <div class="field--item"><a href="/tags/heavy-ion" hreflang="en">Heavy Ion</a></div> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/lhcp-2020-0" hreflang="en">LHCP 2020</a></div> <div class="field--item"><a href="/tags/lhcp-0" hreflang="en">LHCP</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p><strong>Light-by-light scattering is a very rare phenomenon in which two photons – particles of light – interact, producing another pair of photons. Direct observation of this process at high energy had proven elusive for decades, until it was</strong> <strong>first seen by the ATLAS Collaboration in 2016 and established in 2019. In a new measurement, ATLAS physicists are using light-by-light scattering to search for a hyped phenomenon beyond the Standard Model of particle physics: axion-like particles.</strong></p> <p>Collisions of heavy lead ions in the Large Hadron Collider (LHC) provide the ideal environment to study light-by-light scattering. As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated corresponding to an electrical field with strength of up to 10<sup>25</sup> volt per metre. When ions from opposite beams pass next to each other at the centre of the ATLAS detector, their surrounding photons can interact and scatter off one another. Because the lead ions lose only a tiny fraction of their energy in this process, the outgoing ions continue their path around the LHC ring, unseen by the ATLAS detector. These interactions are known as <em>ultra-peripheral collisions</em>. This leads to a distinct event signature, very unlike typical lead ion collision events, with two back-to-back photons and no further activity in the detector.</p> <p>Based on lead–lead collision data recorded in 2015, the ATLAS Collaboration found the <a href="https://atlas.cern/updates/press-statement/atlas-sees-first-direct-evidence-light-light-scattering-high-energy">first direct evidence of high-energy light-by-light scattering</a>. More recently the ATLAS Collaboration reported the <a href="http://atlas.cern/updates/physics-briefing/atlas-observes-light-scattering-light">observation of light-by-light scattering</a> with a significance of 8.2 standard deviations, using a large data sample taken in 2018.</p> <p>In new results <a href="https://indico.cern.ch/event/856696/contributions/3742119/attachments/2044134/3424089/lhcp_2020.pdf">presented today</a>, the ATLAS Collaboration studied the full LHC Run-2 dataset of heavy-ion collisions to measure light-by-light scattering with improved precision and more detail. Out of the more than hundred billion ultra-peripheral collisions probed, ATLAS observed a total of 97 candidate events while 27 events are expected from background processes. In addition to the production rate (<a href="http://atlas.cern/glossary/cross-section">cross section</a>), ATLAS measured the energies and angular distributions of the produced photons (i.e. their kinematics). The result explores a broader range of diphoton masses, increasing the expected signal yield by about 50% in comparison to the previous ATLAS measurements.</p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-009-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-009-2/file?size=large" /></a><figcaption>Figure 1: Differential cross section of γγ→γγ production in lead–lead collisions at 5.02 TeV as a function of the invariant mass of the diphoton system and the cosine of the scattering angle in the photon-photon centre-of-mass frame, as measured by ATLAS. The measurements are compared to the theoretical prediction. (Image: ATLAS Collaboration/CERN)</figcaption></figure> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The measurement of light-by-light scattering is sensitive to processes beyond the Standard Model, such as axion-like particles. The new ATLAS analysis places some of the strongest limits on the production of these particles to date.</h3> <hr class="divider"> <div class="narrow"> <figure class="right mobile-float img-50"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-009-1" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-009-1/file?size=large" /></a><figcaption>Figure 2: Compilation of exclusion limits at 95% confidence level in the photon–a (axion-like particle) coupling (1/Λa) versus a mass (ma) plane obtained by different experiments. The existing limits are compared to the limits extracted from this measurement. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>The measurement of light-by-light scattering is sensitive to processes beyond the Standard Model, such as axion-like particles. These are hypothetical spin-less (scalar) particles with an odd parity quantum number (the Higgs boson, for example, is a scalar with even parity) and typically weak interactions with Standard Model particles. In the new ATLAS result, physicists considered whether the pairs of interacting photons produce axion-like particles (<em>a</em>) as they scatter off each other (<em>γγ </em>→ <em>a</em> → <em>γγ</em>), which would lead to an excess of scattering events with diphoton mass equal to the mass of <em>a</em>. They examined the diphoton mass distribution for a mass range for <em>a</em> between 6 and 100 GeV. No significant excess of events over the expected background was found in the analysis. ATLAS physicists were able to derive, at a 95% <a href="http://atlas.cern/glossary/confidence-level-cl">confidence level</a>, an exclusion bound of the axion-like particles coupling to photons (Figure 2). Assuming 100% of the putative particles decay to photons, this new analysis places the strongest existing limits on the production of axion-like particles in the examined mass range to date.</p> <p>With the much larger dataset expected in the future LHC runs, physicists will continue to explore the sensitivity of light-by-light scattering to phenomena beyond the Standard Model.</p> <hr class="divider"> <h3>Links</h3> <ul> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-010/">Measurement of light-by-light scattering and search for axion-like particles with 2.2 nb<sup>−1</sup> of Pb+Pb data with the ATLAS detector</a> (ATLAS-CONF-2020-010) </li> <li>LHCP 2020 talk: <a href="https://indico.cern.ch/event/856696/contributions/3742119/attachments/2044134/3424089/lhcp_2020.pdf">Recent results on hard and rare probes from ATLAS</a> by M. Przybycien</li> <li><a href="http://doi.org/10.1103/PhysRevLett.123.052001">Observation of light-by-light scattering in ultraperipheral Pb+Pb collisions with the ATLAS detector</a> (Phys.Rev.Lett. 123 (2019) 5, 052001, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2018-19">see figures</a>)</li> <li><a href="http://atlas.cern/updates/physics-briefing/atlas-observes-light-scattering-light">ATLAS observes light scattering off light</a>, <em>Physics Briefing</em>, March 2019 </li> <li><a href="https://www.nature.com/nphys/journal/v13/n9/full/nphys4208.html">Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC</a> (Nature Physics 13 (2017) 852, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2016-05/">see figures</a>)</li> <li><a href="http://atlas.cern/updates/press-statement/atlas-sees-first-direct-evidence-light-light-scattering-high-energy">ATLAS sees first direct evidence of light-by-light scattering at high energy</a>, <em>Press Statement</em>, August 2017</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Mon, 25 May 2020 15:40:00 +0000 Steven Goldfarb 6664 at https://atlas.cern Machine learning qualitatively changes the search for new particles https://atlas.cern/updates/briefing/search-new-particles-machine-learning <span>Machine learning qualitatively changes the search for new particles</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Wed, 05/13/2020 - 18:49</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/undefined" hreflang="en">undefined</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow"> <p><strong>The ATLAS Collaboration is exploring novel ways to search for new phenomena. Alongside an extensive research programme often inspired by specific theoretical models – ranging from quantum black holes to supersymmetry – physicists are applying new model-independent methods to broaden their searches. ATLAS has just released <a href="https://arxiv.org/abs/2005.02983">the first model-independent search for new particles using a novel technique called “weak supervision”</a>.</strong></p> <p>Searches for new particles typically start with a specific theoretical model. Given the model’s phenomenology and parameters, physicists will simulate how new particles would be produced and decay in the ATLAS detector. They then simulate the Standard Model background processes in order to develop classifiers (with or without machine learning) that separate signals from background. These classifiers determine the best phase-space region of the data to be studied, where a hypothetical signal is expected to be enriched. Finally, physicists will compare the data and background prediction in search of anomalies.</p> <p><a href="https://arxiv.org/abs/2005.02983">ATLAS’ new search</a> uses machine-learning classifiers (neural networks) developed directly on data in order to reduce their dependence on a specific model. This is a significant departure from the standard methods because the data are <em>unlabeled</em>: it is not known if a particular proton–proton collision event is background or signal. This method – known as “weak supervision” – exploits structures in the data without needing per-event labels.</p> <figure class="right mobile-float img-40"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-008-2" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-008-2/file?size=large" /></a><figcaption>Figure 1: Diagram illustrating the construction of mixed samples for training a weakly supervised CWoLa classifier in the bump hunt. In the ATLAS search, the resonant feature (mres) is the dijet mass and the other features (y) are the masses of the two jets. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>Alongside with this method, the new ATLAS search uses one of the most traditional simulation-independent anomaly detection strategies: the “bump hunt”. The goal of a bump hunt is to look for a localised “bump” on top of a smooth background. Such bumps are a generic feature of many models of new particles, where the bump happens at the mass of the new particle. The new search builds on this strong foundation to enhance the sensitivity to a wide variety of hypothetical particles without specifying their properties ahead of time.</p> <p>The combination of bump hunt and weak supervision results in an analysis that is mostly free of signal-model and background-model dependence.</p> <h3><strong>Detecting anomalies with weak supervision</strong></h3> <p>ATLAS physicists trained neural networks on data using a technique called “<a href="https://arxiv.org/abs/1708.02949">Classification without labels</a>” (CWoLa, pronounced “Koala”). In this approach, physicists construct two mixed datasets composed of background and potentially also signal. These are identical except for the relative proportions of the potential signal. While the signal-vs-background labels are unknown for each event, the neural networks can be trained to differentiate between the two datasets. With sufficient data and a powerful enough classifier, this is actually optimal for distinguishing signal from background.</p> <p>The CWoLa method is <a href="https://arxiv.org/abs/1805.02664">combined with a bump hunt</a> when creating the mixed datasets above, as shown in Figure 1. Signal events would be characterised by a localised resonance region and a sideband region. These regions would have other features (y) that can also be used to train the neural networks. If there is no signal, a neural network would not learn anything and if there is a signal, it may learn to pick it out over the background.</p> <p>The new ATLAS search is the first application of fully data-driven machine-learning-enhanced anomaly detection. The search examined events with hadronic final states, using the invariant mass of pairs of particle “jets” as the resonant feature and the masses of the individual jets as the features to train the CWoLa classifier. Using this restricted set of features, physicists have successfully established the procedure and have found it is already sensitive to a wide range of new particles.</p> <p>Physicists were able to train the neural networks while avoiding a statistical trials factor which would reduce the sensitivity of the search from training and testing on the same data. The neural network (Figure 2) is mapped to an efficiency. For example, 10% means that 90% of events have a network output that is lower than this value. In the absence of the signal, the network should not learn anything (as the two mixed datasets should be the same), but there must be a region of low efficiency by design. The right plot of Figure 2 shows that the network is able to identify the injected signal, even though it was not told where to look in advance!</p> <figure class=""><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-008-4" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-008-4/file?size=large" /></a><figcaption>Figure 2: The neural network output in one dijet mass bin. As a two-dimensional function, the output can be readily visualised as an image, where the intensity corresponds to the efficiency of the network output in the dijet mass bin. The left plot has no signal injected and the right plot shows the output when a hypothetical particle at 3 TeV that decays into two other particles at 200 GeV is added to the data. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p> </p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">The combination of bump hunt and weak supervision results in an analysis that is mostly free of signal-model and background-model dependence.</h3> <hr class="divider"> <div class="narrow"> <h3><strong>Providing new precision</strong></h3> <figure class="mobile-float img-50 right"><a href="//cds.cern.ch/images/ATLAS-PHOTO-2020-008-3" title="View on CDS"><img alt="Plots or Distributions,Physics,ATLAS" src="//cds.cern.ch/images/ATLAS-PHOTO-2020-008-3/file?size=large" /></a><figcaption>Figure 3: Particular signals are simulated and then added to the data in order to set limits. The models chosen here represent a heavy particle A (with a mass of 3 TeV) decaying to two other new particles B and C with masses written on the horizontal axis. The vertical axis is the limit - lower numbers indicate stronger limits. The new search is compared with two existing results from ATLAS: the inclusive dijet search (red triangles) and a dedicated search for jets produced from W and Z bosons (grey cross). (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>The new search did not result in significant evidence for new particles and quantifying what was <em>not</em> found was its own challenge. Usually, physicists can simply ask how much signal would have to be added to register a significant excess, and then that amount of signal is declared excluded as no excess was observed. Achieving similar exclusions for this analysis required all of the neural networks to be re-trained for each modelled signal type and signal amount.</p> <p>The resulting limits are presented in Figure 3. Producing this plot required training about 20,000 neural networks! Some signals were harder for the neural networks to find than others, with those in regions with a lot of background proving particularly challenging. For other signals, the new limits are stronger than previous limits and improve upon <a href="https://arxiv.org/abs/1910.08447">previous</a> <a href="https://arxiv.org/abs/1906.08589">searches</a> in a similar phase space.</p> <h3><strong>Looking to the future</strong></h3> <p>This new approach taken by ATLAS has many possibilities for extensions. The weakly supervised bump hunt could be applied to additional event topologies and more features could be added to broaden the sensitivity to new particles. More complex neural networks may be needed to accommodate higher-dimensional feature spaces and this will require demanding computational resources. ATLAS physicists are also considering a variety of alternative anomaly-detection techniques, which may be able to complement the CWoLa-based search. It is likely that no one method will cover everything – multiple approaches will be needed to ensure broad, robust, and strong sensitivity to new particles.</p> <hr class="divider"> <h3><strong>Links</strong></h3> <ul> <li><a href="https://arxiv.org/abs/2005.02983">Dijet resonance search with weak supervision using 13 TeV proton-proton collisions in the ATLAS detector</a> (arXiv: 2005.02983, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HDBS-2018-59/">see figures</a>)</li> <li><a href="https://link.springer.com/article/10.1007/JHEP03(2020)145">Search for new resonances in mass distributions of jet pairs using 139 fb<sup>−1</sup> of proton-proton collisions at 13 TeV with the ATLAS detector</a> (JHEP 03 (2020) 145, <a href="https://arxiv.org/abs/1910.08447">arXiv: 1910.08447</a>, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2019-03/">see figures</a>)</li> <li><a href="https://link.springer.com/article/10.1007%2FJHEP09%282019%29091">Search for diboson resonances in hadronic final states in 139 fb<sup>−1</sup> of proton-proton collisions at 13 TeV with the ATLAS detector</a> (JHEP 09 (2020) 091, <a href="https://arxiv.org/abs/1906.08589">arXiv: 1906.08589</a>, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HDBS-2018-31/">see figures</a>)</li> <li>EP-IT seminar: <a href="https://indico.cern.ch/event/853615/">Dijet Resonance Search using Weak Supervision</a> by A. Cukierman</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Wed, 13 May 2020 16:49:00 +0000 Steven Goldfarb 6663 at https://atlas.cern Probing dark matter with the Higgs boson https://atlas.cern/updates/briefing/probing-dark-matter-higgs-boson <span>Probing dark matter with the Higgs boson</span> <div class="field field--name-field-top-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Top HIghlight</b></div> <div class="field--item">False</div> </div> <span><span lang="" about="/user/2" typeof="schema:Person" property="schema:name" datatype="">Steven Goldfarb</span></span> <span>Tue, 04/21/2020 - 14:41</span> <div class="field field--name-field-highlight field--type-boolean field--label-inline"> <div class="field--label"><b>Highlight</b></div> <div class="field--item">False</div> </div> <div class="field field--name-field-update-category field--type-entity-reference field--label-hidden field--item"><a href="/physics-briefing" hreflang="en">Physics Briefing</a></div> <div class="field field--name-field-author field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/authors/atlas-collaboration" hreflang="en">ATLAS Collaboration</a></div> </div> <div class="field field--name-field-tags field--type-entity-reference field--label-hidden field--items"> <div class="field--item"><a href="/tags/higgs-boson" hreflang="en">Higgs boson</a></div> <div class="field--item"><a href="/tags/dark-matter" hreflang="en">dark matter</a></div> <div class="field--item"><a href="/tags/physics-results" hreflang="en">Physics Results</a></div> <div class="field--item"><a href="/tags/run-2" hreflang="en">Run 2</a></div> <div class="field--item"><a href="/tags/cern-seminar-21-april-2020-0" hreflang="en">CERN Seminar 21 April 2020</a></div> </div> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><div class="narrow">Visible matter – everything from pollen to stars and galaxies – accounts for roughly 15% of the total mass of the Universe. The remaining 85% is made of something entirely different from things we can touch and see: <a href="https://atlas.cern/updates/atlas-feature/dark-matter">dark matter</a>. Despite overwhelming evidence from the observation of gravitational effects, the nature of dark matter and its composition remain unknown.</p> <p>How can physicists study dark matter beyond gravitational effects if it is practically invisible? Three different approaches are pursued: indirect detection with astronomical observatories, searching for the decay products of dark-matter annihilation in galactic centres; direct detection with highly sensitive low-background experiments, looking for dark matter scattering off nuclei; and by creating dark matter in the controlled laboratory environment of the Large Hadron Collider (LHC) at CERN.</p> <p>Although successful at describing elementary particles and their interactions at low energies, the Standard Model of particle physics does not include a viable dark-matter particle. The only possible candidates, neutrinos, do not have the right properties to explain the observed dark matter. To remedy this problem, a simple theoretical extension of the Standard Model posits that existing particles, such as the Higgs boson, act as a “portal” between known particles and dark-matter particles. Since the Higgs boson couples to mass, massive dark-matter particles should interact with it. The Higgs boson still has large uncertainties associated with the strength of its interaction with Standard Model particles; up to 30% of the Higgs-boson decays can potentially be invisible, according to the latest <a href="https://journals.aps.org/prd/pdf/10.1103/PhysRevD.101.012002">ATLAS combined Higgs-boson measurements</a>.</p> <p>Could some of the Higgs bosons decay into dark matter? As dark matter does not interact directly with the ATLAS detector, physicists look for signs of “<a href="https://atlas.cern/updates/atlas-feature/dark-matter#invisibleparticles">invisible particles</a>”, inferred through momentum conservation of the proton–proton collision products. According to the Standard Model, the fraction of Higgs bosons decaying to an invisible final state (four neutrinos!) accounts for just 0.1% and is thus negligible. Should such events be observed, it would be a direct indication of new physics and potential evidence of Higgs bosons decaying into dark-matter particles.</p> </div><div style="clear: both; height: 0;"></div> <hr class="divider"> <h3 class="rtecenter">Could the Higgs boson decay into dark matter? The ATLAS Collaboration searched the full LHC Run 2 dataset to set the strongest limits on the Higgs boson decaying to invisible dark-matter particles to date.</h3> <hr class="divider"> <div class="narrow"> <p>At the LHC, the most sensitive channel to search for direct decays of the Higgs boson to invisible particles is via the so-called vector boson fusion (VBF) production of the Higgs boson. VBF Higgs-boson production results in two sprays of particles (called “jets”) that point in a more forward direction in the ATLAS detector. This, combined with a large missing momentum in perpendicular direction (“transverse”) to the beam axis from the invisible dark-matter particles, creates a unique signature that ATLAS physicists can search for.</p> <figure class=""><a href="/sites/atlas-public.web.cern.ch/files/HiggsDM_fig1.png"><img alt="" src="//atlas-public.web.cern.ch/sites/atlas-public.web.cern.ch/files/HiggsDM_fig1.png" /></a><figcaption>Figure 1: Mass of the leading two jets (x-axis) in the search region with all background processes stacked and compared to data. A hypothetical Higgs boson signal decaying to invisible final states is shown in red. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>In <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-008/">new results presented today</a>, the ATLAS Collaboration studied the full LHC Run 2 dataset, collected by the ATLAS detector in 2015–2018 to search for Higgs-boson decays to dark-matter particles in VBF events. No significant excess of events over the expected background from known Standard Model processes was found in the analysis. ATLAS derived, at a 95% <a href="http://atlas.cern/glossary/confidence-level-cl">confidence level</a>, an exclusion bound of the Higgs-boson decay to invisible particles of 13%. This analysis included roughly 75% more data than the previous ATLAS search, and the team implemented several improvements including:</p> <ul> <li>Faster filtering algorithms to generate more simulated collisions with equivalent computing power. Lack of simulated events was the leading uncertainty in the first 13 TeV version of this analysis.</li> <li>Optimised collision selection to accept ~50% more Higgs-boson events on the same dataset.</li> <li>Refined event categorisation to result in a higher signal-to-background ratio in the search regions. This can be seen in Figure 1 as the red curve in the lower panel increases with higher invariant mass of the two leading jets (m<sub>jj</sub>).</li> <li>Improved acceptance for collisions enriched in background processes, allowing the analysts to improve the background-process modelling.</li> </ul> <p><br /> <figure class=""><a href="/sites/atlas-public.web.cern.ch/files/HiggsDM_fig2.png"><img alt="" src="//atlas-public.web.cern.ch/sites/atlas-public.web.cern.ch/files/HiggsDM_fig2.png" /></a><figcaption>Figure 2: Upper limit on the WIMP-nucleon cross section at 90% confidence level derived in this analysis compared to direct detection experiments. (Image: ATLAS Collaboration/CERN)</figcaption></figure> <p>This observed exclusion is consistent with no signs of the Higgs boson decaying to dark matter. The new results advance the search for weakly interacting massive particles (WIMPs), a popular candidate for dark matter. ATLAS set additional exclusion limits for lower WIMP masses, which are compared to other direct-detection experiments in Figure 2. These limits are competitive with the best direct-detection experiments for WIMP masses up to half of the Higgs-boson mass, assuming the Higgs boson interacts directly with dark matter.</p> <p>This new analysis places the strongest existing limits on the Higgs boson decaying to invisible particles to date. As the search goes on, physicists will continue to increase the sensitivity to this fundamental probe of dark matter.</p> <hr class="divider"> <h3>Links</h3> <ul> <li>CERN LHC Seminar: <a href="https://indico.cern.ch/event/868253/">Searches for rare and invisible Higgs boson decays and for high-mass resonances with the ATLAS detector</a> by Douglas Michael Schaefer and Takuya Nobe</li> <li><a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2020-008/">Search for invisible Higgs boson decays with vector boson fusion signatures with the ATLAS detector using an integrated luminosity of 139 fb<sup>−1</sup></a> (ATLAS-CONF-2020-008)</li> <li><a href="https://journals.aps.org/prd/pdf/10.1103/PhysRevD.101.012002">Combined measurements of Higgs boson production and decay using up to 80 fb<sup>−1</sup> of proton-proton collision data at 13 TeV collected with the ATLAS experiment</a> (Phys. Rev. D 101 (2020) 012002, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2018-57">see figures</a>)</li> <li><a href="https://www.sciencedirect.com/science/article/pii/S0370269319302564?via=ihub">Search for invisible Higgs boson decays in vector boson fusion at 13 TeV with the ATLAS detector</a> (Phys. Lett. B 793 (2019) 499, <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2016-37">see figures</a>)</li> <li>See also the full lists of <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/CONFnotes">ATLAS Conference Notes</a> and <a href="https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome#Physics_papers">ATLAS Physics Papers</a>.</li> </ul> </div><div style="clear: both; height: 0;"></div> </div> Tue, 21 Apr 2020 12:41:00 +0000 Steven Goldfarb 6662 at https://atlas.cern