Rare phenomenon observed by ATLAS features the LHC as a high-energy photon collider

During the International Conference on High-Energy Physics (ICHEP 2020), the ATLAS Collaboration presented the first observation of photon collisions producing pairs of W bosons, elementary particles that carry the weak force, one of the four fundamental forces. The result demonstrates a new way of using the LHC, namely as a high-energy photon collider directly probing electroweak interactions. It confirms one of the main predictions of electroweak theory – that force carriers can interact with themselves – and provides new ways to probe it.

Press Statement | 05 Aug 2020

ATLAS observes W-boson pair production from light colliding with light

The ATLAS Collaboration has announced the first observation of two W bosons produced from the scattering of two photons — particles of light – at the International Conference on High-Energy Physics (ICHEP 2020).

Physics Briefing | 05 Aug 2020

CERN experiments announce first indications of a rare Higgs boson process

The ATLAS and CMS experiments at CERN announce new results which show that the Higgs boson decays into two muons. These new results have pivotal importance for fundamental physics because they indicate for the first time that the Higgs boson interacts with second-generation elementary particles.

Press Statement | 03 Aug 2020

New ATLAS result marks milestone in the test of Standard Model properties

The ATLAS Collaboration has released a new study into a key building block of matter: leptons. 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 key measurement of the W-boson decay rates into leptons by the ATLAS Collaboration.

Physics Briefing | 03 Aug 2020

New measurements of the Higgs boson find strength in unity

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.

Physics Briefing | 31 Jul 2020

Looking forward: ATLAS measures proton scattering when light turns into matter

Today, at the International Conference for High Energy Physics (ICHEP 2020), the ATLAS Collaboration announced first results using the ATLAS Forward Proton (AFP) spectrometer. With this instrument, physicists directly observed and measured the long sought-after prediction of proton scattering when particles of light turn into matter.

Physics Briefing | 30 Jul 2020

ATLAS probes interactions between heavyweights of the Standard Model

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.

Physics Briefing | 30 Jul 2020

Jetting into the dark side: a precision search for dark matter

The nature of dark matter remains one of the great unsolved puzzles of fundamental physics. Many 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. Today, at the International Conference in High-Energy Physics (ICHEP 2020), ATLAS presented a new search for novel phenomena in collision events with jets and high missing transverse momentum (MET).

Physics Briefing | 27 Jul 2020

Summary of new ATLAS results for ICHEP 2020

Since the 1950s, one conference has stayed circled in red on every physicist's calendar: the International Conference on High­-Energy Physics (ICHEP). The fortieth edition of ICHEP kicks off today, bringing together particle physicists, astrophysicists and accelerator scientists to share the latest news in their fields. Originally planned as an in-person event in Prague, ICHEP2020 will instead be the very first all-virtual edition of the conference.

News | 27 Jul 2020

ATLAS one step closer in the search for rare Higgs boson decays to muons

The ATLAS Collaboration has released a new paper on the search for the Higgs-boson decay to a pair of muons. 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.

Physics Briefing | 23 Jul 2020

Summary of ATLAS results presented at BOOST 2020

The first all-virtual BOOST workshop kicks off today, bringing together experts from the LHC experiments and the theory community. This is the twelfth conference on "Boosted Object Phenomenology, Reconstruction and Searches in High-Energy Physics" (BOOST 2020), hosting plenary-style talks and virtual poster presentations on the latest developments in hadronic physics.

News | 20 Jul 2020

Keeping the ATLAS Inner Detector in perfect alignment

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 new paper released today, physicists describe the complex solutions they developed to align the Inner Detector, ensuring the continued accuracy of the experiment.

Experiment Briefing | 16 Jul 2020

Connecting during COVID-19: Updates from the (physically but not socially distanced) Early Career Scientist Board

As a community, we need to stay in contact, remain motivated and learn from each other's experiences. The work-from-home situation is one to which everyone has to adjust, balancing personal and professional lives, while accepting the effect of the ongoing pandemic on society. Despite these challenges, the ATLAS Early Career Scientist Board (ECSB) developed a series of events to boost the morale of the ECS community and to help people connect, even when they are sitting miles away from each other. I joined the ATLAS ECSB in March 2020, and to be honest, it has felt great to be a part of something that makes a difference in people’s lives – even if it’s just to laugh together.

Blog | 01 Jul 2020

You want me to present a poster…. remotely?

Though an academic affair, poster sessions are also an opportunity to network and socialise with colleagues. Typically, a large hall will be filled with rows of poster stands, their authors standing anxiously beside them, anticipating whatever question may be posed by a passer-by. Finger food and drinks are usually served. Sometimes these encounters lead to in-depth discussions about a new result but, more often than not, they just serve as ice-breakers for would-be colleagues, or a kind of “physics buffet” for conference attendees to sample subjects outside their specialization. Could such an experience be recreated in an online conference?

Blog | 22 Jun 2020

In conversation with Claudia Gemme, an influential voice in ATLAS detector upgrades

Claudia Gemme, researcher at INFN in Genova, has had a varied career with the ATLAS Collaboration. From her work on the construction and commissioning of the ATLAS Pixel detector, to a career in physics analysis and the ATLAS Publication Committee, she now leads a key upgrade of the ATLAS detector: the ATLAS Inner Tracker (ITk).

Portrait | 15 Jun 2020

Physicists gather online for the Large Hadron Collider Physics conference

The eighth Large Hadron Collider Physics (LHCP 2020) conference concluded today, 30 May, in Zoom rooms around the world. Instead of descending on Paris to meet, particle physicists held the conference fully online for the first time. As a result, LHCP 2020 welcomed some 1300 registered participants – nearly triple its previous record of attendance. A bumper crop of new ATLAS results were prepared for the conference covering a broad range of topics, from precise measurements of the Standard Model to novel searches for new physics. These new results probed the full dataset collected during Run 2 of the LHC (2015-2018) – a proven gold mine for ATLAS’ rich physics programme.

News | 30 May 2020

New ATLAS result addresses long-standing tension in the Standard Model

This week, at the LHCP 2020 conference, the ATLAS Collaboration presented a precise measurement of lepton flavour universality 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 – but there has been long-standing tension with this prediction, ever since it was measured at the Large Electron-Positron (LEP) collider in the 1990s.

Physics Briefing | 28 May 2020

Fantastic decays and where to find them

Supersymmetry offers an elegant solution to the limitations of the Standard Model, extending it to give each elementary particle a “superpartner” with different spin 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 “R-parity” (or “matter-parity”), which incorporates the baryon number, lepton number and spin. 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.

Physics Briefing | 27 May 2020

ATLAS finds evidence of spectacular four-top quark production

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.

Physics Briefing | 26 May 2020

ATLAS measures light scattering on light and constrains axion-like particles

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 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.

Physics Briefing | 25 May 2020

Summary of new ATLAS results from LHCP 2020

The eighth annual conference on Large Hadron Collider physics (LHCP 2020) kicks off today in video-conference rooms around the world. This week-long event is usually an opportunity for physicists from around the world to meet in person and share the latest news from their LHC experiments. This year, due to the COVID-19 pandemic, the conference is being held online.

News | 25 May 2020

Machine learning qualitatively changes the search for new particles

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 the first model-independent search for new particles using a novel technique called “weak supervision”.

Physics Briefing | 13 May 2020

Probing dark matter with the Higgs boson

Could the Higgs boson decay into dark matter? As dark matter does not interact directly with the ATLAS detector, physicists look for signs of “invisible particles”, inferred through momentum conservation of the proton–proton collision products. The ATLAS Collaboration searched the full LHC Run 2 dataset, setting the strongest limits on the Higgs boson decaying to invisible dark-matter particles to date.

Physics Briefing | 21 Apr 2020

ATLAS searches for rare Higgs boson decays into a photon and a Z boson

The ATLAS Collaboration has just released a new result searching for the Higgs-boson decay to a Z boson and a photon. This result uses the full LHC Run 2 dataset, analysing almost four times as many Higgs-boson events as the previous ATLAS result.

Physics Briefing | 21 Apr 2020

Novel probes of the strong force: precision jet substructure and the Lund jet plane

A hallmark of the strong force at the Large Hadron Collider (LHC) is the dramatic production of collimated jets of particles when quarks and gluons scatter at high energies. Particle physicists have studied jets for decades to learn about the structure of quantum chromodynamics – or QCD, the theory of the strong interaction – across a wide range of energy scales. Recent theoretical and experimental advancements in their study is now allowing ATLAS physicists to test the strong force in new ways.

Physics Briefing | 19 Apr 2020