In new results presented today at CERN, the ATLAS Experiment’s search for supersymmetry (SUSY) reached new levels of sensitivity. The results examine a popular SUSY extension studied at the Large Hadron Collider (LHC): the “Minimal Supersymmetric Standard Model” (MSSM), which includes the minimum required number of new particles and interactions to make predictions at the LHC energies.
Physics Briefing | 18 February 2020
The electromagnetic fields of the Lorentz-contracted lead nuclei in heavy-ion collisions at the LHC act as intense sources of high-energy photons, or particles of light. This environment allows physicists to study photon-induced scattering processes, that can not be studied elsewhere. A key process examined by ATLAS physicists involves the annihilation of photons into pairs of oppositely charged muons. The ATLAS Collaboration recently released a new, comprehensive measurement of this process.
Physics Briefing | 19 November 2019
During Run 2, ATLAS achieved an exceptionally high data-quality efficiency for a hadron collider, with over 95% of the 13 TeV proton-proton collision data certified for physics analysis. In a new paper released today, the ATLAS data quality team summarises how this excellent result was achieved.
Physics Briefing | 13 November 2019
Does the Higgs boson follow all of the rules set by the Standard Model? Since discovering the particle in 2012, the ATLAS and CMS Collaborations have been hard at work studying the behaviour of the Higgs boson. Any unexpected observations could be a sign of new physics beyond the Standard Model.
Physics Briefing | 6 August 2019
As the heaviest known particle, the top quark plays a key role in studies of fundamental interactions. Due to its short lifetime, the top quark decays before it can turn into a hadron. Thus, its properties are preserved and transferred to its decay products, which can in turn be measured in high-energy physics experiments. Such studies provide an excellent testing ground for the Standard Model and may provide clues for new physics.
Physics Briefing | 6 August 2019
New particles sensitive to the strong interaction might be produced in abundance in the proton-proton collisions generated by the LHC – provided that they aren’t too heavy. These particles could be the partners of gluons and quarks predicted by supersymmetry (SUSY), a proposed extension of the Standard Model of particle physics that would expand its predictive power to include much higher energies. In the simplest scenarios, these “gluinos” and “squarks” would be produced in pairs, and decay directly into quarks and a new stable neutral particle (the “neutralino”), which would not interact with the ATLAS detector. The neutralino could be the main constituent of dark matter.
Physics Briefing | 5 August 2019
As the heaviest known elementary particle, the top quark has a special place in LHC physics. Top quark-antiquark pairs are copiously produced in collisions recorded by the ATLAS detector, providing a rich testing ground for theoretical models of particle collisions at the highest accessible energies. Any deviations between measurements and predictions could point to shortcomings in the theory – or first hints of something completely new.
Physics Briefing | 5 August 2019
In the Standard Model of particle physics, elementary particles acquire their masses by interacting with the Higgs field. This process is governed by a delicate mechanism: electroweak symmetry breaking (EWSB). Although EWSB was first proposed in 1964, it remains among the least understood phenomena of the Standard Model as a large dataset of high-energy particle collisions is required to probe it.
Physics Briefing | 15 July 2019
This week, at the European Physical Society Conference on High-Energy Physics (EPS-HEP) in Ghent, Belgium, the ATLAS Collaboration at CERN released new measurements of Higgs boson properties using the full LHC Run 2 dataset. Critically, the new results examine two of the Higgs boson decays that led to the particle’s discovery in 2012: H→ZZ*→4ℓ, where the Higgs boson decays into two Z bosons, in turn decaying into four leptons (electrons or muons); and H → γγ, where the Higgs boson decays directly into two photons.
Physics Briefing | 12 July 2019
A key interaction not yet observed by LHC experiments is the production of “double Higgs”. The Standard Model predicts that the Higgs field can interact with itself to create a Higgs boson pair. The rate with which this happens is critical, as it allows physicists to directly probe the potential energy of the Higgs field, which is responsible for mass of particles. Deviations from the expectation would be a strong hint of new physics.
Today, at the European Physical Society Conference on High-Energy Physics (EPS-HEP) in Ghent, Belgium, the ATLAS Collaboration released a new preliminary result searching for Higgs boson decays to a muon and antimuon pair (H → μμ). The new, more sensitive result uses the full Run 2 dataset, analysing almost twice as many Higgs boson events as the previous ATLAS result.
Among the most intriguing particles studied by the ATLAS collaboration is the top quark. As the heaviest known fundamental particle, it plays a unique role in the Standard Model of particle physics and – perhaps – in yet unseen physics beyond the Standard Model. A new ATLAS result, presented today at the European Physical Society Conference on High-Energy Physics (EPS-HEP) in Ghent, Belgium, examines the full Run 2 dataset to find evidence of charge asymmetry in top-quark pair events, with a significance of four standard deviations.
The large amount of data delivered by the LHC in Run 2 (2015-2018) has not only allowed the ATLAS Experiment to probe previously unexplored territory for rare Standard Model processes and new physics, but also to measure already known processes to better precision. In both cases, but particularly the latter, a precise measurement of the integrated luminosity of the dataset is essential. In other words, how many proton collisions actually occurred in ATLAS during Run 2.
Physics Briefing | 1 July 2019
Dipole magnets are probably the best-known source of magnetic fields. They consist of a north and south pole; while one end magnetically attracts, the opposite repels. If you cut a magnet in half, you are left with two magnets, each with its own north and south pole. This apparent absence of an isolated magnetic pole - or “magnetic monopole” - has puzzled physicists for more than a century. It would seem perfectly natural for this particle to be present in our universe; Maxwell’s equations would reflect complete symmetry between electricity and magnetism if particles with magnetic charge were observed. So far the mystery remains: while every known particle in our universe is either electrically charged or neutral, none have been found to be magnetically charged.
Physics Briefing | 3 June 2019
Today, at the Large Hadron Collider Physics (LHCP) conference in Puebla, Mexico, and at the SUSY2019 conference in Corpus Christi, USA, the ATLAS Collaboration presented numerous new searches for SUSY based on the full Run-2 dataset (taken between 2015 and 2018), including two particularly challenging searches for electroweak SUSY. Both target particles that are produced at extremely low rates at the LHC, and decay into Standard Model particles that are themselves difficult to reconstruct. The large amount of data successfully collected by ATLAS in Run 2 provides a unique opportunity to explore these scenarios with new analysis techniques.
Physics Briefing | 20 May 2019
One of the most complete theoretical frameworks that includes a dark matter candidate is supersymmetry. Dark matter is an unknown type of matter present in the universe, which could be of particle origin. Many supersymmetric models predict the existence of a new stable, invisible particle - the lightest supersymmetric particle (LSP) – which has the right properties to be a dark matter particle. The ATLAS Collaboration has recently reported two new results on searches for an LSP where it exploited the experiment’s full “Run 2” data sample taken at 13 TeV proton-proton collision energy. The analyses looked for the pair production of two heavy supersymmetric particles, each of which decays to observable Standard Model particles and an LSP in the detector.
Physics Briefing | 8 April 2019
The Higgs boson was discovered in 2012 by the ATLAS and CMS experiments, but its rich interaction properties (its coupling to other particles) have remained a puzzle. Thanks to an unprecedented amount of Higgs bosons produced at the LHC, all of the main Higgs boson production and decay modes have now been observed.
Physics Briefing | 19 March 2019
At the Rencontres de Moriond (La Thuile, Italy), the ATLAS Collaboration presented an updated measurement of ttH production in the diphoton channel. The result examines the full Run 2 dataset – 139 fb-1 collected between 2015 and 2018 – to observe ttH production in a single channel with a significance of 4.9 standard deviations.
Physics Briefing | 18 March 2019
Today, at the Rencontres de Moriond conference (La Thuile, Italy), the ATLAS collaboration released evidence for the simultaneous production of three W or Z bosons in proton–proton collisions at the Large Hadron Collider (LHC). The W and Z bosons are the mediator particles of the weak force – one of the four known fundamental forces – which is responsible for the phenomenon of radioactivity as well as an essential ingredient to our Sun's thermonuclear process.
Physics Briefing | 18 March 2019
Light-by-light scattering is a very rare phenomenon in which two photons – particles of light – interact, producing again a pair of photons. The ATLAS Collaboration has reported the observation of light-by-light scattering with a significance beyond 8 standard deviations.
Physics Briefing | 17 March 2019
Could a Grand Unified Theory resolve the remaining mysteries of the Standard Model? If verified, it would provide an elegant description of the unification of SM forces at very high energies, and might even explain the existence of dark matter and neutrino masses. ATLAS physicists are searching for evidence of new heavy particles predicted by such theories, including a neutral Z’ gauge boson.
Physics Briefing | 27 February 2019
For several decades, particle physicists having been trying to better understand Nature at the smallest distances by colliding particles at the highest energies. While the Standard Model of particle physics has successfully explained most of the results that have arisen from experiments, many phenomena remain baffling. Thus, new particles, forces or more general concepts must exist and – if the history of particle physics is any indication – they could well be revealed at the high-energy frontier.
Physics Briefing | 6 November 2018
The study of hadrons – particles that combine together quarks to form mesons or baryons – is a vital part of the ATLAS physics programme. Their analysis has not only perfected our understanding of the Standard Model, it has also provided excellent opportunities for discovery. On 20 September 2018, at the International Workshop on the CKM Unitarity Triangle (CKM 2018), ATLAS revealed the most stringent experimental constraint of the very rare decay of the B0 meson into two muons (μ).
Physics Briefing | 25 September 2018
The Brout-Englert-Higgs (BEH) mechanism is at the core of the Standard Model, the theory that describes the fundamental constituents of matter and their interactions. It introduces a new field, the Higgs field, through which the weak bosons (W and Z) become massive while the photon remains massless. The excitation of this field is a physical particle, the Higgs boson, which was discovered by the ATLAS and CMS collaborations in 2012.
Physics Briefing | 5 September 2018
While the Standard Model has proven tremendously successful, much experimental evidence points to it not being a complete description of our universe. The search for “new physics” is therefore an important component of the ATLAS experimental programme, where a number of analyses are looking for signs of new heavy particles decaying to different final states. Though these searches have not yet found a significant signal, they have allowed physicists to place stringent constraints on different new physics scenarios. These can be further tightened by combining different analysis channels and approaches.
Physics Briefing | 14 August 2018