ATLAS delivers new direct measurement of the top-quark decay width with improved precision

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 | 06 Aug 2019

ATLAS releases new search for strong supersymmetry

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 | 05 Aug 2019

Zooming in on top-quark production

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 | 05 Aug 2019

A golden era of exploration: ATLAS highlights from EPS-HEP 2019

Eight years of operation. Over 10,000 trillion high-energy proton collisions. One critical new particle discovery. Countless new insights into our universe. The Large Hadron Collider (LHC) has been breaking records since data-taking began in 2010 – and yet, for ATLAS and its fellow LHC experiments, a golden era of exploration is only just beginning.

News | 20 Jul 2019

New milestone reached in the study of electroweak symmetry breaking

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 Jul 2019

Exploring the Higgs boson “discovery channels"

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 Jul 2019

Double the Higgs for double the difficulty

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.

Physics Briefing | 11 Jul 2019

ATLAS searches for rare Higgs boson decays into muon pairs

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.

Physics Briefing | 11 Jul 2019

ATLAS finds evidence of charge asymmetry in top-quark pairs

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.

Physics Briefing | 11 Jul 2019

Summary of new ATLAS results for EPS-HEP 2019

ATLAS physicists are in Ghent, Belgium, this week for the European Physical Society Conference on High-Energy Physics (EPS-HEP) 2019. Since its establishment in 1971, the EPS-HEP conference has brought together the high-energy particle physics community every two years to discuss the latest results in field. This year, several hundred physicists from around the world are expected to attend.

News | 11 Jul 2019

In conversation with Zachary Marshall, a leading voice in the search for new physics

Simulation and supersymmetry, two things that have defined Zachary Marshall’s career. Zach is a researcher with Lawrence Berkeley National Lab. He is currently the co-convener of the ATLAS Supersymmetry group, leading the team searching for supersymmetry and all its various manifestations, building on his previous work as convenor of the ATLAS Simulation group.

Portrait | 05 Jul 2019

ATLAS delivers its most precise luminosity measurement yet

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 | 01 Jul 2019

ATLAS releases new result in hunt for mysterious magnetic monopoles

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 | 03 Jun 2019

Searching for Electroweak SUSY: not because it is easy, but because it is hard

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

Exploring the scientific potential of the ATLAS experiment at the High-Luminosity LHC

The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) is scheduled to begin colliding protons in 2026. This major improvement to CERN’s flagship accelerator will increase the total number of collisions in the ATLAS experiment by a factor of 10. To cope with this increase, ATLAS is preparing a complex series of upgrades including the installation of new detectors using state-of-the-art technology, the replacement of ageing electronics, and the upgrade of its trigger and data acquisition system.

News | 17 May 2019

Ten days of Trigger and Data Acquisition at ISOTDAQ

This April marked the 10th anniversary of the International School of Trigger and Data Acquisition (ISOTDAQ). It was a fantastic event that united researchers in physics, computing and engineering, ranging from undergraduate students to post-doctoral scientists. The goal of the school was to teach the "arts and crafts" of triggering and data-acquisition for high-energy physics experiments through a series of lectures and hands-on laboratory exercises.

Blog | 26 Apr 2019

ATLAS Management enters new term

The management of the ATLAS experiment begins a new term this Spring, with Spokesperson Karl Jakobs (University of Freiburg) continuing to steer the collaboration through Long Shutdown 2 to the start of Run 3 of the Large Hadron Collider (LHC).

News | 25 Apr 2019

ATLAS sets strong constraints on supersymmetric dark matter

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 | 08 Apr 2019

Celebrating the recipients of the 2019 ATLAS PhD Grant

At an award ceremony in the Globe of Science and Innovation, the recipients of the 2019 ATLAS PhD Grant were celebrated in the presence of CERN & Society donors and members of the ATLAS community. The ATLAS PhD Grant has supported up-and-coming talents in particle physics since 2014 and this year saw a new donor take up its cause.

News | 29 Mar 2019

Highlights from Moriond: ATLAS explores the full Run 2 dataset

This week, particle physicists from around the world gathered in La Thuile, Italy, for the annual Rencontres de Moriond conference on Electroweak Interactions and Unified Theories. It was one of the first major conferences to be held following the recent completion of the Large Hadron Collider’s (LHC) second operation period (Run 2). The ATLAS Collaboration unveiled a wide range of new results, including new analyses using the full Run 2 dataset, as well as some high-profile studies of Higgs, electroweak and heavy-ion physics.

News | 23 Mar 2019

All together now: adding more pieces to the Higgs boson puzzle

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 Mar 2019

ATLAS measures Higgs boson coupling to top quark in diphoton channel with full Run 2 dataset

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 Mar 2019

ATLAS finds evidence of three massive vector boson production

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 Mar 2019

ATLAS observes light scattering off light

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 Mar 2019

Searching for Dark Matter with the ATLAS detector

When we look around us, at all the things we can touch and see all of this is visible matter. And yet, this makes up less than 5% of the universe.

Feature | 05 Mar 2019