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Summary of new ATLAS results from Moriond 2026

Katarina Anthony — Fri, 13 Mar 2026 07:57:08 +0000

Summary of new ATLAS results from Moriond 2026

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ATLAS Collaboration

Moriond 2026

Moriond

For particle physicists, March means one thing: Moriond. Every spring, theorists and experimentalists from around the world gather at the Rencontres de Moriond conferences to present their latest results and exchange ideas. The 2026 edition marks the sixtieth anniversary of these meetings, beginning on 15 March with Electroweak Interactions and Unified Theories, followed by QCD and High Energy Interactions on 22 March.

This year’s conferences will showcase a rich harvest of new results from the ATLAS Collaboration. In total, 45 ATLAS analyses will make their major conference debut, spanning a wide range of topics, from high-precision studies of the top quark and the Higgs boson to record-setting searches for supersymmetry and other new physics phenomena.

The results draw on the powerful datasets collected during both Run 2 (2015–2018) and Run 3 (2022–ongoing) of the Large Hadron Collider (LHC). With many analyses already exploiting the latest Run 3 data, this year’s Moriond programme highlights the strong momentum of the ATLAS Run-3 analysis campaign.

Several results will be featured in upcoming physics briefings, with more presented in conference talks. Follow the Moriond 2026 tag to keep up with the latest announcements, and explore the full list of new ATLAS results below as it grows throughout the conferences.

Latest Physics Briefings and News

ATLAS spots rare high-energy Higgs bosons for the first time, Physics Briefing, 31 March 2026
How “odd” are Higgs boson interactions? Physics Briefing, 27 March 2026
ATLAS extends Higgs boson studies via vector boson fusion into beauty and charm, Physics Briefing, 26 March 2026
ATLAS sets strong limits on supersymmetry, CERN News, 19 March 2026
ATLAS surpasses LEP limits in search for compressed higgsinos, Physics Briefing, 10 March 2026
The curious case of the disappearing track, Physics Briefing, 10 March 2026
The most elusive higgsinos, CERN Courier, 6 March 2026

New results presented at the 2026 Moriond Conferences

B-Physics

Observation of structures in the J/ψ+ψ(2S) mass spectrum with the ATLAS detector (arXiv:2509.13101, see figures)
A search for lepton-flavour violating τ→3μ decays with the ATLAS detector (arXiv:2603.18099)

Exotic new physics

Heavy Ion

Measurements of charged-particle pseudorapidity distributions and mean transverse momenta in O+O and Ne+Ne collisions at 5.36 TeV with the ATLAS detector (ATLAS-CONF-2026-004)

Higgs boson

Performance

Performance and efficiency of a transformer-based quark/gluon jet tagger in the ATLAS experiment (arXiv:2512.03949, see figures)
Calibration of the jet energy scale and resolution of small-radius jets using semileptonic tt¯ events with the ATLAS detector (arXiv:2512.17482, see figures)

Scalar boson and diboson searches

Standard Model

Supersymmetry

Top quark

ATLAS 2025 Thesis Awards spotlight the “soul” of the Collaboration

Katarina Anthony — Thu, 26 Feb 2026 12:57:54 +0000

ATLAS 2025 Thesis Awards spotlight the “soul” of the Collaboration

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The 2025 ATLAS Thesis Award ceremony. From left to right: ATLAS Collaboration Board Chair Davide Constanzo; ATLAS Thesis Awards Committee Chair Jean-François Arguin; ATLAS Thesis Award winners Elena Mazzeo, Stephen Nicholas Swatman, Ryan Roberts, Elliot Watton, Antonio Jesús Gómez Delegido, Takumi Aoki and Simon Florian Koch; and ATLAS Spokesperson Stéphane Willocq. Not pictured: ATLAS Thesis Award winner Kartik Deepak Bhide. (Image: K. Anthony/CERN

On 19 February 2026, the ATLAS Collaboration gathered in CERN’s Main Auditorium for the 2025 Thesis Awards – an annual celebration of the vital role of PhD students within the experiment.

From physics analysis and detector operations to software development and upgrade work, ATLAS PhD students make critical contributions to the Collaboration’s scientific mission while completing their degrees. This year’s ATLAS Thesis Awards drew from more than 200 eligible theses, reflecting both the scale of the collaboration and the breadth of student research. From this pool, the committee reviewed 36 formal applications before selecting eight winners.

This year’s recipients are: Takumi Aoki from the University of Tokyo (Japan), Kartik Deepak Bhide from Albert-Ludwigs-Universität Freiburg (Germany), Antonio Jesús Gómez Delegido from Universitat de València (Spain), Simon Florian Koch from the University of Oxford (UK), Elena Mazzeo from Università degli studi di Milano (Italy), Ryan Roberts from the University of California, Berkeley and Lawrence Berkeley National Laboratory (USA), Stephen Nicholas Swatman from the University of Amsterdam (Netherlands), and Elliot Watton from the University of Glasgow and Rutherford Appleton Laboratory (UK).

“Students are the ‘soul’ of the ATLAS Collaboration,” said Jean‑François Arguin, ATLAS Thesis Awards Committee Chair. “They make up a third of ATLAS authors and carry out much of the essential work that keeps ATLAS at the frontiers of scientific research. The quality and breadth of this year’s nominations made the committee’s decision especially challenging, and we congratulate all nominees for their outstanding work.”

Takumi Aoki from the University of Tokyo (Japan).

Antonio Jesús Gómez Delegido from Universitat de València (Spain).

Kartik Deepak Bhide from Albert-Ludwigs-Universität Freiburg (Germany).

Simon Florian Koch from the University of Oxford (UK).

Elena Mazzeo from Università degli studi di Milano (Italy).

Ryan Roberts from the University of California, Berkeley and Lawrence Berkeley National Laboratory (USA).

Stephen Nicholas Swatman from the University of Amsterdam (Netherlands).

Elliot Watton from the University of Glasgow and Rutherford Appleton Laboratory (UK).

During the ceremony, winners presented highlights of their time as students, offering snapshots of their analyses and operational contributions while sharing the challenges they encountered along the way. As in previous years, the presentations also provided a moment to thank the supervisors, colleagues, friends and family who supported their journeys.

This marks the sixteenth edition of the ATLAS Thesis Awards. Since 2010, the awards have been charting the experiment’s progress through the eyes of its students – several of whom have gone on to important leadership roles.

“In many ways, the future of our Collaboration is visible in our students’ theses,” concluded Arguin. “They showcase the new ideas, energy and leadership that will guide our field in years ahead. On behalf of the Thesis Awards Committee, I can confidently state that the future of particle physics is in capable hands!”

Explore the winning theses:

Summary of new ATLAS results from EPS-HEP 2025

Edite Prado Felgueiras — Sun, 06 Jul 2025 12:00:23 +0000

Summary of new ATLAS results from EPS-HEP 2025

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As summer arrives and the Mediterranean sun lights up Marseille, the particle physics community gets ready for one of the season’s key highlights: the European Physical Society Conference on High Energy Physics (EPS-HEP). From 7 to 11 July, hundreds of researchers will gather by the sea to share the latest breakthroughs in understanding the fundamental building blocks of our Universe.

The ATLAS Collaboration will be making waves at EPS-HEP 2025, unveiling a wide range of new results. These include new insights into the Standard Model and bold searches for new physics — all powered by the LHC Run-2 (2015–2018) and the ever-growing Run-3 (2022–present) datasets.

Key highlights will be explored in upcoming physics briefings throughout the conference, with many more results to discover in conference talks. Follow the 2025 EPS conference tag for the latest updates, and dive into the full list of ATLAS results below.

News and Physics Briefings

Bound to be discovered? ATLAS explores top-quark interactions near threshold, Physics Briefing, 7 July 2025
ATLAS closes in on rare Higgs decays, Physics Briefing, 7 July 2025
ATLAS enters a new era of jet flavour tagging – powered by AI, Physics Briefing, 7 July 2025
Elusive romance of top-quark pairs observed at the LHC, CERN Press Release, 8 July 2025
ATLAS closes in on rare Higgs decays, CERN News Piece, 11 July 2025

New results presented at EPS-HEP 2025

Higgs Boson and Di-Higgs

Searches for New Physics

Standard Model

Top Quark

Observation of tt¯γγ production at 13 TeV with the ATLAS detector (arXiv:2506.05018, see figures)
Measurements of differential cross-sections of WbWb production in the dilepton channel in proton-proton collisions at 13 TeV using the ATLAS detector (arXiv:2506.14700, see figures)
Measurement of the top-quark pole mass in dileptonic tt¯+1-jet events at 13 TeV with the ATLAS experiment (arXiv:2507.02632, see figures)
Observation of a cross-section enhancement near the tt¯ production threshold in 13 TeV proton-proton collisions with the ATLAS detector (ATLAS-CONF-2025-008)

Physics Modelling & Performance

Modelling of Longitudinal-Longitudinal polarisation of the ZZ final state in the four-lepton decay channel at the ATLAS experiment (ATL-PHYS-PUB-2025-021)
Identification of Boosted Higgs Bosons decaying into a pair of collimated τ-leptons decaying hadronically with Transformer Neural Networks in ATLAS (ATL-PHYS-PUB-2025-026)
Explaining the ATLAS GN2 Flavour Tagging Algorithm with Integrated Gradients (ATL-PHYS-PUB-2025-029)
Evaluating the statistical uncertainties and statistical consistency of constraints for the jet η-intercalibration in ATLAS (ATL-PHYS-PUB-2025-026)

Explore all ATLAS results

ATLAS takes a breath of oxygen

Katarina Anthony — Tue, 01 Jul 2025 08:07:38 +0000

ATLAS takes a breath of oxygen

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The ATLAS Experiment at CERN dives into uncharted waters, recording its first-ever collisions of oxygen and neon ions. These are not your typical heavy-ion collisions. Oxygen and neon nuclei are far smaller than lead – the LHC’s usual candidate for ion runs – and offer physicists exciting new opportunities to study the strong force.

Though best known for its high-energy proton–proton collisions, the Large Hadron Collider (LHC) also collides heavy ions. These ions smash together to create quark-gluon plasma (QGP), a unique state of matter that existed shortly after the Big Bang. In the hot, dense state of the QGP, the strong force – which, in “cold” conditions, holds protons and neutrons together – begins to behave differently.

Lead ions, with 82 protons and 126 neutrons, have long served as the LHC’s go-to tool for generating QGP. Oxygen and neon ions, with just 8 and 10 protons and neutrons respectively, are expected to form smaller droplets of QGP when they collide, compared to those in lead collisions – offering a new pool of data for physicists to dive into.

“These collision systems will let us investigate how the properties of the QGP emerge in relation to the system size,” says Riccardo Longo, a physicist with the ATLAS heavy-ion group. “While we understand the strong force well in ‘cold’ conditions, thanks to studies of proton–proton collisions, and in extremely hot and dense environments like lead–lead collisions, the question remains: what happens in between? We hope these lighter systems will let us connect the dots between the two.”

Oxygen and neon ions, with just 8 and 10 protons and neutrons respectively, are expected to form smaller droplets of QGP compared to lead-ion collisions – offering a new pool of data for physicists to dive into.

Figure 1: Comparison of lead-lead (Pb+Pb), proton-lead (p+Pb), xenon-xenon (Xe+Xe) and oxygen-oxygen (O+O) collision system sizes. The ions or particles, represented by light blue and red circles, travel in opposite directions and collide to form systems of varying sizes, shown as solid blue and red circles. (Image: ATLAS Collaboration/CERN)

Not too big, not too small

One of the first phenomena ATLAS physicists will be looking for is jet quenching, where high-energy particle jets lose energy as they traverse the QGP. The jets that emerge are some of the most powerful tools for studying the medium and the ATLAS experiment – purpose-built to measure high-energy jets – is uniquely suited to wield these tools.

Jet quenching was one of the ATLAS Collaboration’s first major observations in the heavy-ion domain. It was later observed in xenon–xenon collisions but – critically – not in proton–lead collisions, which form a much smaller QGP system (see Figure 1). So what’s the tipping point?

“Theory predicts we should see the onset of jet quenching in oxygen–oxygen collisions,” explains Longo. “If we observe even modest suppression, it could pin down the critical system size at which jet quenching begins.”

Interestingly, physicists have observed strong collective flow – a collective motion of particles emerging from the QGP – in collision systems of many different sizes. Even in smaller systems, where signs of energy loss by quarks and gluons have faded, evidence of QCD’s long-range, collective behaviour remains. By colliding light ions like oxygen, physicists can vary the initial conditions of the QGP’s formation, providing insight into how collective behaviour develops over space and time. The degree of collectivity may also help uncover the complex geometrical structure of oxygen nuclei, which are thought to be a composition of four alpha particles.

Neons bring a twist

Though close in size to oxygen, theorists suggest that neon ions have a bowling pin shape rather than the usual sphere (see Figure 2). This shape may impact the initial configurations of the ion collisions, providing new insights into the role of geometry in QGP formation.

“What’s even better is that the oxygen and neon runs will be right after one another,” says Qipeng Hu, ATLAS Heavy Ion Physics Group convenor. “This adds incredible value to the datasets when we do our comparison, as they’ll have exactly the same experimental conditions.”

Figure 2: Comparison between an oxygen ion and a neon ion, showcasing the distinct “bowling pin” shape of the neon ion. (Image: Giacalone et al., arXiv:2402.05995)

Aiming high

The LHC will also collide oxygen ions with protons, providing an unexpected link between collider physics and cosmic rays. As high-energy particles from space hit the oxygen and nitrogen Earth’s atmosphere, they produce “cosmic ray” showers of particles that are still not fully understood. By reproducing such interactions in laboratory conditions, scientists can better understand the composition of these showers. The LHCf experiment, just downstream of the ATLAS interaction point, will use the particles produced in proton–oxygen collisions in the proton-going direction for this purpose – providing valuable input for cosmic-ray experiments like AMS, located on the International Space Station. LHCf will be aided by the ATLAS Zero Degree Calorimeter (ZDC) – also located downstream of the interaction point – which will greatly enhance detector precision when measuring near-beam neutrons.

Proton–oxygen collisions recorded by ATLAS will also provide crucial data for refining nuclear parton distribution functions (nPDFs) — theoretical inputs that describe how quarks and gluons are distributed inside a nucleus. Until now, the nPDFs for oxygen were based only on simulated data.

Studies of these collisions will advance our understanding of the strong force and help shape the future of the LHC heavy-ion programme.

All this, in just a few days

The new ion run kicks off today, 1 July 2025, and concludes just over a week later, on 9 July 2025. To make the most of this rare opportunity, the ATLAS experiment has carefully adapted its data-taking strategy to match the fast-paced schedule.

“One of our biggest challenges was working out how we could maximise our recorded data,” says Tomasz Bold, ATLAS Heavy Ion Physics Group convenor. “Working together with trigger, data acquisition and data preparation experts, we’ve managed to increase the experiment’s data bandwidth by nearly 50%. Instead of sending the collision data to the CERN Computing Centre immediately, we temporarily store it locally at the experiment during each fill, and only transfer it afterward.” To support this, they also developed new event-selection protocols (“triggers”) that carefully balance the search for rare physics signals with the need to keep an unbiased sample of events.

Another key preparation for this run was the installation of the ZDC. The ZDC measures neutral particles that emerge close to the beamline, particularly “spectator neutrons” originating from the evaporation of the non-overlapping regions of the colliding nuclei. For proton–oxygen and oxygen–oxygen data-taking, the ZDC will play a particularly important role in monitoring the beam composition in real time. By analysing the evolution of newly implemented ZDC triggers corresponding to different neutron topologies, experts will be able to estimate any beam contamination due to transmutation and adjust data-taking on the fly.

Getting it right matters. The data gathered during this brief run has the potential to impact the heavy-ion community for years to come. “This special run is a great example of how different physics communities can benefit from the unique flexibility of the LHC,” concludes Longo. “From heavy-ion to cosmic-ray physicists, there is deep and diverse physics potential in these data. Studies of these collisions will advance our understanding of the strong force and help shape the future of the LHC heavy-ion programme.”

About the banner image: Event display of a proton–oxygen collision recorded by the ATLAS Experiment on 1 July 2025. (Image: ATLAS Collaboration/CERN)

Learn more

First-ever collisions of oxygen at the LHC, CERN News, 1 July 2025
The unexpected uses of a bowling pin: exploiting 20Ne isotopes for precision characterizations of collectivity in small systems (Giacalone et al., arXiv:2402.05995)

Celebrating the outstanding achievements of the ATLAS Collaboration

Katarina Anthony — Mon, 30 Jun 2025 03:04:05 +0000

Celebrating the outstanding achievements of the ATLAS Collaboration

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The ATLAS Collaboration celebrated the dedication, ingenuity and collaborative spirit of its members at the 8th Outstanding Achievement Awards.

Winners of the ATLAS Outstanding Achievement Awards 2025 (Image: K. Anthony/ATLAS Collaboration).

Established in 2014, the Outstanding Achievement Awards honour outstanding contributions made to the collaboration outside the field of physics analysis. They have since become a highlight of the community calendar as a chance to celebrate the vital work that keeps ATLAS running. Starting this year, the awards moved from a biennial to an annual format.

“As our community rises to the dual challenge of operating the detector while pushing ahead with upgrade preparations, our members' technical contributions have become more critical than ever,” says Sarah Demers, co-chair of the Awards Committee. “This is reflected in the sheer volume of outstanding nominations we’ve received, which made it clear that the awards needed to be celebrated every year.”

“While many of these contributions happen behind the scenes, they are absolutely central to the success of our experiment.”

A total of 65 nominations were put forward for this year’s awards, recognising work carried out between November 2023 and November 2024. The nominations spanned 15 categories, including detector operation and calibration, trigger, upgrade, outreach, and software and computing. Following careful consideration by the ATLAS Collaboration Board Chair Advisory Group, seven awards were presented to individuals and teams whose efforts stood out among a strong field of nominees.

“While many of these contributions happen behind the scenes, they are absolutely central to the success of our experiment,” says Ricardo Goncalo, co-chair of the Awards Committee. “These awards are a celebration of the fantastic work done by so many people, which enables the great physics output of the ATLAS Collaboration.”

The award ceremony was held at CERN on 19 June 2025, during the ATLAS Collaboration Week meeting. Awardees joined both in person and remotely from their home institutes around the globe, and each received a plaque in recognition of their achievements.

Meet the 2025 ATLAS Outstanding Achievement Award recipients

For the deployment of the complete Phase-I L1 Muon Endcap Trigger, including the NSW triggers, enabling ATLAS to run at higher pileup and gather more data in 2024: Masato Aoki (KEK), Leesa Brown (Victoria and TRIUMF), George Chatzianastasiou (CERN and BNL), Thiago Costa de Paiva (University of Massachusetts Amherst)(not pictured), Nathan Felt (Harvard)(not pictured), Simone Francescato (Harvard), Eleni Kanellaki (Demokritos)(not pictured), Foteini Kolitsi (uniWA), Audrey Kvam (University of Massachusetts Amherst), Callum McCracken (Vancouver UBC and TRIUMF), Tomoyuki Saito (Tokyo ICEPP), Yoshiaki Tsujikawa (Kyoto).

For advancing the experiment’s computing infrastructure through the integration of HPCs, which contribute substantially to ATLAS’s Run 3 computing power: Doug Benjamin (Brookhaven BNL)(not pictured), Andrej Filipcic (Jozef Stefan Institute, Ljubljana)(not pictured), Michal Svatos (Prague AS), Rod Walker (Munich LMU)(not pictured).

For contributing to the installation, commissioning and operation of the ZDC detector and responding to multiple challenges in November 2024: Brian Andrew Cole (Columbia), Riccardo Longo (Urbana UI).

For the successful design and testing campaign of the HGTD LGAD sensors that led to the start of sensor production: Bojan Hiti (Jozef Stefan Institute, Ljubljana)(not pictured), Alissa Howard (Jozef Stefan Institute, Ljubljana)(not pictured), Xuewei Jia (Beijing IHEP), Mengzhao Li (Beijing IHEP)(not pictured), Chihao Li (CERN), Kuo Ma (USTC)(not pictured), Theodoros Manoussos (CERN), Weiyi Sun (IHEP CAS), Guilherme Tomio Saito (Universidade de São Paulo), Iskra Velkovska (Jozef Stefan Institute, Ljubljana), Xiao Yang (USTC)(not pictured), Mei Zhao (Beijing IHEP)(not pictured).

For management of pixel code and databases, improving reliability and speed of detector calibrations, contributions to operations, and substantial improvements to system stability and monitoring in 2024: Tobias Bisanz (Dortmund).

For years of exceptional management of ATLAS operations, including significant interventions in 2024 to manage the recovery of the ATLAS infrastructure in the face of cooling challenges, enabling excellent data-taking efficiency: André Rummler (CERN).

For successful operation of the LAr detector, including contributions to online software and detector control systems, while commissioning the digital trigger: Huacheng Cai (Pittsburgh), Etienne Fortin (CERN)(not pictured), Davide Mungo (Toronto), Pavol Strizenec (Kosice)(not pictured).

ATLAS Highlights from Quark Matter 2025

Katarina Anthony — Mon, 14 Apr 2025 09:34:15 +0000

ATLAS Highlights from Quark Matter 2025

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Members of the ATLAS Heavy Ion team at the Quark Matter 2025 conference. (Image: A. Dimri/ATLAS Collaboration)

Last week, the ATLAS Collaboration presented new results at the Quark Matter 2025 conference, held in Frankfurt am Main (Germany). The conference gathered more than a thousand physicists from around the world to share the latest developments in high-energy heavy-ion physics. This year’s edition featured a wealth of new experimental results from the Large Hadron Collider (LHC) experiments as well as new theoretical work.

ATLAS physicists showcased a wide range of new measurements based on heavy-ion data from LHC Run 2 (2015–2018) and Run 3 (2022–ongoing). From detailed studies of the quark-gluon plasma’s collective motion to new measurements in ultra-peripheral collisions, ATLAS is providing new insight into the fascinating behaviour of heavy-ion collisions and the quark–gluon structure of nuclei. Key new ATLAS results are described below, addressing some of the biggest questions in the field.

How does the QGP flow radially?

When heavy ions collide at the LHC, they produce quark-gluon plasma (QGP) – a hot, dense state of matter that existed shortly after the Big Bang. As this primordial soup expands and cools, it exhibits unique fluid characteristics. While one of these characteristics – anisotropic flow, based on the dynamics of the original heavy-ion collision and the shape of the region where the ions overlap – has been studied for years, another – radial flow, based on the collective expansion of the QGP – has received less attention.

The ATLAS Collaboration presented the first measurements of radial flow in lead-lead collisions. Researchers performed differential measurements looking at the transverse momentum (p_T) and pseudorapidity (η) of charged particles produced in those collisions. They found radial flow to be largely independent of particle transverse momentum and centrality of the lead-lead collisions, with a long-range correlation in pseudorapidity – all key features that point to its collective nature (see Figure 1). By comparing the measurements to a hydrodynamic model, researchers also demonstrated their sensitivity to the bulk viscosity of the QGP. The bulk viscosity is a measure of how a moving fragment of QGP liquid pulls the liquid behind it. These findings establish a new, powerful tool for probing the collective dynamics and properties of the QGP.

Figure 1: The pT-differential radial flow, v0(p_T), measured for different momenta of reference particles (left), different pseudorapidity gap (middle) in the 0-5% most central collisions, and the shape of v0(p_T) in various centrality ranges, obtained by normalizing with p_T integrated value v0 (right). These three scaling behaviours at low p_T (< 4 GeV) are hallmarks suggesting that radial flow is a type of global, collective phenomenon. (Image: ATLAS Collaboration/CERN)

What can near-miss collisions tell us about the strong force?

Not all particles collide head on. Interesting phenomena can also occur when lead-nuclei narrowly miss each other and do not create a QGP. These ultra-peripheral collisions typically involve photons pulled from the intense electromagnetic fields surrounding each nucleus. They were used to observe light-by-light scattering in 2019 and the production of tau pairs in 2022, and are now shedding light on the strong force.

Figure 2: Differential cross section of coherent exclusive J/ψ production in lead–lead collisions at center-of-mass energy per nucleon of 5.36 TeV as a function of the J/ψ rapidity, as measured by ATLAS. The measurements are compared to theoretical predictions, including a parton saturation approach with fluctuating nucleon shapes, which best describes the data. (Image: ATLAS Collaboration/CERN)

Using the lead-lead collision data collected in 2023, ATLAS researchers reported the creation of J/ψ mesons in ultra-peripheral collisions. ATLAS physicists searched for events where a photon from one nucleus creates a quark-antiquark pair, which gently scatters off several virtual gluons in the second nucleus in a special way that creates a single J/ψ meson, without breaking up the nearby nucleus. As this process involves multiple gluons interacting with each other, it allows physicists to study the nonlinear nature of Quantum Chromodynamics (QCD, the theory that describes the strong force).

The data studied used an innovative event selection strategy focusing on signals from the ATLAS Transition Radiation Tracker. Researchers measured the kinematic distributions of J/ψ mesons over the widest rapidity region covered by a single experiment (Figure 2), and found that nonlinear QCD dynamics with additional gluonic fluctuations provide a good description of the data.

These results highlight the power and versatility of the ATLAS experiment in exploring the physics of heavy-ion collisions.

How do particles lose energy in the QGP?

One of the most intriguing features of the QGP is how high-energy “jets” of quarks and gluons lose energy as they pass through and interact with it. Previous studies have found that this jet quenching depends on the opening angle of the jet and the ability of the QGP to resolve that structure.

At this year’s Quark Matter conference, ATLAS physicists presented a new measurement of jet quenching, making significant refinements over previous measurements. To achieve unprecedented resolution, they studied large-radius jets made of clusters of smaller-radius jets. They then considered two variables of jet substructure – the angular separation (dR₁₂) and momentum sharing ( √d₁₂) – evaluated with charged tracks. Figure 3 shows the energy loss-induced suppression in the jet yield as a function of dR₁₂.

They found that jets with wider angular separation are more strongly suppressed. Interestingly, there are hints that this suppression changes behavior near dR₁₂ ≈ 0.1, suggesting an interplay of competing mechanisms that govern how the QGP resolves jet substructure. These precise data will provide new constraints on theoretical models of how particles lose energy in this extreme environment.

Figure 3: Nuclear modification factor R_AA of jets as a function of angular distance between the two hardest sub-jets. The results of the new measurement are compared with ones from previous ATLAS publications. (Image: ATLAS Collaboration/CERN

Learn more

Evidence for the collective nature of radial flow in Pb+Pb collisions with the ATLAS detector (arXiv:2503.24125, see figures)
Measurement of coherent exclusive J/ψ→μ+μ– production in ultraperipheral Pb+Pb collisions at 5.36 TeV with the ATLAS detector (ATLAS-CONF-2025-003/)
Measurement of substructure-dependent suppression of large-radius jets with charged particles in Pb+Pb collisions with ATLAS (arXiv:2504.04805, see figures)

ATLAS prepares for High-Luminosity LHC

Katarina Anthony — Wed, 02 Apr 2025 10:52:31 +0000

ATLAS prepares for High-Luminosity LHC

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News

Andreas Hoecker

HL-LHC

In 2025, the global particle physics community turns its attention to Europe as it embarks on the third update of the European Strategy for Particle Physics. This follows the initial strategy process launched under the CERN Council in 2005–2006, and subsequent updates in 2012–2013 and 2018–2020. While discussions will focus on future projects, such as the next-generation high-energy collider, it is equally essential to reaffirm the critical importance of the upgrades of the LHC and its experiments for the High-Luminosity LHC (HL-LHC), approved to operate from 2030 to 2041. The HL-LHC is a large-scale effort to probe the structure of matter and its interactions more deeply than ever before. It is the highest mid-term priority for particle physics, as emphasised by the 2023 P5 report in the U.S. and the 2020 European Strategy update.

Display of a simulated ttbar event with µ = 200 (1600 tracks) in the ATLAS ITk. (Image: ATLAS Collaboration/CERN)

To contribute to this update, the ATLAS Collaboration submitted four documents to the European Strategy Group (ESG), namely, an overview of the extensive ATLAS detector upgrade for the HL-LHC, a summary of the software and computing preparations to address the HL-LHC challenges, a joint update with the CMS Collaboration on HL-LHC physics expectations, and a report on perspectives in heavy flavour physics in collaboration with Belle II, CMS, and LHCb.

Through enhancements in beam intensity, emittance and final focus, the HL-LHC is expected to achieve a levelled instantaneous proton–proton luminosity of up to 7.5×10³⁴ cm^–2s^–1, leading up to 200 simultaneous inelastic collisions per bunch crossing (also known as "pileup") and a total delivered integrated luminosity of 3,000 fb^–1 by 2041 – including about 520 fb^–1 recorded during the LHC Runs 1–3. With this performance, the LHC and HL-LHC serve as a heavy-particle factory, producing in ATLAS alone 180 million Higgs bosons, 120 thousand Higgs boson pairs, 6.7 billion top quarks, 130 billion (12 billion) W (Z) bosons decaying to electrons or muons, and 360 million W bosons pairs. These quantities provide unparalleled physics potential. In many ways, they complement the cleaner but less abundant production of these particles (except for the Z boson) at a future e⁺e^– collider, where the initial state is precisely defined.

An ambitious ATLAS detector upgrade

The harsh conditions at the HL-LHC demand unprecedented detector and computing technologies, including radiation-hard components, high detection granularity and resolution, precise timing for improved pileup resilience, more selective triggers, higher-bandwidth data acquisition, deeply embedded machine learning, and high-performance heterogeneous software and computing.

To meet these challenges, ATLAS is pursuing an ambitious high-technology upgrade programme. It includes at its heart a new all-silicon based inner tracker (ITk) featuring the world’s largest silicon pixel detector with an active area of 13 m², five billion readout channels, an innermost layer at a radius 33 mm from the beam, and extending forward coverage up to pseudorapidity of 4 (corresponding to a 2-degree polar angle). The pixel detector is surrounded by a 165 m² silicon strip detector with active layers up to 1-metre in radius and 60 million readout channels, ten times that of the current ATLAS silicon strip detector.

ATLAS members visiting HL-ATLAS upgrade sites at CERN in October 2024. (Images: H.P. Beck & S. Guindon/ATLAS Collaboration, C. Krishna/CERN)

Additionally, a high-granularity timing detector based on newly developed 50 µm silicon sensor technology with fast signal rise is inserted behind the ITk into the forward region of ATLAS. It will provide 30–50 ps timing resolution per track. New resistive plate chambers (RPC) in the inner layer of the barrel muon spectrometer, partly accompanied by new small-diameter monitored drift tubes, improve the fast online muon acceptance. Comprehensive upgrades of the real-time event selection system (trigger) will enable ATLAS to maintain low-threshold detection and open up new physics opportunities. It operates with a first-level acceptance rate of 1 MHz, utilising global event selection through programmable hardware, followed by a second-level selection at 10 kHz output rate, implemented via algorithms running on a computing farm. The on-detector and off-detector electronics in the calorimeters and muon systems are being upgraded to accommodate the higher trigger rates and enhance overall performance. Smaller upgrade projects aim at improving the luminosity measurement and the detection of neutral particles along the beam line.

A new cooling system for the silicon detectors developed by CERN together with ATLAS and CMS, along with RPC leak repairs, consolidation, and an improved gas mixture, as well as the replacement of the transition radiation tracker will significantly reduce greenhouse gas emissions during HL-LHC operation.

The successful construction, installation, and commissioning of these upgrades engage thousands of ATLAS members worldwide and are the collaboration’s highest priority.

Wire Bonding at CERN, February 2025. (Image: A. Hoecker/ATLAS Collaboration)

Pixel Module testing at Argonne National Laboratory, December 2024. (Image: A. Hoecker/ATLAS Collaboration)

Pixel Module assembly at CERN. (Image: K. Anthony/ATLAS Collaboration)

ITk Short-strip module, December 2024. (Image: S. Willocq/ATLAS Collaboration)

Pixel module assembly at Argonne National Laboratory, December 2024. (Image: S. Willocq/ATLAS Collaboration)

Pixel module assembly at Argonne National Laboratory, December 2024. (Image: A. Hoecker/ATLAS Collaboration)v

Strip loading gantry at BNL, December 2024. (Image: A. Hoecker/ATLAS Collaboration)

Cleanroom at UC Santa Cruz, December 2023. (Image: J. Kotcher/ATLAS Collaboration)

Rising to the computing challenge

ATLAS operates one of the world’s largest distributed scientific computing systems, with over a million computing cores at peak capacity and more than one exabyte of storage spread across more than 100 sites worldwide. This system must evolve to address the software and computing challenges of the HL-LHC era, focusing on scalability, efficient hardware utilization and adaptation, and the long-term sustainability of the workforce. Investments by the collaboration in software optimisation, heterogeneous computing and data management will not only benefit ATLAS but also shape future collider experiments.

At the HL-LHC, data volumes and processing rates are expected to grow 3 to 5 times, requiring more efficient storage and data management solutions. To prepare for this, network stress tests are already underway, with full-scale evaluations planned by 2029. While high-performance computing centres offer significant computational power, they face storage limitations. To address this, ATLAS is collaborating with the broader HEP software community to develop resource-efficient data formats and compression techniques that could reduce disk usage by 10–50%. Alternative computing resources are also being explored, including commercial cloud services, which provide greater flexibility but at a higher cost, and the integration of trigger computing farms for offline data processing. To boost computing performance, hardware accelerators – especially GPUs in AI-driven computing centres – are being integrated.

ATLAS is modernising metadata and data management by transitioning to a more flexible and interoperable conditions data infrastructure, making database interactions more efficient.

Significant progress has been made in Geant4 based full simulation, which now runs twice as fast as in Run 2, and is complemented by high-fidelity fast simulation employing generative machine learning. Event generation has also been accelerated by up to a factor of 40. Machine learning techniques are enhancing charged particle tracking and calorimeter clustering, further optimising resource efficiency.

HGTD Peripheral Electronics Board prototype. (Image: ATLAS Collaboration/CERN)

TDAQ L0 Global Common Module version 3. (Image: ATLAS Collaboration/CERN)

LAr electronics integration tests at BNL. (Image: ATLAS Collaboration/CERN)

HGTD demonstrator at CERN. (Image: S. Guindon/ATLAS Collaboration)

A new physics landscape

The 3,000 fb^–1 integrated proton–proton luminosity of the HL-LHC offer an extraordinarily rich spectrum of physics opportunities, particularly for the exploration of rare processes. ATLAS and CMS have updated several of their HL-LHC physics projections for this year’s European Strategy update.

Projected ATLAS and CMS combined uncertainties of the Higgs to particle coupling strength modifiers (measured relative to predicted coupling strength). (Image: ATLAS Collaboration/CERN)

Higgs boson physics remains a central focus of the HL-LHC physics programme. Assuming Standard Model values and a realistic – though likely conservative – systematic uncertainty evolution, both rare Higgs decay channels into µµ and Z𝛄 will be measured with a combined ATLAS and CMS cross-section uncertainty of 6% and 14%, respectively.

Higgs boson couplings to Standard Model particles will be measured with remarkable experimental precision ranging from 0.9% to 1.2% (1.7% for the bottom quark) for the W and Z bosons, photon, gluon, tau lepton, and top quark, when fixing the Higgs boson width to its predicted value. These measurements will be limited by theoretical uncertainties, currently projected between 1.3% and 3.2%, highlighting the need for theoretical developments to match the experimental precision.

Not updated in this edition, earlier projections indicate that the Higgs boson width can be determined via off-shell production in the four-lepton channel with better than 20% precision, and that the branching ratio to invisible decays (such as those involving dark matter) can be constrained to 2.5%.

Di-Higgs production, with a predicted rate of less than a permille of single Higgs production, may be individually observed by both ATLAS and CMS, with their combined sensitivity exceeding 7σ. The measurement of the triple Higgs coupling or Higgs self interaction – a fundamental parameter of the Brout-Englert-Higgs (BEH) potential and predicted to be 0.13 in the Standard Model – is expected to achieve better than 30% precision. Any deviation greater than about 75% from the predicted central value would allow ATLAS and CMS to exclude the Standard Model prediction, offering a crucial and unprecedented test of the Higgs sector.

Constraints on the triple Higgs coupling modifier. (Image: ATLAS Collaboration/CERN)

ATLAS and CMS have also investigated their sensitivity to the nature of the electroweak phase transition during the early universe. A strong first-order, that is non-continuous, phase transition would enable the nucleation and expansion of “bubbles” containing the broken phase within the surrounding gauge-symmetric phase. New CP-violating interactions at the bubble walls could potentially generate baryogenesis, as baryon- and lepton-number-violating sphaleron processes would be prevented from reaching equilibrium due to the rapid bubble expansion. However, in the Standard Model, the relatively large Higgs boson mass leads to a continuous phase transition, ruling out this mechanism. Analyses of several well-motivated extensions of the BEH energy potential indicate that, with a dataset of 3,000 fb^–1, ATLAS and CMS together may have the sensitivity to exclude all these scenarios. This conclusion remains robust even in the presence of an additional scalar singlet field.

ATLAS and CMS have also updated their projections for W and Z boson scattering, a crucial test of the BEH mechanism. The Higgs boson regulates the interaction of longitudinally polarized W and Z bosons at high energy — polarization states that exist only due to the BEH mechanism generating the W and Z masses — thereby ensuring unitarity. The new analysis projects joint longitudinal W polarization can be observed above 5σ, with a cross-section measured to better than 20% precision.

ATLAS and CMS have further explored their measurement potential for rare top-quark processes, including four-top production and top–antitop production in association with a photon or a Z boson. They examined constraints on new physics using an effective field theory approach, where new, heavy particles interact only through virtual exchanges, parameterised via an operator product expansion. The analysis shows that sensitivity to energy scales above 2 TeV can be achieved for couplings of order unity.

Only a subset of studies was updated for this year’s European Strategy update. Earlier projections, particularly for direct new physics searches, can be found in here and here.

A tremendous opportunity, and a tremendous challenge

The HL-LHC offers extraordinary physics opportunities over the next two decades, addressing crucial questions on electroweak symmetry breaking and providing unmatched potential for discovering new physics for decades to come.

The necessary accelerator and experiment upgrades are ambitious global projects now under construction. Their successful and timely completion is a conditio sine qua non for high-energy particle physics and any future large-scale project of the field. Achieving this goal requires the collaborations to remain fully committed and focused on this number one priority, and relies on the continuing support and allocation of the essential resources by our funding agencies.

We count on the European Strategy Group and the CERN Council to reaffirm this as a top priority for the years ahead.

Submissions to the European Strategy Group

The ATLAS Upgrade for the HL-LHC (ATL-UPGRADE-PUB-2025-001, see figures)
ATLAS Software and Computing for the Future (ATL-SOFT-PUB-2025-002)
ATLAS and CMS Collaborations: Highlights of the HL-LHC physics projections by ATLAS and CMS (ATL-PHYS-PUB-2025-018, see figures)
ATLAS, Belle II, CMS, LHCb Collaborations: Projections for Key Measurements in Heavy Flavour Physics (arXiv:2503.24346)

Summary of new ATLAS results from Moriond 2025

Edite Prado Felgueiras — Sun, 23 Mar 2025 08:04:52 +0000

Summary of new ATLAS results from Moriond 2025

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As the days grow longer and the seasons shift, particle physicists spring forward into conference mode! Every Spring, all eyes turn to La Thuile (Italy) for the arrival of the annual Recontres de Moriond conferences. Starting on 23 March with the Electroweak Interactions & Unified Theories conference, followed by the QCD and High Energy Interactions conference on 30 March, these meetings bring together theorists and experimentalists from around the world to share their latest findings.

Results from the ATLAS Collaboration will be at the forefront, with ATLAS physicists presenting high-precision measurements of the W boson and top quark, novel searches for exotic particles, and new insights into the Higgs boson — all made possible using the LHC Run 2 (2015–2018) and Run 3 (2022–ongoing) datasets.

Several of these ATLAS analyses will be featured in upcoming physics briefings, with many more presented in conference talks. Keep up with the latest by following the 2025 Moriond conference tag. Explore the full list of new ATLAS results below, which will be updated as the conferences progress.

News and Physics Briefings

Boosting precision of top-quark mass measurement with ATLAS, Physics Briefing, 25 March 2025
Learning from the Tau, Physics Briefing, 31 March 2025
Displaced but not unnoticed: ATLAS in pursuit of Long-Lived Particles, Physics Briefing, 1 April 2025
ATLAS probes the Higgs mechanism in the scattering of W boson, Physics Briefing, 4 April 2025
ATLAS gets under the hood of the Higgs mechanism, CERN Media Update, 10 April 2025
Exploring the W boson at extreme mass scales, Physics Briefing, 15 April 2025

New results presented at the 2025 Moriond Conferences

B-Physics

Differential cross-section measurements of D^± and D^±_s meson production in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2412.15742, see figures)

Exotic new physics

Higgs boson

Measurements and interpretation of the Higgs boson differential and production mode cross sections in the H→ ZZ*→ 4l channel at 13.6 TeV with the ATLAS detector (ATLAS-CONF-2025-002)

Scalar boson and diboson searches

Standard Model

Supersymmetry

Prospects for a search for direct pair production of top squarks at the high-luminosity LHC with the ATLAS detector (ATL-PHYS-PUB-2024-001)

Top Quark

Measurement of the top quark mass with the ATLAS detector using tt¯ events with a high transverse momentum top quark (arXiv:2502.18216, see figures)

Explore all ATLAS results

Steering ATLAS forward with new management

Katarina Anthony — Mon, 10 Mar 2025 09:08:31 +0000

Steering ATLAS forward with new management

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The ATLAS Collaboration embarks on a new chapter under the leadership of Spokesperson Stéphane Willocq (University of Massachusetts Amherst).

New ATLAS Spokesperson Stéphane Willocq (University of Massachusetts Amherst) (Image: M. Brice/CERN)

Taking the helm in March 2025, Stéphane succeeds Andreas Hoecker, under whom he previously served as Deputy Spokesperson. A long-standing ATLAS member since 2004, he brings extensive leadership experience, having also served as ATLAS Physics Coordinator (2020–2021) and Chair of the Publications Committee (2017–2018). Elected for a two-year term, Stéphane will steer ATLAS through the final phase of LHC Run 3 and navigate the transition into the High-Luminosity LHC (HL-LHC) era. Joining him in the new management team are Deputy Spokespersons Anna Sfyrla (University of Geneva) and Guillaume Unal (CERN), alongside returning members Technical Coordinator Martin Aleksa (CERN), Resources Coordinator David Francis (CERN), and Upgrade Coordinator Benedetto Gorini (CERN).

"I want to express my deep gratitude to Andreas for his leadership and dedication to ATLAS," says Stéphane. “During his two terms as Spokesperson and two preceding terms as Deputy Spokesperson, he played a key role in advancing our scientific programme and strengthening our collaboration. We are building on a solid foundation thanks to his efforts. I also want to extend my appreciation to Manuella Vincter for her six years as Deputy Spokesperson. Her commitment to ATLAS and its members has been invaluable.”

Stéphane takes the wheel at a pivotal moment, as ATLAS members wrap up data-taking from LHC Run 3 and prepare for extensive upgrades to the experiment.

Stéphane takes the wheel at a pivotal moment, as ATLAS members wrap up data-taking from LHC Run 3 and prepare for extensive upgrades to the experiment. “The HL-LHC will provide an unprecedented volume of data, offering new opportunities for discovery,” says Benedetto Gorini, ATLAS Upgrade Coordinator. “To harness these data, we are undertaking the most ambitious upgrade in ATLAS history. Every layer of the detector is affected — from intricate new electronics to the complete replacement of the inner tracker. As with ATLAS’ original construction, this colossal effort is distributed across institutes worldwide, and all teams are working tirelessly to ensure these new systems are ready for installation in the coming LHC shutdown.”

With some systems set for installation as early as next year, preparations in the ATLAS cavern and services are already underway. “We have an intense period ahead, ensuring that every part of our infrastructure is shipshape for the coming upgrades.” says Martin Aleksa, ATLAS Technical Coordinator. “We are working out every detail of the installation of new detector components in the cavern during the planned four-year shutdown, as the cavern will be teeming with activity during this period.”

On the physics front, ATLAS is also at a critical period. The LHC continues to operate at record luminosities, delivering high-quality collision data that will shape future analyses. “With just over a year left in Run 3, it is essential that we maximize this final stretch of data-taking to secure the best and largest possible dataset,” says Stéphane. “By the winter of 2027, we aim to have the first results using the full Run 3 dataset — marking the true beginning of ATLAS’ broad Run 3 physics programme and providing an opportunity to explore uncharted waters in fundamental physics.”

With so much on the horizon, it’s all hands on deck! "We have our work cut out for us, but this is a challenge the ATLAS Collaboration is ready to meet," concludes Stéphane. "The strength of ATLAS lies in its people. Our collaboration spans the globe, bringing together individuals from diverse backgrounds and areas of expertise to tackle some of the most profound questions in nature. As we set course for the HL-LHC era, we can do so with confidence, knowing that our collective efforts have consistently driven major scientific advances.”

About the image banner: From left to right: Technical Coordinator Martin Aleksa (CERN), Deputy Spokespersons Guillaume Unal (CERN) and Anna Sfyrla (University of Geneva), Upgrade Coordinator Benedetto Gorini (CERN), Spokesperson Stéphane Willocq (University of Massachusetts Amherst), and Resources Coordinator David Francis (CERN).

Meet the new members of the ATLAS Management team

Anna Sfyrla, Deputy Spokesperson

Anna Sfyrla is an Associate Professor at the Nuclear and High Energy Physics Department of the University of Geneva. She studied physics at the National and Kapodistrian University of Athens, specialising in particle physics with a thesis on the CMS experiment. She then moved to the University of Geneva for her PhD, contributing first to the construction and surface commissioning of the ATLAS SCT and later to the Fermilab CDF experiment, where she focused on diboson physics and helped commission a diamond-based beam condition monitoring system. After completing her PhD in 2008, she returned to ATLAS, first as a postdoc at UIUC (USA), then as a CERN research fellow, and later as CERN staff. During this time, her work centred on the ATLAS trigger system, and she became involved in operations. She also conducted searches for new physics in hadronic final states.

She joined the University of Geneva as a faculty member in 2015 and, soon after, became involved in the TDAQ Phase-II upgrade, contributing to the development of the trigger system for the HL-LHC. With her research group at the University of Geneva, they primarily search for new physics in hadronic final states on ATLAS through both data analysis and trigger advancements for Run 3 and the HL-LHC. They are also involved in the FASER experiment and explore the potential of FCC-ee. Beyond research, she is actively engaged in education, outreach, and initiatives promoting equal opportunities in academia.

Guillaume Unal, Deputy Spokesperson

Guillaume Unal received his PhD in 1991 from Orsay, working on the W boson mass measurement with the UA2 experiment. He then spent a year conducting early studies on Higgs boson decays to gamma-gamma at the LHC and on the accordion liquid argon calorimeter test beam. From 1992 to 1996, he was a member of the CDF collaboration in the University of Pennsylvania group, working on the observation of the top quark and its first cross-section measurement. From 1996 to 2002, he was part of the NA48 collaboration at CERN in the Orsay group, working on the experiment’s liquid krypton calorimeter and the precise measurement of direct CP violation in the neutral kaon system. In 2002, he joined the ATLAS Collaboration, initially with the Orsay group and, since 2006, with the CERN team.

Since 2002, he has contributed to the calibration, reconstruction, and simulation software of the ATLAS liquid argon calorimeter, as well as to electron and photon identification and energy measurement performance. He has worked on several physics analyses using photons, including Higgs boson searches in the gamma-gamma channel and Higgs boson mass measurements. He served as Higgs group convener (2004-2006), ATLAS Data Preparation Coordinator (2012-2013), e/gamma group convener (2014-2016), Publication Committee Chair (2019-2020), and Physics Coordinator (2021-2022). On 1 March 2025, he began his new role as ATLAS Deputy Spokesperson.

ATLAS celebrates excellence in doctoral research

Edite Prado Felgueiras — Thu, 27 Feb 2025 11:12:58 +0000

ATLAS celebrates excellence in doctoral research

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Meet the winners of 2024 ATLAS Thesis Awards

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The 2024 ATLAS Thesis Award ceremony. From left to right: ATLAS Spokesperson Andreas Hoecker; ATLAS Thesis Award winners Christian Appelt, Shalini Epari, Kaito Sugizaki, Ana Luisa Moreira de Carvalho, Martino Tanasini, Makayla Vessella and Emily Ann Smith; ATLAS Thesis Awards Committee Chair Jean-François Arguin; and ATLAS Collaboration Board Chair Maria Jose Costa. Not pictured: ATLAS Thesis Award winner Aric Tate. (Image: K. Anthony/CERN)

Among the more than 5500 members of the ATLAS Collaboration, PhD students play a vital role in advancing the experiment while pursuing their degrees. Every year, the ATLAS Thesis Awards celebrate their outstanding achievements, recognising the significant impact of their research on physics analyses, detector advancements, and software development.

On 20 February 2025, CERN’s main auditorium hosted the 2024 ATLAS Thesis Awards, celebrating the next generation of leading researchers. This year’s recipients were: Christian Appelt from Humboldt University (Germany), Ana Luisa Moreira de Carvalho from Universidade de Lisboa, Instituto Superior Técnico and Laboratório de Instrumentação e Física Experimental de Partículas (Portugal), Shalini Epari from Universitat Autònoma de Barcelona (Spain), Emily Ann Smith from the University of Chicago (USA), Kaito Sugizaki from the University of Tokyo (Japan), Martino Tanasini from Università di Genova (Italy), Aric Tate from the University of Illinois at Urbana-Champaign (USA), and Makayla Vessella from the University of Massachusetts Amherst (USA).

“The ATLAS Thesis Awards not only recognise the dedication and excellence of our early-career researchers but also underscore the critical role they play in advancing the experiment’s scientific mission,” said Jean-François Arguin, Chair of the 2024 Thesis Awards Committee. “This year’s submissions were of an exceptional standard, reflecting the depth and breadth of research within ATLAS. We are delighted to celebrate the outstanding achievements of the winners.”

Christian Appelt from Humboldt University (Germany).

Ana Luisa Moreira de Carvalho from Universidade de Lisboa, Instituto Superior Técnico and Laboratório de Instrumentação e Física Experimental de Partículas (Portugal).

Shalini Epari from Universitat Autònoma de Barcelona (Spain).

Emily Ann Smith from the University of Chicago (USA).

Kaito Sugizaki from the University of Tokyo (Japan).

Martino Tanasini from Università di Genova (Italy).

Aric Tate from the University of Illinois at Urbana-Champaign (USA).

Makayla Vessella from the University of Massachusetts Amherst (USA).

During the award ceremony, each winner presented a brief overview of their thesis work, highlighting their contributions to detector technology and data analysis. The winners also took a moment to reflect on their journey as PhD students, acknowledging the challenges they overcame, the skills they developed, and the mentors, peers, and family members who supported them along the way.

The achievements of these students are a testament to their dedication and the collaborative spirit of ATLAS. As Jean-François Arguin highlighted: “Each thesis represents years of rigorous research and a meaningful contribution to the ongoing success of the ATLAS Experiment. On behalf of the Awards Committee, I extend my congratulations to the winners and look forward to seeing their continued impact on the field of particle physics.”

Explore the winning theses:

ATLAS releases first open data from heavy-ion collisions

Katarina Anthony — Fri, 13 Dec 2024 09:41:21 +0000

ATLAS releases first open data from heavy-ion collisions

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The ATLAS Collaboration has released its first open data of heavy-ion collisions for research purposes. This dataset features lead-ion (Pb-Pb) collisions at an energy of 5 TeV per nucleon pair, recorded in 2015 as part of the Large Hadron Collider’s second operation period (LHC Run 2). Today’s release is the highest energy heavy-ion collision data ever made publicly available and the first LHC Run 2 heavy-ion open dataset. Researchers worldwide can now explore over four terabytes of open data – comprising 221 million collision events – unlocking new opportunities for scientific discussions.

Heavy-ion collisions, such as those between lead nuclei, are markedly different from proton-proton collisions. “Their massive size leads to very different types of interactions,” says Zach Marshall of the ATLAS Open Data team. “While colliding protons is akin to smashing two garbage cans together, colliding lead ions is what happens when you drive two garbage trucks at each other.”

Unlike colliding trucks, when lead ions smash together they can create a droplet of quark-gluon plasma (QGP). This is the state of matter believed to have existed in the early universe, shortly after the Big Bang. As this droplet of plasma cools and evaporates, it leaves behind a distinct energy signature in the ATLAS experiment that researchers can analyse.

“Using the new open dataset,” continues Zach, “researchers can study these energy signatures to learn how the QGP forms and evaporates, as well as the intricate dynamics of particle interactions within this extreme environment.” In particular, this dataset offers the opportunity to study jet suppression, a phenomenon where high-energy particle jets lose energy as they pass through the QGP. Such suppression provides researchers with a window into the properties of the quark-gluon plasma.

Today’s release is the highest energy heavy-ion collision data ever made publicly available and the first LHC Run 2 heavy-ion open dataset.

Releasing heavy-ion open data posed unique challenges for the ATLAS Open Data team. “Unlike proton-proton data, we needed to include lots of extra collision event information – from charged particle tracks to energy measurements in our far-forward detectors – as that is where details about the QGP’s behaviour are hidden,” explains Zach. “To achieve this, we developed a new open data format (DAOD_HION14), which includes all this information but can also be studied with the same tools as our proton-proton datasets.”

Today’s release features “Minimum Bias” heavy-ion data, which includes a wide range of collision events (see image banner) from central, head-on collisions to glancing, ultra-peripheral interactions. Next year, ATLAS will release “Hard Probes” heavy-ion data collected in 2015, focusing on collision events triggered by high-energy particles such as Z bosons or top-quark pairs. To help researchers understand the unique characteristics of each open dataset, the release is also accompanied by documentation and simulated data.

Earlier this year, ATLAS made public over 65 TB of proton-proton collisions in its first open dataset for research. Today’s release adds to this legacy, comprising approximately 20% of ATLAS’ total Pb-Pb dataset, and more releases are already in the pipeline. This commitment to openness reflects core values of the high-energy physics community: fostering accessibility, enhancing reproducibility, and driving scientific progress.

As ATLAS continues to make its data publicly available, the potential for collaboration grows. Researchers worldwide are invited to explore this new dataset and join the exploration of the universe’s most extreme conditions.

Are you a non-ATLAS researcher looking to collaborate with ATLAS? Get access to the most recent ATLAS data by becoming a short-term associate. Click here to learn more.

About the image: Two heavy-ion collision events from the new ATLAS Open dataset, occurring in close succession. On the left, a head-on collision of lead ions; on the right, a peripheral interaction where the ions glanced off each other. (Image: ATLAS Collaboration/CERN)

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Summary of new ATLAS results from BOOST 2024

Katarina Anthony — Mon, 29 Jul 2024 06:31:34 +0000

Summary of new ATLAS results from BOOST 2024

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The 16th International Workshop on Boosted Object Phenomenology, Reconstruction, Measurements, and Searches at Colliders (BOOST 2024) kicks off today in Genova (Italy). This year’s edition continues the tradition of bringing together leading experts from both theoretical and experimental backgrounds to discuss latest developments in the study of particle “jets”.

The ATLAS Collaboration is set to unveil several new results at BOOST 2024. These include advancements in the study of jet properties, with a particular focus on the application of machine learning. These pioneering studies promise to offer deeper insights into Quantum Chromodynamics (QCD) and enhance the search for physics beyond the Standard Model.

The conference will cover various topics crucial to the field, including new particle searches, QCD calculations and measurements, jet reconstruction and calibration, pile-up mitigation, new observables and techniques, and considerations related to future collider experiments. Check the BOOST 2024 tag for the latest updates. A comprehensive list of ATLAS results presented at BOOST 2024 is given below

Latest Physics Briefings

Breaking through (jet) barriers, ATLAS Physics Briefing, 29 July 2024
Transforming bottom-jets: machine learning for improved bottom-jets measurements, ATLAS Physics Briefing, 30 July 2024

New results presented at BOOST 2024

Using pile-up collisions as an abundant source of low-energy hadronic physics processes in ATLAS and an extraction of the jet energy resolution (arXiv:2407.10819, see figures)
Performance and calibration of quark/gluon-jet taggers using 140 fb-1 of proton-proton collisions at 13 TeV with the ATLAS detector (Chinese Phys. C 48 (2024) 023001, arXiv:2308.00716, see figures)
A precise measurement of the jet energy scale derived from single-particle measurements and in situ techniques in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2407.15627, see figures)
Transformer networks for constituent-based-jet calibration with the ATLAS detector (ATL-PHYS-PUB-2024-015)
TST systematics and calibration (JETM-2024-01)
Comparison of top tagging performance with different ML algorithms (JETM-2024-02)
Run-3 (2022+2023) Jet and MET Performance (JETM-2024-03)

Explore all ATLAS results

Summary of new ATLAS results from ICHEP 2024

Katarina Anthony — Thu, 18 Jul 2024 05:29:37 +0000

Summary of new ATLAS results from ICHEP 2024

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The 2024 International Conference on High Energy Physics (ICHEP) kicks off this week in Prague, Czech Republic. For over 70 years, ICHEP has gathered physicists from around the world to share the latest advancements in particle physics, astrophysics, cosmology, and accelerator science, and to discuss plans for major future facilities.

The ATLAS Collaboration will be unveiling a wide range of new results at ICHEP 2024, delving into the extensive dataset collected during LHC Run-2 (2015-2018) and presenting first-of-their-kind studies of Run 3 data (collection ongoing). Anticipate exciting new studies of the Higgs boson, pioneering searches for elusive magnetic monopoles and other new-physics phenomena, and much more.

Key results will be explored in ATLAS Physics Briefings, with many more presented in conference talks and posters. For all news and physics briefings, follow the ICHEP 2024 tag and explore all of the latest ATLAS results through the links below.

Latest News and Physics Briefings

Hunting for magnetic monopoles with the Universe's strongest magnetic fields, ATLAS Physics Briefing, 18 July 2024
ATLAS probes uncharted territory with improved trigger, ATLAS Physics Briefing, 20 July 2024
The beauty and the charm of the Higgs boson, ATLAS Physics Briefing, 22 July 2024
ATLAS releases precise new measurement of Higgs boson production in association with top quarks, ATLAS Physics Briefing, 23 July 2024
Breaking “R-parity” in new searches for supersymmetry, ATLAS Physics Briefing, 25 July 2024
ATLAS probes uncharted territory with LHC Run 3 data, CERN News, 26 July 2024

New results presented at ICHEP 2024

Exotic New Physics

Scalar boson and diboson searches

Higgs boson

Standard Model

Measurement of the Lund jet plane in hadronic decays of top quarks and W bosons with the ATLAS detector (arXiv:2407.10879)

Heavy Ions

Search for magnetic monopole pair production in ultraperipheral Pb+Pb collisions at 5.36 TeV with the ATLAS detector at the LHC (ATLAS-CONF-2024-009)

Top Quark

Combined Performance

Explore all ATLAS results

ATLAS releases 65 TB of open data for research

Katarina Anthony — Mon, 01 Jul 2024 12:19:01 +0000

ATLAS releases 65 TB of open data for research

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Explore over 7 billion LHC collision events – from home

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The ATLAS Experiment at CERN has made two years’ worth of scientific data available to the public for research purposes. The data include recordings of proton–proton collisions from the Large Hadron Collider (LHC) at a collision energy of 13 TeV. This is the first time that ATLAS has released data on this scale, and it marks a significant milestone in terms of public access and utilisation of LHC data.

“Open access is a core value of CERN and the ATLAS Collaboration,” says Andreas Hoecker, ATLAS Spokesperson. “Since its beginning, ATLAS has strived to make its results fully accessible and reusable through open access archives such as arXiv and HepData. ATLAS has routinely released open data for educational purposes. Now, we’re taking it one step further — inviting everyone to explore the data that led to our discoveries.”

Released under the Creative Commons CC0 waiver, ATLAS has made public all the data collected by the experiment during the 2015 and 2016 proton–proton operation of the LHC. This is approximately 65 TB of data, representing over 7 billion LHC collision events. In addition, ATLAS has released 2 billion events of simulated “Monte Carlo” data, which are essential for carrying out a physics analysis.

Today's release underscores the ATLAS Collaboration's long-standing commitment to open access principles.

External researchers, in particular, are encouraged to explore the ATLAS open data. “Along with the data, we have provided comprehensive documentation on several of our analyses, guiding users through our process step-by-step,” says Zach Marshall, ATLAS Computing co-Coordinator. “These guides provide first-hand experience of working on a real ATLAS result, allowing anyone to test our tools, and evaluate the systematic uncertainties associated with the result for themselves.”

ATLAS traditionally collaborates with non-ATLAS scientists through short-term associations, granting them full access to ATLAS data, internal tools, and information. Through the open data, ATLAS researchers hope to further nurture this dialogue and collaboration. “In particular,” adds Zach, “we’d like to encourage phenomenologists and also computer scientists to explore our datasets, instead of relying on mock-ups.”

Today’s release builds upon previous open data releases for educational use (in 2016 and 2020). “All of our open data releases are now available through the ATLAS open data website,” says Dilia Portillo, ATLAS Outreach and Education co-Coordinator. “The website includes multi-level documentation, video tutorials and online tools aimed at the full-spectrum of users, from high school students to senior particle physics researchers. In addition, the software used to create the education-use open data has been released. This provides a seamless transition from the research open data to all the tutorials for outreach and education, including newly updated Higgs-boson discovery documentation. With a bit of time and dedication, you can go from being a relative novice to carrying out your own analysis.”

The ATLAS open data website also serves as a hub for the community, which includes teachers, students, enthusiasts and, now, scientists. Anyone diving into the open data can also directly engage with ATLAS physicists, who are available to respond to user feedback and take suggestions.

This release marks the start of more to come, with ATLAS’ first release of lead-lead-nuclei collision data up next. The ATLAS Collaboration, along with the other main LHC experiment collaborations, has committed to making all of its data publicly accessible after a certain time. Openness is deeply ingrained in the culture of high-energy physics, enabling greater accessibility, reproducibility and better science.

Begin your journey with ATLAS open data by following the tutorial below.

Get started with ATLAS open data:

Quick Start: beginners guide to the 13 TeV dataset released for education. No downloads required.
Deep Dive: more in-depth tutorial designed for students or teachers who want to use ATLAS open data resources over multiple sessions and download data.
Researcher Toolkit: documentation for expert users looking to perform detailed analyses.

Find out more

ATLAS and CMS celebrate a decade of innovation by the RD53 Collaboration

Katarina Anthony — Mon, 01 Jul 2024 09:53:53 +0000

ATLAS and CMS celebrate a decade of innovation by the RD53 Collaboration

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HL-LHC

Starting in 2029, the High-Luminosity LHC (HL-LHC) will provide much higher collision intensities to the ATLAS and CMS experiments, with up to 200 concurrent collisions (or pileup) per bunch crossing expected. This will create very complicated collision events with many overlapping tracks. Consequently, the ATLAS and CMS Collaborations are preparing to equip their tracking detectors with high-resolution inner tracking layers based on silicon pixel detectors. These new detectors will measure the inner points of the particle tracks as close as possible to the collision point.

The close proximity of the detectors to the interaction point — just 3 cm radial distance — means they will experience extreme data rates and radiation levels, and require a specialised design. The detectors’ front-end readout chips are a crucial component. They must be very fast, low-noise and radiation-hard, and specifically designed to avoid readout errors caused by charge build-up within the chips themselves — all while maintaining a low power budget. In addition, the new detectors have increased their number of pixels per area by about five times compared to the previous generation, moving from a pixel cell area of 50x250 μm² for ATLAS or 100x150 μm² for CMS, to 50x50 μm².

The challenge of designing a chip capable of meeting these requirements motivated ATLAS and CMS experts to join forces. In 2013, they founded the RD53 Collaboration with some twenty institutes from both experiments. CERN served as the central hub for the RD53 Collaboration.

In 2013, ATLAS and CMS joined forces to found the RD53 Collaboration, which set out to design critical read-out chips for the experiments' upgrades.

Over the last ten years, RD53 has been characterising the effects of radiation on a novel electronics technology for high-energy physics detectors. Their work has made it possible to design detectors with circuits four times smaller than those of the previous generation. New building blocks were achieved and a large size demonstrator was submitted in 2017 using 65 nm technology. This development process led to the creation of prototypes and subsequently to the final versions of the ATLAS and CMS pixel read-out chips for the HL-LHC Inner Tracker detectors (respectively called ITkPix and CROC). The ITkPix chip is now being produced in its final version by industrial partner TSMC/IMEC, with excellent yield after wafer probing.

On 24 June 2024, during an awards ceremony in CERN’s Main Auditorium, the ATLAS and CMS Collaborations marked this important achievement with a special recognition of the RD53 Collaboration. The representatives of the RD53 Collaboration were presented with a plaque recognizing RD53’s “exceptional contribution to the HL-LHC upgrade with the design, submission and testing of the ATLAS ITkPix and the CMS CROC pixel readout chips for the Inner Tracker detectors”.

The ATLAS and CMS Collaborations celebrate the achievements of the RD53 Collaboration. From left to right: Francesca Cavallari and Ulrich Heinz (Chairs of the CMS Awards Committee), Lucia Di Ciaccio (ATLAS Collaboration Board Deputy Chair), Flavio Oddo (Project Engineer, RD53 Collaboration), Elisabetta Gallo (CMS Collaboration Board Chair), Sarah Demers (co-Chair ATLAS Outstanding Achievement Awards), and Patricia McBride (CMS Spokesperson). (Image: CERN)

About the image banner: Simulation of a typical HL-LHC event with 200 pileup in the ATLAS experiment. (Image: ATLAS Collaboration/CERN)

The RD53 Collaboration comprises 22 institutes coming from ATLAS and/or CMS: Politecnico di Bari (Italy), INFN Sezione di Pavia and Università di Bergamo (Italy), University of Bergen (Norway), University of Bonn (Germany), CERN (Switzerland), CPPM (France), University of Applied Sciences and Arts Dortmund (Germany), Fermilab (USA), Lawrence Berkeley National Laboratory (USA), LPNHE Paris (France), INFN Sezione di Milano and Università degli Studi di Milano (Italy), NIKHEF (Netherlands), IJCLab Orsay (France), Oxford University (UK), INFN Sezione di Padova and Università di Padova (Italy), INFN Sezione di Perugia and Università di Perugia (Italy), INFN Sezione di Pisa (Italy), Czech Technical University in Prague (Czech Republic), Rutherford Appleton Laboratory (UK), Universidad de Sevilla (Spain), INFN Sezione di Torino (Italy), Universidad de Zaragoza (Spain).

Celebrating excellence at the 7th ATLAS Outstanding Achievement Awards

Katarina Anthony — Mon, 01 Jul 2024 09:26:44 +0000

Celebrating excellence at the 7th ATLAS Outstanding Achievement Awards

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Winners of the ATLAS Outstanding Achievement Awards 2024.

The ATLAS Collaboration held the 7th Outstanding Achievement Awards Ceremony on 20 June 2024. These biennial awards recognize the invaluable technical work performed across the Collaboration in various fields.

The awards received an impressive 116 nominations, each meticulously evaluated with input from expert groups in each category by the ATLAS Collaboration Board Chair Advisory Group, serving as the Awards Committee. This year, the Committee honoured 2 individuals and 7 groups for their exceptional contributions to detector operation, upgrade, software, computing and combined performance during the period from February 2022 to October 2023.

“The Outstanding Achievement Awards recognize but a part of the extensive efforts made across the ATLAS Collaboration,” acknowledged Ricardo Goncalo, co-chair of the Awards Committee. “Selecting the ‘outstanding’ from the many high-quality nominations we received was a challenge.”

“The work of the awardees demanded great creativity and determination,” said Sarah Demers, co-chair of the Awards Committee. “Not only were their contributions crucial during this recent data-taking period, they also set the stage for high-quality physics analyses and operations in the years to come.”

This year’s award ceremony took place at the Aristotle University of Thessaloniki, Greece, as part of the ATLAS Collaboration Week meeting. Awardees, based in institutes around the world, participated in the ceremony both in person and remotely. Each recipient was honoured with a plaque to celebrate their remarkable contributions.

Here are the 2024 ATLAS Outstanding Achievement Award recipients:

Luca Canali (CERN)
For outstanding contributions to the ATLAS database infrastructure.

Jackson Barr (University College London), Alexander Froch (Albert-Ludwigs-Universität Freiburg), Philipp Gadow (CERN), Dan Guest (Humboldt University Berlin), Nilotpal Kakati (Weizmann Institute of Science), Dmitrii Kobylianskii (Weizmann Institute of Science), Nikita Ivvan Pond (University College London), Samuel Van Stroud (University College London)
For outstanding contributions to heavy flavour tagging algorithms based on Graph Neural Networks.

Liang Guan (Michigan), Ioannis Mesolongitis (University of West Attica), Michelle Solis (University of Arizona), Aaron White (Harvard University)
For understanding the problem of randomly dropping e-links in the NSW and for finding a very effective mitigation for this problem.

Jakub Kremer (DESY), Agnieszka Ogrodnik (Charles University), Martin Rybar (Charles University)
For outstanding contributions to the Heavy Ions operation and trigger.

Koji Nakamura (KEK), Hideyuki Oide (KEK), Manabu Togawa (KEK)
For outstanding contributions to the ITk Pixel project in sensor production, hybridisation and module assembly.

Johannes Junggeburth (University of Massachusetts Amherst), Patrick Scholer (Carleton University)
For outstanding contributions to the Run 3 muon software.

Sara Alderweireldt (University of Edinburgh), Rafal Bielski (University of Oregon), Francesco Giuli (CERN), Ralf Gugel (University of Mainz), Claudia Merlassino (University of Udine), Stefanie Morgenstern (CERN), Gabriel Palacino (Indiana University), Aleksandra Poreba (CERN), Antonia Strubig (Stockholm University), Daniele Zanzi (Albert-Ludwigs-Universität Freiburg)
For outstanding contributions to the Trigger operation.

Julien Maurer (Bucharest IFIN-HH)
For outstanding contributions to the ATLAS prompt reconstruction operation.

Anthony Affolder (UC Santa Cruz), Ian Dyckes (Berkeley LBNL), Vitaliy Fadeyev (UC Santa Cruz), Cole Helling (University of British Columbia, Vancouver), Jacob Wayne Johnson (UC Santa Cruz), Matthew Kurth (Beijing IHEP), Masahiro Morii (Harvard University), Peter Phillips (Rutherford Appleton Laboratory), Luise Poley (TRIUMF), Craig Sawyer (Rutherford Appleton Laboratory)
For outstanding contributions to the identification of the vibrational source of cold noise on ITk Strip modules.

ATLAS dives deeper into di-Higgs: a combined search

Katarina Anthony — Thu, 06 Jun 2024 16:29:02 +0000

ATLAS dives deeper into di-Higgs: a combined search

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Two Higgs bosons appearing in the same proton collision is much more than a simple coincidence. Studies of this process, known as “di-Higgs” production, allows physicists to measure the strength of the Higgs boson’s “self-coupling” – a fundamental aspect of the Standard Model that connects the Higgs mechanism and the stability of our Universe.

However, measuring di-Higgs production is an especially challenging task. It’s a very rare process, about 1000 times rarer than the production of a single Higgs boson. During Run 2 of the LHC (2015-2018), only about 4000 di-Higgs events are expected to have been produced, compared to the 40 million collisions that happened every second.

Figure 1: Upper limits at the 95% confidence level on the di-Higgs signal strength, defined as the ratio of the production cross section to the Standard-Model theoretical prediction. The vertical black bars are the observed limits and the dashed lines the expected ones. The blue and yellow bands represent the regions where the actual observed limits are expected to be set with about 68 and 95% confidence level. (Image: ATLAS Collaboration/CERN)

So can physicists find these rare needles in the data haystack? To maximise their chances, researchers study the most promising decays of the two Higgs bosons. The most probable way two Higgs bosons decay is into four bottom quarks (HH → bbbb). However, many other processes also create four bottom quarks in a collision, thus this signature has the highest likelihood of being mimicked by other Standard-Model processes (“background”). The di-Higgs decay to two bottom quarks and two tau leptons (HH → bb𝜏𝜏) has moderate background interference but is five times less common and has neutrinos that escape undetected in the final state, complicating physicists’ ability to reconstruct the di-Higgs decay fully. By contrast, the di-Higgs decay to two bottom quarks and two photons (HH → bb𝛾𝛾) is one of the rarest accessible decay modes, but the presence of two photons makes it clearly identifiable in the ATLAS detector. There’s also the di-Higgs “soup” to consider, where two Higgs bosons leave a multi-lepton signature in the detector. Finally, another rare decay mode is the di-Higgs final state containing two bottom quarks, two leptons, and neutrinos (HH → bbll+ET_miss).

Each of these decay modes has its pros and cons, and all have sensitivity to di-Higgs production. What if physicists could combine their power to achieve a more sensitive result? In a new analysis from the ATLAS Collaboration, physicists merged five di-Higgs studies of LHC Run 2 data. By combining the power of all these decay channels, researchers achieved the most sensitive probe of di-Higgs production and the Higgs self-coupling. These new determinations provide crucial information to help theoretical physicists in developing new models.

The ATLAS Collaboration merged five analyses to achieve the most sensitive probe yet of di-Higgs production and the Higgs self-coupling.

Achieving this combined result required a careful study of all the analyses, ensuring the statistical models were compatible and that none of the selected ATLAS events overlapped between channels. Though ATLAS physicists had previously performed a combination of di-Higgs analyses in 2021, this new combination includes, for the first time, the multi-lepton and bbll+ET_miss decay channels, as well as improvements to analyses of the other channels. This new result is thus stronger and more comprehensive, covering over half of all possible di-Higgs decays.

The analysis excludes di-Higgs production cross-section values that are more than 2.9 times larger than the Standard-Model prediction (Figure 1). Researchers also constrained, relative to the Standard Model prediction, the magnitude of the Higgs self-interaction (κλ) and the interaction strength of two Higgs bosons and two vector bosons (κ2V). This is shown in Figure 2, where κλ is constrained between -1.2 and 7.2, and κ2V between 0.57 and 1.48. Both measurements are consistent with the value of 1 which corresponds to the Standard-Model predictions.

Figure 2: Expected (dashed lines) and observed (solid line) κλ and κ2V ranges for the individual channels and the combination. All values outside the ellipse contour are excluded at the 95% confidence level. The cross is the combination of parameters favoured by the data, or best fit, and the star represents the Standard-Model prediction. (Image: ATLAS Collaboration/CERN)

Figure 3: Expected (filled shapes) and observed (black lines) allowed ranges for the c_tthh and c_gghh parameters after the combination. These parameters denote the interactions between a di-Higgs pair and a pair of top quarks or gluons. All values outside the ellipse contours are excluded at the 68 or 95% confidence level. The cross is the combination of parameters favoured by the data, or best fit, and the star represents the Standard-Model prediction. (Image: ATLAS Collaboration/CERN)

Physicists also considered how physics beyond the Standard Model would affect di-Higgs production. Using the “Effective Field Theory” approach, physicists predicted that new interactions, like the one between two Higgs bosons and two top quarks (ttHH) or two gluons (ggHH), would appear in the data. This latest analysis sets the most stringent constraints on the strength of these interactions, as shown in Figure 3. The observed data are in tension with the Standard Model, with a probability that the results could have occurred by chance (p-value) of 3.1%.

This combined result sets a milestone in the study of di-Higgs in Run 2 data, providing the best-yet sensitivity to the strength of the Higgs boson self-coupling. Now, ATLAS researchers set their sights on data from the ongoing LHC Run 3 and upcoming High-Luminosity LHC operation. With these, physicists will at last observe these elusive Higgs-boson pairs.

About the event display: Candidate HH → bbɣɣ event in ATLAS data taken in 2017. Charged-particle tracks are shown in green, the two candidate b-jets are shown as red cones, and the two candidate photons are shown as cyan towers. (Image: ATLAS Collaboration/CERN)

Learn more

Combination of searches for Higgs boson pair production in proton-proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2024-006)
LHCP2024 presentation by Arely Cortes Gonzalez: Non-resonant di-Higgs searches and measurements with the ATLAS detector
Probing new physics with pairs of Higgs bosons, ATLAS Physics Briefing, July 2021
Search for pair production of boosted Higgs bosons via vector-boson fusion in the bb¯bb¯ final state using proton-proton collisions at 13 TeV with the ATLAS detector (Phys. Rev. D 108, 052003, arXiv:2404.17193, see figures)
Two Higgs bosons are better than one, ATLAS Physics Briefing, July 2021
Search for the non-resonant production of Higgs boson pairs via gluon fusion and vector-boson fusion in the bb¯τ+τ− final state in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2404.12660, see figures)
Shedding light on Higgs-boson self-interactions, ATLAS Physics Briefing, September 2023
Studies of new Higgs boson interactions through nonresonant HH production in the bb¯γγ final state in proton-proton collisions at 13 TeV with the ATLAS detector (JHEP 01 (2024) 066, arXiv:2310.12301, see figures)
Search for non-resonant Higgs boson pair production in the 2b+2ℓ+ETmiss final state in proton-proton collisions at 13 TeV with the ATLAS detector (JHEP 02 (2024) 037, arXiv:2310.11286, see figures)
Menu of the day: Di-Higgs soup!, ATLAS Physics Briefing, April 2024
Search for non-resonant Higgs boson pair production in final states with leptons, taus, and photons in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2405.20040, see figures)

Summary of new ATLAS results from LHCP 2024

Edite Prado Felgueiras — Mon, 03 Jun 2024 12:46:13 +0000

Summary of new ATLAS results from LHCP 2024

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The twelfth annual conference on Large Hadron Collider physics (LHCP 2024) kicks off today in Boston, USA. This week-long event will feature a detailed review of the latest experimental and theoretical results in collider physics, including several final studies of the LHC Run-2 dataset (collected in 2015-2018) and discussions on future research directions for the high-energy particle-physics community.

Members of the ATLAS Collaboration will be in attendance, presenting several new analyses of the LHC Run-2 dataset alongside early studies of the Run-3 dataset (2022-ongoing). Look forward to innovative new studies of the Higgs-boson pair production, cutting-edge searches for Dark Matter particles, and much more.

Key results will be explored in ATLAS Physics Briefings, to be released throughout the conference. Check the LHCP 2024 tag for the latest updates. The full list of new ATLAS results presented at LHCP 2024 is given below and will be continually updated this week.

Latest Physics Briefings

ATLAS chases long-lived particles with the Higgs boson, ATLAS Physics Briefing, 3 June 2024
ATLAS dives deeper into di-Higgs: a combined search, ATLAS Physics Briefing, 6 June 2024
ATLAS searches for the building blocks of dark matter, ATLAS Physics Briefing, 8 June 2024
ATLAS dives deeper into di-Higgs, CERN News, 18 June 2024
Jetting into a new era of Higgs studies, ATLAS Physics Briefing, 20 June 2024

New results presented at LHCP 2024

Exotic New Physics

Scalar boson and diboson searches

Standard Model

Underlying-event studies with strange hadrons in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2405.05048, see figures)
Measurements of jet cross-section ratios in 13 TeV proton-proton collisions with ATLAS (arXiv:2405.20206, see figures)
A simultaneous unbinned differential cross section measurement of twenty-four Z+jets kinematic observables with the ATLAS detector (arXiv:2405.20041, see figures)

Supersymmetry

Top Quark

Observation of tt production in lepton+jets and dilepton channels in p+Pb collisions at 8.16 TeV with the ATLAS detector (ATLAS-CONF-2023-063)

Explore more ATLAS results

ATLAS status and overview (LHCP 2024 plenary talk by Heather Gray)
ATLAS results using Run 3 data
ATLAS results using the full Run 2 dataset
All ATLAS Public Results

ATLAS mourns the loss of Peter Higgs

Katarina Anthony — Wed, 10 Apr 2024 05:23:52 +0000

ATLAS mourns the loss of Peter Higgs

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Peter Higgs tours the ATLAS Experiment in 2008. (Image: CERN)

The ATLAS Collaboration mourns the passing of renowned physicist Peter Higgs, whose theoretical work on elucidating the fundamental mechanism by which elementary particles acquire mass reshaped our understanding of the Universe. The manifestation of this mechanism as a new particle – the Higgs boson – laid the foundation for decades of research.

The mechanism, relying on a new field and spontaneous symmetry-breaking, was first proposed in 1964 by Peter Higgs at the University of Edinburgh and, independently, by Robert Brout and François Englert at the Université Libre de Bruxelles and physicists at Imperial College London. It would take nearly half a century for their theory to be experimentally confirmed. In 2012, the ATLAS and CMS Experiments at CERN’s Large Hadron Collider (LHC) detected the long-sought Higgs boson. Peter Higgs and François Englert themselves were present at the announcement of this historic discovery at CERN (Robert Brout had passed away in 2011). This momentous achievement not only confirmed the existence of the particle but also solidified a place in science history for Higgs, Brout and Englert, as well as other theorists publishing related work in 1964. In recognition of their contributions, Peter Higgs and François Englert were jointly awarded the Nobel Prize in Physics in 2013. Their theoretical work and the discovery of the Higgs boson stand out as an example of the deep consequences of mathematical symmetries in the physical world.

“Peter Higgs leaves behind a monumental legacy, defined not only by his scientific achievements but also by his inspiring personality,” says Andreas Hoecker, ATLAS Spokesperson. “The ATLAS Collaboration aspires to honour his memory by embodying the spirit of curiosity he exemplified — especially in our efforts to unlock the secrets of the particle named after him and the mysteries of the underlying mechanism of mass generation.”

Peter Higgs (right) and François Englert (left) speak at the 2012 announcement of the discovery of the Higgs boson at CERN. (Image: CERN)

Peter Higgs signs a LEGO model of the ATLAS Experiment. (Image: CERN)

Learn more

Statement on the death of Professor Peter Higgs, University of Edinburgh Statement, April 2024
CERN pays tribute to Peter Higgs, CERN News, April 2024
10 years of discovery with the Higgs boson, ATLAS Press Statement, July 2022
Englert and Higgs get the Nobel, ATLAS News, October 2013
CERN experiments observe particle consistent with long-sought Higgs boson, CERN Press Release, July 2012

Summary of new ATLAS results from Moriond 2024

Edite Prado Felgueiras — Fri, 22 Mar 2024 14:04:06 +0000

Summary of new ATLAS results from Moriond 2024

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This year, the start of Spring marks the beginning of the conference season for particle physicists! All eyes turn to La Thuile, Italy, where the annual Recontres de Moriond conferences will be held. Starting on 24 March with the Electroweak Interactions & Unified Theories conference, followed by the QCD and High Energy Interactions conference on 31 March, these gatherings bring together theorists and experimentalists from around the world to share their latest findings.

ATLAS Collaboration members will be at the forefront, unveiling new analyses of the full LHC Run-2 dataset (collected 2015-2018) and diving into studies of the Run-3 dataset (2022-ongoing). Look forward to measurements of key Standard-Model parameters, novel searches for new heavy particles, precision studies of familiar particles and more.

Several of these ATLAS analyses will be featured in upcoming physics briefings, with many more presented in conference talks. Keep up with the latest by following the 2024 Moriond conference tag. Explore the full list of new ATLAS results below, which will be updated as the conferences progress.

Latest Physics Briefings and News

ATLAS explores Z boson production with heavy-flavour quarks, Physics Briefing, 15 April 2024
ATLAS provides first measurement of the W-boson width at the LHC, CERN News, 10 April 2024
ATLAS provides first measurement of the W-boson width at the LHC, Physics Briefing, 5 April 2024
First ATLAS measurement of W and Z boson production using Run-3 data, Physics Briefing, 28 March 2024
The delicate balance of lepton flavours, CERN News, 26 March 2024
Measuring the delicate balance of lepton flavours, Physics Briefing, 25 March 2024

News

Summary of new ATLAS results from Moriond 2026

Latest Physics Briefings and News

New results presented at the 2026 Moriond Conferences

ATLAS 2025 Thesis Awards spotlight the “soul” of the Collaboration

Summary of new ATLAS results from EPS-HEP 2025

News and Physics Briefings

New results presented at EPS-HEP 2025

Higgs Boson and Di-Higgs

Searches for New Physics

Standard Model

Top Quark

Physics Modelling & Performance

Explore all ATLAS results

ATLAS takes a breath of oxygen

Oxygen and neon ions, with just 8 and 10 protons and neutrons respectively, are expected to form smaller droplets of QGP compared to lead-ion collisions – offering a new pool of data for physicists to dive into.

Not too big, not too small

Neons bring a twist

Aiming high

Studies of these collisions will advance our understanding of the strong force and help shape the future of the LHC heavy-ion programme.

All this, in just a few days

Learn more

Celebrating the outstanding achievements of the ATLAS Collaboration

“While many of these contributions happen behind the scenes, they are absolutely central to the success of our experiment.”

Meet the 2025 ATLAS Outstanding Achievement Award recipients

ATLAS Highlights from Quark Matter 2025

How does the QGP flow radially?

What can near-miss collisions tell us about the strong force?

These results highlight the power and versatility of the ATLAS experiment in exploring the physics of heavy-ion collisions.

How do particles lose energy in the QGP?

More questions lie ahead

Learn more

ATLAS prepares for High-Luminosity LHC

An ambitious ATLAS detector upgrade

Rising to the computing challenge

A new physics landscape

A tremendous opportunity, and a tremendous challenge

Submissions to the European Strategy Group

Further reading

Summary of new ATLAS results from Moriond 2025

News and Physics Briefings

New results presented at the 2025 Moriond Conferences

B-Physics

Exotic new physics

Higgs boson

Scalar boson and diboson searches

Standard Model

Supersymmetry

Top Quark

Explore all ATLAS results

Steering ATLAS forward with new management

Stéphane takes the wheel at a pivotal moment, as ATLAS members wrap up data-taking from LHC Run 3 and prepare for extensive upgrades to the experiment.

Meet the new members of the ATLAS Management team

Anna Sfyrla, Deputy Spokesperson

Guillaume Unal, Deputy Spokesperson

ATLAS celebrates excellence in doctoral research

ATLAS releases first open data from heavy-ion collisions

Today’s release is the highest energy heavy-ion collision data ever made publicly available and the first LHC Run 2 heavy-ion open dataset.

Find out more

Summary of new ATLAS results from BOOST 2024

Latest Physics Briefings

New results presented at BOOST 2024

Explore all ATLAS results

Summary of new ATLAS results from ICHEP 2024

Latest News and Physics Briefings

New results presented at ICHEP 2024

Exotic New Physics

Scalar boson and diboson searches

Higgs boson

Standard Model

Heavy Ions

Top Quark

Combined Performance

Explore all ATLAS results

ATLAS releases 65 TB of open data for research

Today's release underscores the ATLAS Collaboration's long-standing commitment to open access principles.

Find out more

ATLAS and CMS celebrate a decade of innovation by the RD53 Collaboration

In 2013, ATLAS and CMS joined forces to found the RD53 Collaboration, which set out to design critical read-out chips for the experiments' upgrades.

Celebrating excellence at the 7th ATLAS Outstanding Achievement Awards