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ATLAS turns into a cosmic-ray laboratory with proton-oxygen collisions

Katarina Anthony — Wed, 15 Apr 2026 14:53:29 +0000

ATLAS turns into a cosmic-ray laboratory with proton-oxygen collisions

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Physics Briefing

ATLAS Collaboration

physics results

light ion

cosmic ray

The ATLAS Collaboration reports its first measurement of proton-oxygen collisions at the Large Hadron Collider (LHC). These results transform the ATLAS experiment into a cosmic-ray laboratory, helping to unravel the nature of high-energy particle showers in the sky.

Tens of kilometres above ground, high-energy particles from outer space constantly bombard the Earth. When they strike the atmosphere, these “cosmic rays” create showers of energetic secondary particles that rain down from the sky. Approximately one of these particles passes through your head every second.

Cosmic rays were discovered over a century ago by physicist Victor Hess in experiments conducted aboard hot-air balloons. Today, astrophysicists use detectors on the ground to image cosmic-ray showers and depend on computer simulations of the showers to understand the data. However, these simulations require knowledge of the strong force that is difficult to model accurately. Current models disagree with one another, making it difficult for astrophysicists to interpret their measurements.

To solve this problem, physicists at the ATLAS experiment set out to recreate cosmic-ray collisions in the laboratory – effectively putting them under a microscope. In July 2025, for a few days, they transformed the LHC into a giant cosmic-ray machine. For the first time in its history, the LHC was configured to collide protons with oxygen ions. The beam of protons acted as the cosmic ray, while the beam of oxygen ions played the role of the atmosphere.

These results establish a novel way to use the LHC as a cosmic-ray laboratory, opening the path for detailed experimental studies of proton-oxygen interactions.

Figure 1: Graph showing how often proton-oxygen collision events are measured for the number of charged particles on the horizontal axis. Black circles show data with the shaded band being its uncertainty. Lines show predictions from various computer simulation models. The lower panel shows how different the models are from data with ATLAS-measurement uncertainties given by the shaded band. (Image: ATLAS Collaboration/CERN)

ATLAS physicists analysed these collisions by measuring the tracks left in the experiment from electrically charged particles (yellow lines in the event display above). They measured key properties of the collision, including how often the particles were created, how many were created, and the energies and angles at which they flew out.

Figure 1 shows the measured number of charged particles compared to simulations typically used to interpret data from cosmic-ray observatories. These simulations, which are tuned to reproduce previous data from collisions of protons and heavier nuclei, disagree with one another. The new measurements achieve a precision at the level of a few percent, significantly improving the knowledge of proton-oxygen collisions. ATLAS physicists now pass the baton to theorists, who can use this input to refine their models to better match experimental measurements.

These results establish a novel way to use the LHC as a cosmic-ray laboratory, opening the path for detailed experimental studies of proton-oxygen interactions. This can help shed light on the mysterious high-energy particles arriving from the cosmos.

About the banner image: Event display showing nineteen charged-particle tracks (yellow lines) recorded by the ATLAS experiment during proton-oxygen collisions in July 2025. (Image: ATLAS Collaboration/CERN)

Explore the interactive event display

Dynamic view of the proton-oxygen collision event recorded by the ATLAS experiment in July 2025. (Image: ATLAS Collaboration/CERN)

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Measurement of charged-particle production in 9.62 TeV proton-oxygen collisions as a probe of cosmic-ray air showers with the ATLAS detector (arXiv:2604.05512, see figures)
SM@LHC conference talk by Ynyr Harris: Soft QCD and multi-parton interactions with the ATLAS and CMS experiments
ATLAS takes a breath of oxygen, ATLAS News, July 2025

ATLAS spots rare high-energy Higgs bosons for the first time

Katarina Anthony — Tue, 31 Mar 2026 12:22:55 +0000

ATLAS spots rare high-energy Higgs bosons for the first time

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The ATLAS Collaboration reports the first evidence of Higgs-boson production at high transverse momentum in decays to a pair of bottom quarks.

Almost 15 years since its discovery by the ATLAS and CMS Collaborations, the Higgs boson has become a powerful tool in the search for new phenomena beyond the Standard Model. Among the many avenues of exploration, the study of Higgs bosons produced at very large transverse momentum is particularly promising. New physics phenomena can have a strong impact on this production rate, making it a sensitive avenue of exploration, complementary to “precision” measurements of Higgs-boson properties. In this regime, the production rate becomes sensitive to the quantum structure of Higgs-boson interactions – a possibility first proposed by theorists as early as 1988, before the LHC was even being designed.

Despite the potential, evidence of this process has eluded physicists. Out of the roughly 60,000 Higgs bosons produced daily at the LHC presently, only 30 have transverse momentum in excess of 450 GeV. This low yield requires large datasets and makes the most abundant Higgs-boson decay mode, into a pair of bottom quarks, the best channel for study. This decay leaves a distinct signature in the experiment, characterised by two energetic “jets” of particles. At very high energies, the two b-quarks originating from the Higgs boson are produced so close together that they appear as a single large-radius jet, balanced by a second “recoil” jet from momentum conservation (see event display).

However, this Higgs-boson signal sits on top of a background originating from strong-force interactions that is 50,000 larger. Extracting the signal therefore requires not only very large datasets, but also new reconstruction tools and signal-selection techniques.

At the Recontres de Moriond conference, the ATLAS Collaboration presented the first evidence of this process, examining 301 fb^-1 of data collected during LHC Run 2 (2015-2018) and the first three years of Run 3 (2022-2024). During this period, the performance of key detector components – in particular the thin silicon pixel detector closest to the proton beam, which is critical to identifying heavy bottom quarks – evolved due to radiation damage and had to be carefully modelled.

The new ATLAS result finds the first evidence of Higgs bosons at high transverse momentum from the analysis of 301 fb^-1 of data, collected during LHC operation from 2015 to 2024.

Figure 1: Reconstructed mass distribution of selected jets with transverse momentum in excess of 450 GeV in the analysis signal region with backgrounds subtracted after the profile-likelihood fit. The points with error bars represent the data with the filled histogram indicating the expected shape of the Higgs boson signal at the fitted yield and the band the uncertainty from the subtraction of the backgrounds.

ATLAS researchers employed new transformer-based neural networks to improve the isolation of the subtle Higgs signal from background and the measurement of the Higgs boson jet. Originally developed for language translation, these algorithms have since enabled breakthroughs from protein folding in drug discovery to analyses spanning the smallest particles and the largest cosmic structures, demonstrating unprecedented pattern-recognition power across science. In ATLAS, the challenge of jet reconstruction is different but mathematically similar, as a single jet can contain nearly 100 particles with strongly correlated trajectories and energies. The transformer network architecture resolves these correlations by combining these tracks into a global representation of the jet, which is used to classify it as signal or background and to sharpen the mass and transverse momentum resolution in the analysis.

The use of these tools contributed to an overall improvement in analysis sensitivity of a factor of seven to ten compared to the first inclusive ATLAS search for Higgs bosons at high transverse momentum, published in 2022 and based on approximately half the amount of data. As a result, physicists reported an excess of more than 2000 events observed in the Higgs-boson mass region (see Figure 1), corresponding to a significance of 3.8 standard deviations over the background-only hypothesis. This is the first evidence at the LHC for Higgs-boson production at high transverse momentum (above 450 GeV). The measured rate is consistent with Standard Model predictions, and extends earlier studies in other channels.

This result and the associated analysis techniques establish a foundation for future studies using the full Run-3 dataset and, in the longer term, the much larger datasets expected from the High-Luminosity LHC. Studies of Higgs-boson production at high transverse momentum are unique to hadron colliders, and the sensitivity achieved by the ATLAS and CMS Collaborations will be crucial in the exploration of physics beyond the Standard Model with the Higgs boson. Such results are part of the long-term physics legacy of the LHC and will stay in the textbooks for a very long time.

About the banner image: Display of a signal event, recorded in the ATLAS detector in 2024, showing the candidate Higgs-boson decay in the lower hemisphere with a measured transverse momentum of 1200 GeV and an invariant mass of 126 GeV, consistent with a H → bb decay. The recoil jet, in the upper hemisphere, balances the candidate Higgs candidate jet system in the plane transverse to the colliding beams. The Higgs candidate jet system shows the substructure originating from the two quarks produced in the Higgs boson decay. (Image: ATLAS Collaboration/CERN)

Learn more

Evidence of Higgs boson inclusive production at high transverse momentum decaying to a pair of b-quarks with the ATLAS detector (arXiv:2603.19369, see figures)
Constraints on Higgs boson production with large transverse momentum using H→bb¯ decays in the ATLAS detector (Phys. Rev. D 105 (2022) 092003, arXiv:2111.08340, see figures)
Measurement of the associated production of a Higgs boson decaying into b-quarks with a vector boson at high transverse momentum in proton-proton collisions at 13 TeV with the ATLAS detector (Phys. Lett. B 816 (2021) 136204, arXiv:2008.02508, see figures)
Moriond EW presentation by Fabio Cerutti: Experimental Summary
Moriond QCD presentation by Henri Bachacou: Higgs cross section and couplings measurements at ATLAS and CMS
Transforming bottom-jets: machine learning for improved bottom-jets measurements, Physics Briefing, July 2024
ATLAS Pixel detector readies to tackle Run 3, Physics Briefing, July 2022
Studying the Higgs boson in its most common – yet uncommonly challenging – decay channel, Physics Briefing, December 2020

How “odd” are Higgs boson interactions?

Katarina Anthony — Fri, 27 Mar 2026 14:13:46 +0000

How “odd” are Higgs boson interactions?

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One of the biggest mysteries in physics is why anything exists at all. According to the current understanding of the Big Bang, matter and antimatter should have been produced in equal amounts. Yet the visible Universe today is made almost entirely of matter – so where did all the antimatter go?

The answer may lie in the violation of charge-parity (CP) symmetry. CP symmetry states that the laws of physics should remain unchanged if particles are replaced with their antiparticles and their spatial coordinates are inverted. While CP violation has been observed in quark interactions involving the weak force, the amount predicted by the Standard Model of particle physics is far too small to explain the observed matter-dominated Universe.

At the 2026 Recontres de Moriond conferences, physicists from the ATLAS Collaboration presented new searches for additional sources of CP violation linked to the Higgs-boson interactions with W and Z bosons (carriers of the weak force collectively known as vector bosons). In the Standard Model, this interaction is expected to be CP-even, meaning its interactions conserve CP symmetry. Any evidence of CP-odd contributions would be a game-changer, signaling new physics beyond the Standard Model.

The ATLAS Collaboration has released two new searches for new sources of CP violation linked to Higgs boson interactions with W and Z bosons.

Figure 1: From left to right: Feynman diagrams for vector boson fusion, VH associated production and Higgs boson decays to vector bosons, each highlighting the HVV vertex. (Image: ATLAS Collaboration/CERN)

To explore this possibility, ATLAS physicists studied the point where the Higgs boson and vector bosons meet: the HVV vertex. This can be examined in several processes, including vector boson fusion (VBF), where two quarks emit vector bosons that fuse to produce a Higgs boson and leave particle “jets” in opposite regions of the experiment. This distinctive signature makes VBF studies particularly powerful. The HVV vertex can also be studied in associated production (VH), where a Higgs boson is produced alongside a vector boson, and in Higgs-boson decays to a vector-boson pair (see Figure 1).

But how can physicists search for signs of new physics in this interaction? The answer lies in Effective Field Theories (EFTs), which provide a framework to describe how new particles – too heavy to be produced directly at the LHC – could still influence measurements through subtle quantum effects. In this framework, additional parameters modify Higgs-boson interactions and change the shapes of certain observable distributions, such as the angle between the two jets in VBF events. One such parameter is the coefficient c_HW̃, which would modify the HVV vertex and introduce CP-violating effects.

Figure 2: Summary of ATLAS measurements constraining the CP-odd parameter c_HW̃, showing the best-fit values and the expected and observed 95% confidence-level (CL) intervals. All results assume a new physics scale Λ = 1 TeV and include only linear contributions from the relevant CP-odd EFT operator. (Image: ATLAS Collaboration/CERN)

In a new analysis, ATLAS physicists combined several previous Run-2 measurements of c_HW̃, each using 140 fb^-1 of proton-proton collision data at 13 TeV centre-of-mass energy. These include studies of VBF Higgs-boson decays to photons, tau leptons, and W or Z bosons, as well as VH Higgs-boson decays to bottom quarks. Assuming an energy scale (Λ) of 1 TeV for these new physics effects, they set the most stringent limits to date on c_HW̃, constraining it between −0.14 and 0.49 at the 95% confidence level (see Figure 2). No significant deviation from the Standard Model was observed. Notably, the combined analysis improved sensitivity by more than 40% compared to the best individual measurement – highlighting the immense value of these collaborative and complementary studies.

In a parallel effort, the ATLAS Collaboration performed its first search for CP-violating effects in the Higgs sector using LHC Run-3 data (164 fb^-1 of proton-proton collision data at 13.6 TeV, collected between 2022 and 2024). The study focused on Higgs-boson decays to two photons – a decay channel that accounts for only 0.2% of Higgs decays, yet leaves a crystal-clear signature allowing for very precise measurements. Benefiting from the larger Run-3 dataset and sophisticated machine-learning techniques to better isolate the Higgs signal, researchers achieved a nearly 40% improvement in sensitivity over the previous ATLAS study of this channel (see Figure 2).

This is the first ATLAS result to rely entirely on the new, high-speed detector simulation AtlFast3. Its success demonstrates that this tool can deliver world-class physics results, marking a significant step towards a more ecologically and economically sustainable computing model for future LHC analyses.

About the banner image: Event display of a 13.6 TeV proton-proton collision in the ATLAS experiment, where a candidate Higgs boson produced via vector boson fusion (VBF) decays into two photons (H→γγ). The two photons from the Higgs-boson decay are shown as purple cones, accompanied by two forward jets shown as yellow cones. Charged-particle trajectories reconstructed in the inner detector are displayed as orange lines. Yellow/orange and green/cyan boxes indicate energy deposits in the hadronic and electromagnetic calorimeters, respectively. (Image: ATLAS Collaboration/CERN)

Learn more

Combination of measurements of CP properties of Higgs boson interactions with vector bosons using Run 2 proton–proton collisions at 13 TeV with the ATLAS detector (arXiv:2603.20117, see figures)
Search for anomalies in vector-boson fusion production of the Higgs boson in H(→γγ)jj events using 164 fb^–1 of proton-proton collision data collected at 13.6 TeV with the ATLAS detector (arXiv:2603.20087, see figures)
Moriond EW presentation by Lailin Xu: Recent single Higgs measurements with ATLAS

ATLAS extends Higgs boson studies via vector boson fusion into beauty and charm

Katarina Anthony — Thu, 26 Mar 2026 13:11:45 +0000

ATLAS extends Higgs boson studies via vector boson fusion into beauty and charm

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An invisible field fills the Universe, giving mass to fundamental particles and shaping the building blocks of matter – the Higgs field. The Higgs boson is its observable manifestation and a cornerstone of the Standard Model. By studying it with increasing precision, physicists are not only refining their understanding of the current theory but also searching for new phenomena that extend beyond it. Such effects may appear as subtle deviations in rare or difficult-to-measure Higgs processes.

Two new results from the ATLAS Collaboration push into this challenging territory, improving the sensitivity to Higgs-boson decays to bottom (H→bb) and charm quarks (H→cc). Though most Higgs bosons transform into quark-antiquark pairs – with H→bb making up about 58% of all decays, and H→cc accounting for 3% – these processes are notoriously difficult to study in hadron colliders. Bottom and charm quarks produce collimated sprays of particles called “jets” – the most common signature observed in the ATLAS experiment, produced in huge numbers via strong-force interactions. Isolating a Higgs signal amid this background is a challenging task, particularly for the H→cc decay, as charm jets are significantly more difficult to identify than their bottom-quark counterparts.

To tackle this challenge, researchers focused on Higgs bosons produced via the fusion of two W or Z bosons – a process called vector boson fusion (VBF). Although this production mode accounts for just 7% of Higgs bosons, compared to 87% from the dominant gluon fusion production mode, it can leave a distinctive signature in the experiment: two energetic jets produced in the forward regions of the experiment with a large angular separation. This feature made it easier for researchers to separate the signal from background and is used in both analyses.

Figure 1: Neural network classifier output distribution after fitting the simulated signal and background templates to data, in the signal region of invariant di-b-jet mass between 100 and 150 GeV. (image: ATLAS Collaboration/CERN)

Looking for a light

For their first new result, ATLAS physicists looked for a (literal) beacon of light! In roughly 1% of VBF Higgs events, the Higgs boson is accompanied by a high-energy photon. The dominant multi-jet background processes rarely produce such a “shine”, making this a clean signature for the event-selection algorithm ("trigger") to identify collision events.

Building on previous work, the ATLAS Collaboration analysed LHC Run-2 data (2016-2018) to search for VBF H→bb produced in association with a photon. To maximise their sensitivity, physicists trained a neural network to distinguish signal events from background events based on the kinematics of the collisions. Neural networks assign each event a score between 0 and 1, reflecting how signal-like it appears. In this analysis, rather than selecting only the highest-scoring events, physicists fit the distribution of neural-network scores directly and used the mass of the two jets originating from bottom quarks to define the signal and control regions (see Figure 1).

Researchers measured a Higgs-boson signal strength of μ_bb = 0.2 ± 0.7, corresponding to an observed significance of 0.3 standard deviations (1.5 expected). Although this accounts for just 20% of the expected rate, the measurement is fully compatible with the Standard Model.

The ATLAS Collaboration finds evidence of the Higgs boson in one of the “noisiest” environments in which to study the particle.

Figure 2: Mass distribution of H→bb using LHC Run-3 data only. Regions are combined, weighting each region based on the relative size of signal and background in each region. The H→bb signal is shown in red. (Image: ATLAS Collaboration/CERN)

Searching for subtle patterns

But what happens when there is no beacon? For their second result, physicists searched without the help of a guiding photon, instead relying on subtle patterns in the event topology and a dedicated trigger. They focused on the characteristic VBF signature described above, a strategy that enabled searches for both VBF H→bb and VBF H→cc.

The H→cc search used a mix of LHC Run-2 and Run-3 data (2018, 2022 and 2023). No significant excess above the background was observed. At the 95% confidence level, researchers set an upper limit of 41 times the Standard Model prediction for the H→cc rate in this channel (28 times expected). When combined with an earlier ATLAS search for H→cc (via VH production), a direct constraint was set on the interaction strength of the charm quark and Higgs boson, κc, to be less than 4.7 times the Standard Model value (3.9 expected).

For their search for H→bb, researchers focused on Run-3 data from 2022 and 2023. They measured a Higgs-boson signal strength of μ_bb = 0.97 ± 0.57 – right where one would expect a signal consistent with the Standard Model (see Figure 2). The measurement was combined with previous Run-2 results as well as the H→bb search described above, the signal strength reached an observed combined significance of 3.2 standard deviations (3.6 expected). This constitutes the first evidence for VBF H→bb in a fully hadronic final state.

The ATLAS Collaboration has now established evidence of the Higgs boson in one of the “noisiest” environments in which to study the particle. Higgs research has evolved, with focus shifting from the most accessible signatures to increasingly subtle and challenging measurements. If new physics is to be uncovered at the LHC, it may appear as deviations in precisely such studies.

About the banner image: A candidate event from the Run-3 H→cc analysis. The two purple triangles denote jets from charm quarks forming the Higgs boson candidate, while the two yellow triangles denote jets associated with the vector boson fusion (VBF) process. (Image: ATLAS Collaboration/CERN)

Learn more

Search for Higgs bosons produced in association with a high-energy photon via vector-boson fusion and decaying to a pair of b-quarks in the ATLAS detector (arXiv:2509.14005, see figures)
Search for H→cc and measurement of H→bb in vector-boson fusion production with the ATLAS Detector (arXiv:2511.21911, see figures)
Measurements of WH and ZH production with Higgs boson decays into bottom quarks and direct constraints on the charm Yukawa coupling in 13TeV pp collisions with the ATLAS detector (JHEP 04 (2025) 075, arXiv:2410.19611, see figures)
Search for Higgs boson production in association with a high-energy photon via vector-boson fusion with decay into bottom quark pairs at 13 TeV with the ATLAS detector (JHEP 03 (2021) 268, arXiv:2010.13651, see figures)
The beauty and the charm of the Higgs boson, Physics Briefing, July 2024
Studying the Higgs boson in its most common – yet uncommonly challenging – decay channel, Physics Briefing, December 2020

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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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 surpasses LEP limits in search for compressed higgsinos

Katarina Anthony — Tue, 10 Mar 2026 07:35:09 +0000

ATLAS surpasses LEP limits in search for compressed higgsinos

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ATLAS sets new limits on the masses of supersymmetric higgsinos, surpassing results by the Large Electron–Positron (LEP) collider experiments over 20 years ago.

Supersymmetry (SUSY) is a compelling theory that predicts every Standard Model particle has a “super-partner.” Among these are higgsinos, the supersymmetric partners of the Higgs boson. If they exist, higgsinos could help explain why the Higgs boson has its observed mass. In many supersymmetric models, higgsino-like particles would also naturally account for the dark matter left over from the early Universe, making them attractive targets in searches for physics beyond the Standard Model.

Finding them, however, is easier said than done. Higgsinos would not appear as pure particles; instead, they would mix with other supersymmetric particles to produce physical states called neutralinos and charginos, whose masses can vary widely. Although extremely rare and difficult to detect, LHC experiments have been on the hunt for these particles for many years.

When the mass difference – or “mass splitting” – between charginos and neutralinos is small, the resulting experimental signatures become even harder to identify. Explorations of this “compressed mass spectrum” have long been limited by challenges in particle reconstruction and identification.

Equipped with new machine-learning techniques, the ATLAS Collaboration has met the challenge, setting new constraints on compressed higgsinos in regions last explored by the LEP experiments. The new result examines the full LHC Run-2 dataset, with targeted searches for the lightest higgsino-like states – namely, a chargino (χ̃^±₁) and two neutralinos (χ̃⁰₁ and χ̃⁰₂) – which are pair-produced. The mass splitting of these states strongly affects how they would appear in the ATLAS experiment. Researchers therefore conducted two distinct searches targeting different mass-splitting regimes.

The ATLAS Collaboration has now established constraints across the full range of higgsino mass splittings, closing gaps left by previous LHC results.

Figure 1: Observed and expected limits at 95% confidence CL for the higgsino model from 1L1T (purple) and displaced track (red) searches. The limits are shown in the Δm(χ̃±1, χ̃01) vs. m(χ̃±1) plane, along with previous limits from the LEP2 experiments (grey) and the ATLAS experiment (blue, light green, and yellow). (Image: ATLAS Collaboration/CERN)

The “displaced track" search focused on mass splittings of 0.3–1 GeV between the chargino and the neutralino χ̃⁰₁. In such scenarios, the chargino would travel a few millimetres before decaying into an invisible neutralino χ̃⁰₁ and a low-momentum charged pion. The resulting signature is a pion track that is “displaced” from the original collision point having a large transverse impact parameter and high missing transverse momentum from the presence of neutralinos. To enhance the signal sensitivity, physicists used two dedicated neural networks: one focusing on the overall event kinematics and another on the displaced track characteristics.

The “one-lepton-one-track” (1L1T) search targeted larger mass splittings between 1 GeV and 3 GeV. Here the heavier neutralino χ̃⁰₂ promptly decays into the lightest neutralino χ̃⁰₁ and two low-momentum leptons, one of which evades standard ATLAS identification algorithms. To spot these elusive tracks, researchers developed neural-network-based identification algorithms capable of spotting lepton-like tracks with momenta as low as 0.5 GeV for electrons and 1 GeV for muons. The resulting signature therefore consists of one lepton and one lepton-like-track. A parameterized neural network was then used for event selection, enhancing the signal sensitivity by focusing on the kinematic features that depend strongly on the mass splitting.

The observed data are consistent with Standard Model predictions. Physicists therefore set new limits on higgsino masses at the 95% confidence level (Figure 1). The 1L1T search excluded scenarios where the mass difference between the chargino and the lightest neutralino χ̃⁰₁ lies between about 0.8 and 2 GeV – extending previous LEP limits up to a chargino mass of 132 GeV for a mass splitting of 1.8 GeV. The displaced track search extends previous ATLAS exclusion limits by about 30 GeV, reaching chargino masses up to 199 GeV for a mass splitting of 0.6 GeV. Both searches excluded charginomasses below 126 GeV at the 95% confidence level in the targeted mass splitting range. These new limits supersede LEP experiment results in all mass splitting regimes.

The ATLAS Collaboration has now established constraints across the full range of higgsino mass splittings, closing gaps left by previous LHC results. This is an important step forward in the search for supersymmetry. The new Run-3 dataset and evolving analysis techniques will allow the ATLAS Collaboration to further advance these searches, potentially paving the way to the discovery of physics beyond the Standard Model.

About the event display: Event display of a 1L1T event, consisting of an electron and an electron-track candidate. Energy deposits in the electromagnetic and hadronic calorimeters are presented as green and yellow blocks, respectively, while tracks reconstructed by the inner-detector are shown in orange. An identified jet is shown with the yellow cone, while the missing transverse momentum is indicated by the dashed white line. An electron with a transverse momentum of 8 GeV satisfying the standard ATLAS reconstruction and identification algorithms is shown in blue. The low-energy electron-track candidate with a transverse momentum of 2.5 GeV is shown in purple. (Image: ATLAS Collaboration/CERN)

Learn more

Search for higgsinos in compressed mass spectra using low-momentum tracks in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2511.20042, see figures)
CERN LHC Seminar presentation by J. Shahinian: Closing the gap in compressed SUSY searches at ATLAS
The most elusive higgsinos, CERN Courier, 6 March 2026

The curious case of the disappearing track

Katarina Anthony — Tue, 10 Mar 2026 06:52:56 +0000

The curious case of the disappearing track

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Physics Briefing

Now you see it, now you don't

They say a good magician never reveals their secrets. If that’s the case, then Nature is an excellent magician indeed! One of its most elusive tricks could be the ability to make particle tracks seemingly vanish inside the ATLAS experiment. Hunting for this disappearing act could reveal new physics phenomena that lie beyond the Standard Model.

In a new result analysing the full LHC Run-2 dataset (collected 2015-2018), the ATLAS Collaboration studied a unique experimental signature known as "disappearing tracks". This subtle trail could be left by a heavy, long-lived charged particle that travels a very short distance in the innermost layers of the ATLAS experiment before decaying into undetectable particles. Supersymmetry (SUSY) models predict just such a particle, called a chargino, which decays into a very low-energy pion and an invisible neutralino, a candidate for dark matter. Since low-energy pions follow highly curved trajectories in the inner detector, they are extremely difficult to identify in a busy proton-proton collision – causing the chargino’s track to "disappear".

Figure 1: Representative reconstruction efficiencies of the short track algorithm (left) and soft-pion reconstruction algorithm (right). Due to these algorithm developments it is possible to identify both the long-lived chargino leaving only three or four hits in the innermost layers of the ATLAS experiment and the soft pion. (Image: ATLAS Collaboration/CERN)

The ATLAS Collaboration has now set the most stringent limits to date on the masses and lifetimes of charginos.

Figure 2: Exclusion limits as a function of the chargino lifetime (y-axis) and mass (x-axis). The region within the red curve is considered as excluded. The dashed-dotted grey line indicates the lifetime and masses expected if the mass difference between the chargino and neutralino are only due to loop contributions from Standard-Model particles. (Image: ATLAS Collaboration/CERN)

To reveal the chargino's magic trick, physicists first set out to reconstruct the short track left by the chargino before it decays. Standard ATLAS tracking techniques require at least seven “hits” in the silicon layers of the inner detector to reconstruct a particle’s track. For this analysis, physicists deployed a new algorithm capable of identifying tracks from just three or four hits (see event display). This enabled the team to investigate shorter chargino lifetimes than in previous analyses. The ATLAS team then used machine-learning techniques to identify the very low-energy (“soft”) pions produced by the chargino’s decay, with momenta of only 200 MeV – revealing the “disappeared” track! The reconstruction capabilities of both the short track algorithm and the soft-pion reconstruction technique are shown in Figure 1.

No events consistent with chargino production and decay were observed, despite a small excess of events over the expected background. The result extends the sensitivity to charginos into the low-mass region, where the mass difference between charginos and neutralinos is primarily determined by quantum loop effects from Standard Model particles. As shown in Figure 2, the ATLAS Collaboration has now set the most stringent exclusion limits to date on the masses and lifetimes of charginos: up to 225GeV for charginos with lifetimes below 0.03ns and up to 720GeV for a lifetime of 1ns.

The novel techniques deployed in this search will be built upon in the coming years, as researchers continue the search for disappearing charginos using the full LHC Run-3 dataset (2022–ongoing). The next act awaits!

About the event display: Display of a candidate disappearing track event from proton-proton collisions recorded by the ATLAS detector in 2018. Pink crosses indicate hits in the pixel detector layers and red lines indicate the reconstructed tracklet. The cyan line indicates a reconstructed soft pion that continues to the SCT. The green, yellow and cyan boxes indicate energy deposits in calorimeter cells. The yellow cone denotes the reconstructed jet, while the dashed white line indicates the missing transverse momentum. (Image: ATLAS Collaboration/CERN)

Learn more

CERN LHC Seminar presentation by J. Shahinian: Closing the gap in compressed SUSY searches at ATLAS
Search for long-lived charginos and tau-sleptons using final states with a disappearing track in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2603.08315, see figures)
Search for long-lived charginos based on a disappearing-track signature in proton-proton collisions at 13 TeV with the ATLAS detector (JHEP 06 (2018) 022, arXiv:1712.02118, see figures)
Quest for the lost arc, ATLAS Physics Briefing, March 2017
CMS Collaboration: A disappearing act in CMS, CMS Briefing, 2020

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:

Getting to the top of jet calibration

Katarina Anthony — Mon, 16 Feb 2026 02:33:15 +0000

Getting to the top of jet calibration

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When particles burst from an LHC collision, they rarely leave a simple, single trail for physicists to follow. More often, they are high-energy quarks and gluons that erupt into jets, collimated sprays of particles that make up the most common "footprints" left in the ATLAS experiment. A precise determination of jet properties is vital for several Standard Model studies, such as top-quark mass measurements, as well as searches for new physics phenomena. However, accurate jet measurements can be notoriously challenging, and ATLAS physicists are continuously developing new techniques to improve their precision.

Key to this is calibration. Physicists consider two quantities: the jet energy scale (JES), which describes how accurately the average reconstructed jet momentum reflects the true momentum; and the jet energy resolution (JER), which characterises the repeatability of that measurement. JES and JER calibrations are obtained using both simulation and data-driven correction factors. Simulation-based corrections account for information that is not measured by the ATLAS experiment, while data-driven corrections address possible imprecise modeling of the detector material and particle interactions. Traditionally, physicists have relied on the principle of momentum conservation to guide data-driven corrections. By studying collision events in which a jet recoils against a well-measured reference object (i.e. a lepton or photon), they can precisely infer a jet’s momentum. While highly effective, this approach can face limitations in certain situations.

Physicists have pioneered a novel technique to calibrate jets – collimated sprays of particles that make up the most common "footprints" left in the ATLAS experiment.

Figure 1: Reconstructed W-boson mass templates for jets with transverse momentum (pT) ranging between 50 and 70 GeV. JES variations are shown on the left (represented by the parameter s), and JER variations are shown on the right (represented by the parameter r). In both cases, the alternate parameter is set to 1. The differences between the total yields for each distribution are due to acceptance effects. (Image: ATLAS Collaboration/CERN)

In a new result released by the ATLAS Collaboration, researchers describe a new JES and JER calibration method using top quarks. Specifically, they considered top-quark decays to a W boson and a b-quark, where the W boson subsequently decays to two jets. As the W-boson mass has been precisely measured, it is the ideal reference for in-situ jet calibration. The ATLAS team created templates of the reconstructed W-boson mass distribution with different assumptions for the JES and JER (see Figure 1). They then fit the mean and width of these distributions to the data in order to extract the optimal corrections.

Figure 2: Pre-fit (dotted line) and post-fit (solid line) predictions for the W-mass distribution with reconstructed jet transverse momentum (pT) between 70 and 100 GeV in the JES fit to the Run-2 data. The hatched band denotes the uncertainty. Bottom panel shows the ratio of the data without the JES in-situ calibration (PFlow+JES) to the post-fit prediction, with the dashed horizontal line representing the ratio of one. Arrows indicate a data value outside of the displayed range. (Image: ATLAS Collaboration/CERN)

Researchers studied data collected during both LHC Run 2 (2015–2018) and the first years of Run 3 (2022–2023). Considering these periods separately was vital, as detector aging, changes in detector material and differences in LHC operating conditions could affect how jets are recorded. Additionally, significant improvements had been made to Run-3 simulations, informed by Run 2 experience and enhanced physics modeling. Because a detector's response depends on a particle's momentum, the team extracted separate corrections for the JES and JER calibrations across different jet momentum intervals. Figure 2 shows the W-mass distribution before and after applying corrections fitted from data, clearly demonstrating the impact of accurate calibration.

The new ATLAS method was found to be competitive with established techniques for jets with momenta between 35 and 200 GeV, achieving uncertainties of about 1% for JES and about 15 to 20% for JER. When combined with standard ATLAS techniques, it will allow researchers to reach new precision in jet measurements.

While a similar approach was successfully used in top-quark-mass measurements, this is the first application of the technique on jet calibration in ATLAS. As the precision of top-quark measurements improves, this new method has the potential to become one of the most powerful approaches to calibrate jets.

About the image banner: Event display of a top-antitop quark candidate in the ATLAS experiment, representing the type of event used in this new jet calibration method. A muon is shown by the red line, while the green and yellow bars represent energy deposits in the liquid argon and tile calorimeters. From these deposits, four jets are identified with transverse momenta between 25 and 80 GeV. (Image: ATLAS Collaboration/CERN)

Learn more

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)
Boosting precision of top-quark mass measurement with ATLAS, Physics Briefing, March 2025

ATLAS maps the top quark–Higgs boson interaction with multileptons

Katarina Anthony — Fri, 06 Feb 2026 15:23:42 +0000

ATLAS maps the top quark–Higgs boson interaction with multileptons

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Katarina Anthony Fri, 06/02/2026 - 16:23

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One of the central questions in particle physics is how fundamental particles acquire mass. The top quark, as the heaviest known elementary particle, interacts most strongly with the Higgs boson. As such, it may play a crucial role in understanding how the Higgs field gives rise to particle masses.

The ATLAS Collaboration is studying the production of the Higgs boson together with a top-quark pair (“ttH production”). First observed by the ATLAS and CMS Collaborations in 2018, this process accounts for just 1% of all Higgs-boson production and yet provides a unique opportunity for researchers to directly measure the top–Higgs interaction.

Using the complete LHC Run-2 dataset (collected in 2015–2018), the ATLAS Collaboration has carried out a new measurement of ttH production. The analysis focuses on “multilepton” events, where particle collisions leave several leptons (electrons, muons or taus) in the final state.

Multilepton needles in the haystack

Figure 1: The six categories used in the analysis as a function of the number of light leptons (electrons or muons) and hadronically-decaying tau leptons. 2lSS refers to two leptons of the same sign. (Image: ATLAS Collaboration/CERN)

To maximise sensitivity, ATLAS physicists divided their data into six exclusive categories based on the number and charge of the leptons (Figure 1). Categories containing one or two hadronically-decaying tau leptons would be particularly sensitive to Higgs boson decays into taus, while the other categories would focus on Higgs-boson decays into W bosons. All six categories were analysed simultaneously, allowing the small ttH signal to be separated from much more common background processes.

The new ATLAS result improves on the previous Run-2 analysis by combining a larger dataset with enhanced particle identification and reconstruction techniques. Researchers also drew upon more accurate simulations, dedicated studies of the dominant background processes, data-driven methods to constrain background rates, and a refined treatment of experimental uncertainties to bolster their result.

Testing predictions

The ATLAS team assessed how well their measurement matched the Standard Model using the so-called “signal strength”, defined as the observed ttH production rate divided by the predicted rate. The ttH production signal strength (μ_ttH) was measured to be 0.63 ^+0.20_−0.19 While slightly lower than the Standard Model prediction (1.0), the value is compatible within experimental uncertainties. The analysis provides evidence of ttH production in multilepton final states with a statistical significance of 3.3 standard deviations (σ), compared with an expected significance of 5.3σ.

A complementary measurement of Higgs-boson production in association with a single top quark (tH) was also performed. The measured signal strength, μ_tH = 7.2^+4.6_−4.0, is slightly above the Standard Model expectation. A similarly high value was reported in previous ATLAS and CMS studies of this process.

Figure 2: The observed best-fit values of the ttH cross-section relative to the Standard-Model expectation and their uncertainties in the simplified template cross-section measurement. The inclusive theory systematics in the Standard-Model prediction are shown as a grey dashed band, but are not included in the uncertainties of the measurement. (Image: ATLAS Collaboration/CERN)

At the ATLAS experiment, scientists are exploring the relationship between the heavyweights of the subatomic world: the top quark and the Higgs boson.

The Higgs boson in motion

Figure 3: The observed exclusion contours of the CP-even and CP-odd components of the top-Higgs interaction. Regions contained in the dashed, and in the solid lines are compatible with the best-fit result at 68% and 95% confidence level. κt' quantifies a deviation of the interaction amplitude from its Standard Model value and α is the mixing angle between CP-odd and CP-even states. The orange star represents the Standard Model (α = 0°, κt'=1) and the yellow star represents a pure CP-odd interaction with no modification to the amplitude of the coupling (α = 90°, κt'=1). (Image: ATLAS Collaboration/CERN)

Researchers also studied the transverse momentum of the Higgs boson, which can be used to probe different Higgs production mechanisms and possible deviations from the Standard Model interactions. Since the observed leptons can come from either Higgs-boson or top-quark decays and several neutrinos escape detection, the kinematic system cannot be fully reconstructed. Instead, the Higgs boson’s transverse momentum must be inferred statistically using a Graph Neural Network trained on the full event topology of simulated ttH events. Applying a simplified template cross-section (STXS) framework, the ttH production rate was measured in three ranges of transverse momentum (Figure 2).

Testing the "symmetry" of the Higgs boson

Having studied the ttH production rate, physicists turned their attention to more detailed properties of the top-Higgs interaction. The Standard Model predicts that the Higgs boson’s interactions with other particles should have even charge-parity (CP) symmetry. Any evidence of CP-violating interactions (CP-odd) would indicate the presence of as-yet undiscovered phenomena.

In their new analysis, ATLAS researchers tested whether the top–Higgs interaction could contain a mixture of different CP components. The degree of CP mixing is parameterised by the mixing angle α, where α = 0° corresponds to a purely CP-even interaction and α = 90° to a purely CP-odd one. Values of α > 62° were excluded at a 68% confidence level, supporting the Standard Model description of the Higgs boson and remaining consistent with the best ATLAS constraint to date. Constraints were set on possible CP-odd components of the top-Higgs interaction (Figure 3).

As larger datasets from LHC Run 3 and the High-Luminosity LHC are analysed, ATLAS researchers will further refine ttH measurements and sharpen their knowledge of the top–Higgs interaction.

About the banner image: Event display for a ttH candidate event, with an electron (green), muon (red) and two hadronically-decaying tau leptons (pink cone) in the final state. (Image: ATLAS Collaboration/CERN)

Learn more

Measurement of the Higgs boson production in association with top quarks in multilepton final states in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2510.23755, see figures)
Evidence for the associated production of the Higgs boson and a top quark pair with the ATLAS detector (Phys. Rev. D 97 (2018) 072003, arXiv:1712.08891, see figures)
Search for the production of a Higgs boson in association with a single top quark in proton-proton collisions 13 TeV with the ATLAS detector (JHEP 10 (2025) 093, arXiv:2508.14695, see figures)
CMS Collaboration: Measurement of the Higgs boson production rate in association with top quarks in final states with electrons, muons, and hadronically decaying tau leptons at 13 TeV (Eur. Phys. J. C81 (2021) 378, arXiv:2011.03652)
Searching for new sources of matter–antimatter symmetry breaking in Higgs boson interaction with top quarks, ATLAS Physics Briefing, April 2020
New ATLAS result establishes production of Higgs boson in association with top quarks, ATLAS Physics Briefing, June 2018

ATLAS expands its reach into the high-rate frontier

Katarina Anthony — Fri, 12 Dec 2025 11:31:47 +0000

ATLAS expands its reach into the high-rate frontier

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Katarina Anthony Fri, 12/12/2025 - 12:31

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Nature loves symmetry. Nowhere is this more evident than at the Large Hadron Collider (LHC), where the vast majority of proton-proton collisions result in a strikingly symmetrical signature: two concentrated sprays of hadrons (jets) emerging back-to-back with nearly equal momentum. These “dijet” events are a key hunting ground for physics beyond the Standard Model. Thanks to a fundamental feature of particle interactions, almost any new particle that can be produced in a collision of two protons should be able to decay into a pair of gluons or a quark-antiquark pair, leaving the characteristic dijet signature.

Figure 1: Distribution of dijet invariant mass (mjj) for events (black) collected via two TLA streams (J50 and J100) with partial reconstruction within the HLT, and (red) the nominal main stream with full offline reconstruction. It is apparent that the TLA streams allow the retention of a significantly higher number of events with dijet masses much lower than 1 TeV. (Image: ATLAS Collaboration/CERN)

Yet the abundance of dijet events presents a challenge. The ATLAS experiment relies on its “trigger” system to select the most interesting of the billion collisions that occur every second. The first-level (L1) trigger, implemented in custom hardware, reduces the LHC’s 40 MHz rate to 100 kHz. Then, the software-based high-level trigger (HLT) used during Run 2 lowers this further to just 1.2 kHz. As a result, the vast majority of dijet events have to be discarded to manage bandwidth constraints.

The ATLAS Collaboration’s innovative trigger-level analysis (TLA) approach provides a way to overcome these limits. Instead of recording the full collision event data, the TLA stream saves only the essential information reconstructed by the HLT, such as jet momenta and variables used for calibration and validation. This reduces the average event size from 1 MB to just 6.5 kB, allowing far more events to be recorded, particularly at low dijet masses (Figure 1). Similar strategies have been used by the CMS Collaboration (e.g. Data Scouting) and the LHCb Collaboration (e.g. Turbo Stream), and in an ATLAS study of a smaller subset of the Run 2 dataset.

ATLAS researchers are delving into an extraordinary dataset – 60 billion trigger-level events collected during LHC Run 2 – to search for new particles.

In a new paper published in Physical Review D, the ATLAS Collaboration presents a search for new particles decaying into dijets, based on 60 billion trigger-level events collected during Run 2 of the LHC (2015-2018). This is more than double the total number of fully reconstructed ATLAS events from LHC Run 1 and 2 combined (25 billion). Using the TLA approach, physicists were able to record data at over 20 times the standard HLT readout rate (up to 27 kHz vs. 1.2 kHz).

Figure 2: Observed mjj distribution for two different TLA streams (J50 and J100). The red and blue histograms indicate a functional form fit to the data. The data is found to be compatible with the background-only description. Predictions for two potential signals of a new particle (Z’) are shown above the fit (open markers). The bottom panel shows the significance of the deviations of the observed data from the background estimate. (Image: ATLAS Collaboration/CERN)

Dijet searches look for a small excess in the reconstructed dijet mass spectrum, arising from a potential beyond-the-Standard-Model (BSM) resonance on top of the smoothly falling Standard Model (SM) background. While conventional dijet searches study masses above 1 TeV, this trigger-level analysis extends this sensitivity down to 375 GeV (Figure 2). This is the lowest inclusive dijet mass ever studied at the LHC, without relying on additional constraints on the dijet system (such as initial-state radiation, boosts or b-tagging). The data were found to be compatible with the smoothly falling SM background; the most significant excess is observed for a Z’ signal with a mass of m_Z’ = 650 GeV with a global significance of 2.2σ. Following extensive efforts to improve the precision of the background estimate and jet calibration, physicists set world-leading limits on the mass and interaction (“coupling”) strength of new particles in several simplified BSM models.

Looking ahead, trigger-level analyses of Run 3 data promise even greater potential. An upgraded TLA readout stream has allowed ATLAS to record more complex collision event signatures in an even larger dataset, further expanding the experiment’s sensitivity to new physics at the high-rate frontier.

About the banner image: ATLAS event display showing a 13 TeV proton-proton collision event with an especially low reconstructed dijet mass of 356 GeV. This event was fully reconstructed in the nominal ATLAS readout stream, allowing the display of the full event, including individual energy deposits in the calorimeters. This information is not available in the TLA stream. Because of bandwidth limitations, only a very small fraction of events with such low dijet masses are recorded in the nominal readout stream, but the TLA stream captures them in large numbers. (Image: ATLAS Collaboration/CERN)

Learn more

Search for electroweak-scale dijet resonances using trigger-level analysis with the ATLAS detector in 132 fb-1 of proton-proton collisions at 13 TeV (Phys. Rev. D 112 (2025) 092015, arXiv:2509.01219, see figures)
Search for low-mass dijet resonances using trigger-level jets with the ATLAS detector in proton-proton collisions at 13 TeV (Phys. Rev. Lett. 121 (2018) 081801, arXiv:1804.03496, see figures)
New data-collection method aids in the hunt for new physics, Physics Briefing, March 2018

Transforming sensitivity to new physics with single-top-quark events

Katarina Anthony — Thu, 04 Dec 2025 09:23:42 +0000

Transforming sensitivity to new physics with single-top-quark events

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Physics Briefing

Fourier techniques increase sensitivity to effective field theories

The top quark, the heaviest known elementary particle, has captivated physicists since its discovery in the 1990s and holds a privileged place within the Standard Model of particle physics. It has a very short lifetime and is thus the only quark that decays before forming bound states, allowing physicists to study its properties directly through its decay products. The top quark’s strong interaction to the Higgs boson also makes it a powerful probe of the Higgs mechanism, which gives particles their mass. Because of its large mass, it could also be especially sensitive to interactions with as-yet-unknown particles or forces at higher energy scales. If such effects exist, the Large Hadron Collider (LHC), often called a “top-quark factory”, is the ideal place to uncover them. Searching for subtle deviations in how top quarks are produced and decay therefore offers unique clues about physics beyond the Standard Model.

While most top quarks at the LHC are produced in pairs, proton–proton collisions can also yield single top quarks through electroweak interactions. During LHC Run 2 (2015–2018), the ATLAS experiment recorded more than 42 million single-top-quark events at a centre-of-mass energy of 13 TeV. In a new analysis by the ATLAS Collaboration, researchers used this vast dataset to test whether the top quark is hiding signs of new physics. Their findings set comprehensive limits on a broad class of new effects in top-quark physics, encompassing potential new forces of nature and the particles that transmit them. Those effects are described by a theory known as Effective Field Theory (EFT).

A window into new physics

EFTs provide a model-independent framework to describe the possible influence of new particles that are too heavy to be produced directly by an accelerator. New particles and interactions would modify the way elementary particles interact with each other. Studies of single-top-quark production in the t-channel are especially powerful for investigating these effects. It is the only process that produces a strongly polarised top quark – that is, where the top quark’s spin is aligned in a given direction – which allows researchers to precisely study the angular structure of its decay. Any deviation from Standard-Model expectations in these angular correlations could signal new interactions, some of which might even violate the combined symmetry of charge and parity (CP violation).

Figure 1: Feynman diagrams for t-channel single-top-quark process, where initial quark (q) originates from one of the colliding protons and the initial b-quark (b) typically arises from a gluon splitting into a bb̄ pair. The top quark decays into a W boson and a bottom quark, with the W boson subsequently decaying into a lepton and its associated neutrino (t → Wb → ℓνb). Five EFT operators are illustrated at the production and decay interactions and from them seven EFT parameters (CtW, CitW, CbW, CibW, Cφtb and CφQ on the left and CQq on the right) are measured in this analysis. (Image: ATLAS Collaboration/CERN)

Mapping angular information with Fourier precision

To fully exploit this sensitivity, ATLAS physicists developed a novel technique to analyse the pattern of decay products radiated during the decay of top quarks. The underlying formalism, published in 2017, provides a complete mathematical description of the most common decay of a single top quark: where the top quark decays into a W boson and a bottom quark, with the W boson subsequently decaying into a lepton and its associated neutrino (t → Wb → ℓνb, see Figure 1).

Figure 2: The radiation of decay products from a top quark follows patterns like those shown here. Each pattern is represented by a spherical function modulated with a frequency determined by the indices j and m. The colours correspond to the strength of the radiation. An analysis of these patterns is used to measure EFT parameters in this analysis. (Image: ATLAS Collaboration/CERN).

Each single-top-quark event is characterised by four angles that describe the decay geometry and capture the spin correlations and possible CP-violating effects. The novel technique developed by ATLAS analyses the four-dimensional data in the domain of frequencies, a method made famous by the French mathematician Joseph Fourier. Some examples of possible radiation patterns in top-quark decay are shown in Figure 2. Fourier techniques are widely used in fields such as cosmology, medicine, image processing, and music, helping identify patterns hidden in complex data by, for example, separating signals from noise, analysing brain scans, efficiently encoding visual and audio information, or breaking down musical sounds into their fundamental tones. Fourier techniques also play a key role in detector signal processing, helping to clean, reconstruct and interpret the fast electronic pulses produced in the ATLAS subdetectors. Similarly, at the accelerator level, Fourier analysis is essential for monitoring beam stability, identifying oscillations in the proton bunches, and optimising LHC performance.

Beyond its elegance and efficiency, ATLAS’ Fourier-based method provides a complete description of the decay kinematics, capturing both the total production rate and subtle angular distributions.

The new result represents the most complete study of single-top-quark production and decay carried out by the ATLAS Collaboration. Physicists achieved groundbreaking levels of sensitivity to potential new interactions in the top-quark sector, establishing a new benchmark. Through comparisons with Standard-Model predictions, researchers also placed stringent constraints on multiple parameters governing top-quark interactions.

The new result represents the most complete study of single-top-quark production and decay carried out by the ATLAS Collaboration.

Tightening the bounds on new interactions

ATLAS researchers studied seven EFT parameters that describe the fundamental interaction of top quarks, which can be observed in their production and decay. Those interactions appear as dark circles in Figure 1. Some of these parameters can take both a magnitude and a phase. If the phase were non-zero, it would point to a difference between matter and antimatter known as CP violation.

The results agree with the Standard Model predictions (see Figure 3), and they set the most stringent constraints so far on several of these parameters. The analysis improves upon previous ATLAS and CMS results by combining information from both angular distributions and total production rates. It is also the first study to simultaneously extract several of these parameters sensitive to CP violation using top-quark events.

Figure 3: Summary of confidence intervals extracted from the likelihood fits for seven EFT parameters (CtW, CitW, CQq, CφQ on the left and Cφtb, CbW, CibW on the right). The EFT parameter 68% confidence interval (CI) (thick line) and 95% CI (dotted line) intervals are shown for different assumptions. (Image: ATLAS Collaboration/CERN)

A framework for the future

This work demonstrates a powerful new way to explore the top quark’s interactions and to test the Standard Model with high precision. The Fourier-based framework can be readily extended to other processes where angular information carries key insights, including top-quark-pair production, top-quark production associated with a W boson, or even Higgs-boson production.

As the LHC enters its high-luminosity phase, the much larger datasets expected from Run 3 and beyond will allow ATLAS to refine this method further, perhaps revealing the first hints of new physics through the delicate angular fingerprints left by nature’s heaviest known particle.

About the event display: Candidate event for a t-channel single top-quark production in the muon plus jets channel. The muon is represented by a red line, and two jets are represented by yellow cones with the central jet originating from a b-quark. The missing transverse momentum is represented by a dotted white line. The green and yellow bars indicate energy deposits in the calorimeters. (Image: ATLAS Collaboration/CERN)

Learn more

Constraints on effective field theories via quadruple-differential angular decay rates from t-channel single-top-quark production at 13 TeV with the ATLAS detector (arXiv:2510.23372, see figures)

Searching for flavourful violations of the Standard Model

Katarina Anthony — Mon, 27 Oct 2025 13:05:27 +0000

Searching for flavourful violations of the Standard Model

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Katarina Anthony Mon, 27/10/2025 - 14:05

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Feature

Lidia Dell'Asta

Jacob Julian Kempster

Daniele Zanzi

lepton flavour violation

lepton flavour universality

Every aspect of our world – from atomic nuclei and molecular chemistry to biology and astrophysics – emerges from the interactions of particles so small they are considered point-like. While some particles, like the electron, were discovered over a century ago, most remained hidden until the advent of modern particle accelerators. Particle physicists are dedicated to investigating the laws of nature that dictate how fundamental particles interact and combine to shape our everyday lives. By colliding particles at unprecedented energies inside massive detectors, physicists are able to create powerful magnifying lenses.

Accelerators led to the discovery of W and Z bosons, carriers of the weak fundamental force, in 1983 at CERN, as well as to the discovery of the top quark, the heaviest known particle, a decade later at Fermilab (USA). These discoveries supported the experimental foundation of the Standard Model (SM) of particle physics, the theory that describes how all known fundamental particles interact. The greatest triumph of the SM came in 2012 when its last missing piece – the Higgs boson – was discovered by the ATLAS and CMS Collaborations at CERN.

The SM is one of the most precisely tested scientific theories, with thousands of experimental results confirming its predictions to remarkable precision. Despite this success, the SM is not the “ultimate theory” of nature. It cannot explain experimental observations such as dark matter, an enigmatic form of matter detectable only through gravitational effects; the origin of neutrino masses, extremely light particles involved in processes like nuclear radioactive decay; nor the striking imbalance between matter and antimatter in the Universe.

These unanswered questions have driven researchers to develop theoretical frameworks that go beyond the Standard Model, with many predicting the existence of additional particles and interactions. Despite intensive searches, these hypothetical particles have yet to be observed. This could mean they are too heavy (requiring collision energies beyond the reach of the LHC), that their associated new interactions are too rare to be detected in the current amount of recorded data, or that the theory is simply not borne out by nature.

The focus of experimental particle physicists is, therefore, to study higher energies and collect more data, searching for any discrepancies between experimental measurements and SM predictions that could lead towards an ultimate theory of nature.

Nothing is truly forbidden in the quantum world. Even "forbidden" transitions are possible so long as they follow several intermediate, rule-following steps.

The Standard Model, its flavours and the search for new physics

One powerful way to search for new physics phenomena is to look for processes that are expressly forbidden by the SM, occurring only via new interactions. For example, physicists often search for processes that change the properties of a fundamental particle in an unexpected way. In the SM, quarks and leptons (known as fermions) come in different “flavours”. For example, electrons and muons are both charged leptons, but they differ in mass and are therefore considered different flavours. The SM sets strict limits on how fermions can change flavour and which flavours they are allowed to transition to. While the top quark may transition into a bottom, down or strange quark through a weak interaction using the electromagnetically-charged W boson (to conserve charge overall), it is forbidden from transitioning into an up or charm quark, even through an interaction with a neutral boson such as a Z boson, gluon, photon or Higgs boson. Equivalently, while the tau lepton may produce a muon or electron through a W boson interaction (including with the associated neutrinos), it cannot produce a muon or electron through a neutral boson interaction. These rules apply to all quarks and leptons alike.

Despite these rules, nothing is truly forbidden in the quantum world. Many of these transitions are physically possible so long as they follow several intermediate steps that adhere to the rules. But each additional step has a cost, reducing the overall probability of the process occurring. Thus, while these “forbidden” flavour-changing processes could occur according to the SM, they would be so rare as to never be detected by the ATLAS experiment, occurring in fewer than one in every 10¹⁴ interactions.

But what if ATLAS did detect them? Any observation of these processes would be indisputable evidence of new physics phenomena, indicating that something unknown has drastically increased the probability of these interactions. Particle physicists have even given these forbidden processes a name: those involving quarks are flavour-changing neutral currents (FCNCs), and those involving leptons are lepton flavour violation (LFV) (see Figure 1).

Figure 1: The production of a top-quark pair with a subsequent FCNC decay (left) and the production of a Z boson with a subsequent LFV decay (right, where the lepton ‘l’ is an electron or a muon, but not another tau). The forbidden interactions are highlighted with red circles. The pairs of gluons (g) and quarks (q) in the initial state come from the colliding protons. (Image: ATLAS Collaboration/CERN)

The search for FCNC interactions in top-quark processes

The LHC is the most powerful particle accelerator ever built, colliding bunches of protons every 25 nanoseconds at energies up to 13.6 TeV. Thanks to this unprecedented energy and collision rate, the LHC is often considered a particle “factory”, producing millions of top quarks and billions of Z bosons for study.

That’s great news for physicists looking beyond the Standard Model, where the heavier the particle involved in the interaction, the better. Since new particles and interactions likely require large amounts of energy to manifest, they may favour a connection with the heaviest and thus most inherently energetic known particle – the top quark. Searching for FCNC processes involving the top quark is thus considered a powerful technique for uncovering new physics phenomena.

In 2023, ATLAS physicists searched for FCNC interactions between a top quark and a Z boson, considering both modified production and decay mechanisms for the top quark (Figure 2). Both would generate a Z boson, a W boson and a bottom quark, with the modified decay also producing an up or charm quark. Physicists focused their search on leptonic decays of the W and Z bosons, with the W decaying to a charged lepton and a neutrino and the Z to a pair of oppositely-charged leptons of the same flavour. In practical terms, this meant looking for collision events with three charged leptons, one neutrino, one bottom quark and, in some cases, one additional up or charm quark.

Figure 2: The LHC production of a single top quark via a weak interaction utilising an FCNC vertex (left), and the production of a pair of top quarks via the strong interaction, including a subsequent FCNC decay vertex (right). The FCNC vertices are highlighted with the shaded circles. (Image: ATLAS Collaboration/CERN)

While the ATLAS detector is very good at measuring leptons, neutrinos pass through it completely undetected. Their presence can only be inferred from the missing transverse momentum in the collision. Quarks, meanwhile, produce showers of particles in the detector’s calorimeter called jets. Top quarks almost always decay into a bottom quark, which is extremely useful for physicists. Bottom quarks combine briefly with other quarks when they are formed, and so travel farther from the collision point before they decay. The resulting b-jets they produce have unique characteristics. To identify these b-jets (a process called b-tagging) physicists use machine-learning algorithms trained to spot these characteristics. Each jet is scored based on its likelihood of originating from a b-quark. High b-tagging efficiency is crucial for finding top-quark events.

However, all algorithms carry an inherent risk of misidentifying jets initiated by other quarks as b-jets. In their 2023 measurement, physicists calibrated their b-taggers to reach an efficiency of 77% for true b-jets, with a rejection factor of 5 for charm-quark-initiated jets (c-jets) and of 170 for lighter-quark-initiated jets. This means that one out of every five c-jets will be misidentified as coming from a bottom quark, and only one out of every 170 light-quark jets.

If ATLAS were to observe these flavour-changing processes, it would be indisputable evidence of new physics phenomena.

Figure 3: Distributions of recorded events (black points) and of simulated events (filled histograms) as a function of the Boosted Decision Tree output value. The distributions of the expected signal events, considering an FCNC coupling in single top quark production (dashed lines) or an FCNC process in the top quark decay (solid lines) are shown. The signals with an FCNC coupling between a top quark, a Z boson and an up quark (charm quark) are displayed in pink (pale blue). The lower panels show the ratio in each bin of event yields in data to the event yields from simulated background processes. A value of unity indicates that the events in data are in agreement with the predicted backgrounds. (Image: ATLAS Collaboration/CERN)

Physicists then defined an event selection tool to search for FCNC signatures, resulting in two signal regions (SR1 and SR2) that would be enriched in these processes. Figure 3 shows the results for the FCNC pair-produced top quarks and FCNC single-produced top quarks respectively, separated only by the number of jets required. The SM background processes resulting in similar final states are the production of a top-quark pair in association with a Z boson (see the light blue histogram in Figure 3) and the production of a pair of W/Z bosons in association with jets (see the orange histograms in Figure 3). Within each signal region, Boosted Decision Trees (BDT, a machine-learning technique) were employed to further separate the FCNC signatures from these SM backgrounds. The output of these BDTs determine the final shape of the distributions, with the aim being to push the FCNC signatures to one side of the distribution and the SM signatures to the other. This procedure was performed for FCNC processes between a top quark and an up quark (pink lines in Figure 3) and, separately, for FCNC processes between a top quark and a charm quark (blue lines in Figure 3).

Two interesting features can be observed in the distributions in Figure 3. First, the signal of the FCNC process in top-quark-pair decays is very well separated from backgrounds in the region targeting its signal (SR1), but not in the region targeting the FCNC production of a single top quark (SR2), and vice versa. Second, the sensitivity to the FCNC production of single top quarks is very dependent on whether an up quark or a charm quark initiates the process. As the colliding protons are themselves composed of up quarks and down quarks, the ingredients for an up-quark-initiated process are readily available. This results in good experimental sensitivity. By contrast, the charm-quark-initiated process arises from the constantly fluctuating mixture of “sea” quarks and gluons surrounding each proton, which carry a smaller proportion of the proton’s mass and energy. This reduces the probability for the charm-initiated process occurring, and subsequently reduces the experimental sensitivity.

After applying the event selection, physicists compared the data to the SM background prediction, searching for an excess of events that could be an FCNC signal. A statistical analysis was performed to determine whether the number of data events in each bin was larger and statistically incompatible with the predicted number of SM events. No excess was found and limits on the probability of an FCNC top-quark decay into a Z boson and a light quark were obtained. The most stringent limits were set for the top-quark decay into a Z boson and an up quark, which is excluded at probabilities greater than approximately 6 in 100,000 top-quark decays.

Figure 4: Summary of the current 95% confidence level observed limits on the branching ratios of the top quark decays via flavour-changing neutral currents (FCNC) to a quark and a neutral boson t → Xq (X = g, Z, ɣ or H; q = u or c) by the ATLAS and CMS Collaborations compared to several new physics models. Each limit assumes that all other FCNC processes vanish. The limits are expressed as FCNC top decay branching ratios, but several are obtained considering both FCNC top quark decay and FCNC top quark production vertices. (Image: ATLAS Collaboration/CERN)

The ATLAS Collaboration has performed many further FCNC searches for top quarks in association with gluons, photons and Higgs bosons, and for FCNC interactions of other particles such as the bottom quark. Figure 4 summarises the limits on the top-quark FCNC decay probabilities (branching ratios) obtained by the ATLAS (in blue) and the CMS (in red) Collaborations for various forbidden processes.

The search for lepton flavour violation in Z-boson decays

Searches for new physics phenomena have already led to the discovery of LFV in neutrinos, through the observation of neutrino oscillations. Neutrino oscillations could also induce LFV in electromagnetically-charged leptons. However, as the probability of this occurring is less than 1 in 10⁵⁴, any observation would indicate an exciting additional discovery.

A variety of experiments are currently searching for LFV processes with different lepton flavours and neutral bosons. This diversity of approaches is important, as new physics phenomena may interact with different leptons and bosons at different rates, and no single experiment can efficiently detect all LFV processes.

At high-energy colliders, the most promising opportunities to study LFV processes involve the production of tau leptons alongside massive neutral bosons, such as the Z boson. The LHC, in its role as a Z-boson “factory,” provides fantastic opportunities to search for flavour-violating decays involving the Z boson, such as Z→e𝜏 or Z→μ𝜏, where a tau lepton is produced alongside a charged lepton of a different flavour. While searches for LFV involving lighter leptons and massless photons are also viable at the LHC, they can be searched for with greater accuracy in low-energy experiments, i.e. in searches for muon decays into an electron and a photon (μ → eɣ).

The ATLAS Collaboration carried out a detailed search for flavour-violating Z→e𝜏 and Z→μ𝜏 processes. It was extremely challenging for several reasons, starting with the complicated task of spotting tau leptons. While electrons and muons can be directly detected by ATLAS, tau leptons have a lifetime of just 10^–13 seconds and decay before they reach the experiment. The most common tau decay modes include leptonic decays to electrons or muons and their associated neutrinos, or to jets of quarks as described above. Thus, the signature of a flavour-violating Z→e𝜏 or Z→μ𝜏 process may well mimic other perfectly viable SM signatures, such as Z→ee.

Figure 5: Diagrams of a LFV Z-boson decay (left) and of the two main SM processes which can produce final states experimentally similar to the LFV decays: a Z-boson decay into a pair of tau leptons (middle) and a W-boson decay into a light lepton and neutrino in association with an additional jet (right). The green arrows represent electrons or muons (l), the blue triangles are the jets produced by either the hadronic tau decay (𝜏 jet) or by a quark or a gluon (q/g jet), and the dashed blue lines indicate undetected neutrinos. (Image: ATLAS Collaboration/CERN)

Figure 5 (left) illustrates a flavour-violating Z-boson decay on a plane perpendicular to the proton beam direction. The unique feature of the LFV process is the alignment of the undetected neutrino (dashed line) and the tau lepton. This configuration, without any additional detected particles, cannot be produced by SM processes. Nevertheless, due to the random nature of quantum processes in the Universe and imperfections in experimental measurements, many SM processes can still look similar enough to cause a problem!

The main SM lookalike is a Z-boson decay into a pair of tau leptons, where one tau lepton subsequently decays into a light lepton and two neutrinos and the other into a tau-jet and a neutrino. While the number of detected particles in this SM process matches that of the LFV decay, there’s a greater number of undetected neutrinos (see Figure 5, middle). The second most important SM lookalike involves a W boson produced in association with a jet, where the W-boson decays into a light lepton and a neutrino, and the jet originates from a quark or gluon misidentified as a hadronically decaying tau. In this case, the detected final state is also the same as in the LFV decay, but the neutrino is typically emitted in a direction close to the light lepton instead of the jet (see Figure 5, right).

These kinematic differences are the key tool for isolating the LFV signature. Physicists designed a deep neural network (NN) to “learn” the kinematic properties of the signal events and how they differ on average from background events. NNs are extremely effective in this task as they can learn complex correlations among multiple features of an event. Figure 6 shows a histogram of events binned in a NN output (after applying the initial event selections). As with the BDTs in the FCNC searches, the events with high NN scores are more signal-like and the events with low values are more background-like. The red dashed line shows the expected distribution of signal events under an assumption that LFV decays occur as frequently as five in every 10,000 Z-boson decays.

As with all previous measurements, no excess above the SM prediction was observed in the data. The ATLAS Collaboration set limits on LFV Z-boson decays into a tau and a light lepton that are stronger than those set by experiments on the Large Electron Positron (LEP) collider. At the 95% confidence level, the ATLAS result established that if these LFV decays do occur in nature, their probability must be less than about five in every one million Z-boson decays. Using similar analysis methods, physicists also set even more stringent limits on LFV Z-boson decays into an electron and a muon.

The ATLAS Collaboration’s state-of-the-art searches for FCNC and LFV interactions have produced powerful probes of new physics phenomena.

Figure 6: Distribution of recorded events (black points) and of simulated events (filled histograms) as a function of the NN output value. The yellow and blue histograms are the events from the W and Z boson decays shown in Figure 5, respectively. The distribution of the expected signal events is shown with the red dashed line. The lower panel shows the ratio in each bin of event yields in data to the event yields from simulated background processes. A value of unity indicates that the events in data are in agreement with the predicted backgrounds. (Image: ATLAS Collaboration/CERN)

Outlook

The ATLAS Collaboration’s state-of-the-art searches for FCNC and LFV interactions have produced powerful probes of new physics phenomena. Dedicated FCNC studies set limits on the FCNC top-quark decay branching ratio as tight as 10^–5 (Figure 4), while further LFV searches focusing on Higgs-boson decays were also performed. The upper limits on the LFV decay probabilities of the Higgs boson are two orders of magnitude weaker than those for Z bosons, due to the much smaller number of Higgs bosons produced in LHC collisions and the more challenging task of distinguishing events with a Higgs boson from background processes. This sensitivity is expected to improve significantly with the collection of more data. ATLAS physicists also performed a search for a simultaneous FCNC and LFV process using Effective Field Theory in top-quark interactions with light quarks, muons and taus. Although this analysis is also statistically limited, it represents an exciting new direction for future studies.

Since 2022, the LHC experiments have been collecting data from proton-proton collisions at a centre-of-mass energy of 13.6 TeV. This data-taking period is expected to provide approximately 1.5 times more Z bosons, Higgs bosons and top-quark pairs than in all previous runs, thanks to both the increased collision energy and the larger dataset ATLAS is recording.

But larger datasets aren’t the only factor boosting searches for LFV and FCNC processes! Ongoing advancements in experimental methodologies and the ATLAS Collaboration’s deepening understanding of its ever-evolving detector will further improve sensitivity, driving searches for even rarer interactions. One continuous area of development is ATLAS’ in-house algorithms for identifying charm-quark-initiated jets, analogous to the existing and well-honed b-tagging capabilities. As mentioned above, limits on couplings involving the top and charm quarks are less stringent than those involving the up quark, and these algorithms will boost ATLAS’ charm-quark sensitivity.

Looking ahead, further searches will be conducted following the completion of the High-Luminosity LHC (HL-LHC), for which the ATLAS experiment will undergo major upgrades to cope with the much higher luminosity. At the current LHC energy of 13.6 TeV, every time two bunches of protons cross each other an average of 50 overlapping collisions occur. This already makes event reconstruction very challenging. At the HL-LHC, this number will increase to approximately 200 simultaneous collisions. To address this complexity, a new silicon tracking detector, the Inner Tracker (ITk), will be installed in the ATLAS experiment.

The ITk features new technologies that provide higher detection granularity, improved radiation hardness, faster readout speeds and a reduced material budget compared to the current inner detector – all of which will enable similar event reconstruction efficiencies to current data-taking, despite the much harsher future conditions. Moreover, the ITk’s extended coverage will enable ATLAS to reconstruct events in regions of the detector that are not currently equipped with a tracker, further extending and improving the data collection. The integrated luminosity that will be collected by the ATLAS experiment during the HL-LHC is expected to be 10 times greater than the total recorded at the LHC. This means that searches for very rare or forbidden processes, such as LFV and FCNC, will remain powerful, exciting and ever improving probes for physics beyond the Standard Model.

About the Authors

Lidia Dell'Asta joined the ATLAS Collaboration in 2007 while at the University of Milano. She then worked as a postdoctoral fellow at Boston University and as a research fellow at the University of Roma2. She is currently an associate professor at the University of Milano. In addition to her work on the detector trigger system, where she coordinated the muon trigger group, she has been active in data analysis. She has worked on Standard Model measurements as well as the measurement of the coupling of the Higgs boson to tau leptons. Over the past ten years, her research has focused on rare production processes of single top quarks, and she has served as a convener of the Single Top group. She has contributed to the measurement of single top quark production in association with a Z boson and to the search for top–Z FCNC couplings. Jacob Julian Kempster joined the ATLAS Collaboration in 2011 while at Royal Holloway, University of London. He subsequently worked as a Research Fellow at the University of Birmingham and is currently a Senior Research Fellow at the University of Sussex. His research focuses on using the top quark as a tool to search for new physics beyond the Standard Model. He performed the first ATLAS search for lepton flavor violating (LFV) couplings of the top quark to muons and tau leptons, and previously served as a subgroup convener for the Top+X working group. His other primary research area is Effective Field Theory (EFT), where he leads efforts on global EFT fits and recently completed his term as Chair of the LHC EFT Working Group. Daniele Zanzi was an active member of the ATLAS Collaboration until 2024. As researcher at the University of Melbourne, at CERN, and at the University of Freiburg, he has focussed on searching for LFV interactions. He has also contributed to the development and operation of the ATLAS trigger system, to measurements of the Higgs boson properties and to searches for dark matter and supersymmetric particles.

About the banner image: Visualisation of a H →μτ candidate event, with the μτ_e (top) and μτ_had (bottom) channel. An electron track is shown in green, a red line indicates a muon. A τ_had-vis candidate is displayed in purple, the ET_miss is shown by a white dashed line. (Image: ATLAS Collaboration)

Scientific articles

Search for flavor-changing neutral-current couplings between the top quark and the 𝑍 boson with proton-proton collisions at 13 TeV with the ATLAS detector (Phys. Rev. D 108 (2023) 032019, arXiv:2301.11605, see figures)
Search for flavour-changing neutral-current interactions of a top quark and a gluon in proton-proton collisions at 13 TeV with the ATLAS detector (Eur. Phys. J. C 82 (2022) 334, arXiv:2112.01302, see figures)
Search for flavour-changing neutral-current couplings between the top quark and the photon with the ATLAS detector at 13 TeV (Phys. Lett. B 842 (2023) 137379, arXiv:2205.02537, see figures)
Search for flavour-changing neutral-current couplings between the top quark and the Higgs boson in multi-lepton final states in 13 TeV proton-proton collisions with the ATLAS detector (Eur. Phys. J. C 84 (2024) 757, arXiv:2404.02123, see figures)
Top Quarks + X Summary Plots April 2024 (ATL-PHYS-PUB-2024-005)
Search for charged-lepton-flavour violation in Z-boson decays with the ATLAS detector (Nature Phys. 17 (2021) 819, arXiv:2010.02566, see figures)
Search for the charged-lepton-flavor-violating decay 𝑍 →𝑒⁢𝜇 in proton-proton collisions at 13 TeV with the ATLAS detector (Phys. Rev. D 108 (2023) 032015, arXiv:2204.10783, see figures)
Searches for lepton-flavour-violating decays of the Higgs boson into eτ and μτ in 13 TeV proton-proton collisions with the ATLAS detector (JHEP 07 (2023) 166, arXiv:2302.05225, see figures)
Search for charged-lepton-flavor violating 𝜇⁢𝜏⁢𝑞⁢𝑡 interactions in top-quark production and decay in proton-proton collisions at 13 TeV with the ATLAS detector at the LHC (Phys. Rev. D 110 (2024) 012014, arXiv:2403.06742, see figures)
The ATLAS ITk detector system for the Phase-II LHC upgrade (Nucl. Instrum. Methods Phys. Res., Sect. A 1045 (2023) 167597)
Search for flavour-changing neutral tqH interactions with H→γγ in proton-proton collisions at 13 TeV using the ATLAS detector (JHEP 12 (2023) 195, arXiv:2309.12817, see figures)
Search for flavour-changing neutral current interactions of the top quark and the Higgs boson in events with a pair of τ-leptons in proton-proton collisions at 13 TeV with the ATLAS detector (JHEP 06 (2023) 155, arXiv:2208.11415, see figures)

Shining light on the Weak force: ATLAS observes WWγ production

Katarina Anthony — Wed, 08 Oct 2025 11:11:34 +0000

Shining light on the Weak force: ATLAS observes WWγ production

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The ATLAS Collaboration reports its first observation of WWγ production – a rare process involving the simultaneous production of two W bosons and a photon.

The weak force is one of the four fundamental forces of nature. Carried by the W and Z bosons, the influence of the weak force extends from the subatomic to the astronomical, driving the formation of heavy elements and shaping the nuclear processes that power stars. The W and Z bosons were first observed by the UA1 and UA2 experiments at CERN in the early 1980s, and continue to serve as vital probes of the Standard Model.

Among the most sensitive tests of the Standard Model are multi-boson production processes, where two or more force carriers are produced together. These interactions are precisely predicted by theory, and any deviation in their strengths or rates could signal the presence of new physics phenomena.

In a new analysis of the full LHC Run-2 dataset (collected 2015–2018), ATLAS researchers identified events consistent with WWγ production, with a statistical significance of 5.9 standard deviations. This confirms a previous result from the CMS Collaboration and marks the first such observation by ATLAS.

Among the most sensitive tests of the Standard Model are multi-boson production processes, where two or more force carriers are produced together.

Despite the large dataset available, identifying this process posed significant challenges. WWγ production closely resembles more common background processes, such as top-quark-pair production with a photon or Z-boson production associated with photons. Background processes with misidentified photons further complicate the analysis.

Figure 1: Comparison between data and the post-fit predictions from the distribution of the boosted decision tree output in the signal region. The uncertainty band includes both the statistical and systematic uncertainties as obtained from the fit. (Image: ATLAS Collaboration/CERN)

To better isolate the WWγ signal, the ATLAS team focused their search on the most characteristic signature of WWγ decay, containing an oppositely-charged electron and muon, missing transverse momentum from undetected neutrinos and a high-energy photon. They also excluded events containing jets of particles originating from b-quarks (b-jets) to suppress background top-quark processes. Leveraging the developments in lepton, photon, and b-jet identification achieved with the Run-2 data, they were able to reduce the rate of particle misidentification and improve signal purity.

A Boosted Decision Tree (BDT) was trained to identify subtle differences between signal and background events, using kinematic features. A statistical fit was then performed across the signal region and background control regions. Figure 1 shows the BDT output distribution in the signal region. The data exhibit an excess over the background prediction, well described by the WWγ signal.

The measured cross section is 6.2 ± 0.8 (stat.) ± 0.6 (syst.) fb, in agreement with the Standard Model prediction of 6.1 ± 1.0 fb. This result represents the most precise measurement of the WWγ cross section to date, with a relative uncertainty of about 16%.

Using the framework of Effective Field Theory (EFT), physicists also set new constraints on possible new particles or interactions beyond the LHC’s direct reach. These results will provide inputs to the global EFT combinations, helping to tighten the overall limits on anomalous interactions.

This new observation by the ATLAS Collaboration marks another triboson process observed at ATLAS, joining WWW, γγγ, Wγγ, Zγγ, WZγ and VVZ production. With LHC Run 3 underway and more data on the horizon, these measurements will become increasingly precise, offering crucial tests of the Standard Model’s predictions for gauge-boson interactions.

About the event display: Candidate event display of two oppositely-charged W bosons produced in association with a photon. The photon is represented by the yellow bar. One W boson decays into an electron (shown in green) and a neutrino, while the other decays into a muon (shown as a red track) and a neutrino. The presence of the neutrinos is inferred from the missing transverse momentum, indicated by the dashed line. (Image: ATLAS Collaboration/CERN)

Learn more

Observation of W+W-γ production in proton-proton collisions at 13 TeV with the ATLAS detector and constraints on anomalous quartic gauge-boson couplings (arXiv:2509.14070, see figures)
CMS Collaboration: Observation of 𝑊⁢𝑊⁢𝛾 Production and Search for 𝐻⁡𝛾 Production in Proton-Proton Collisions at 13 TeV
Lepton Photon 2025 presentation by Diego Baron: Measurement of rare electroweak processes including vector boson scattering and triboson in ATLAS

Shape-shifting collisions probe secrets of early Universe

Katarina Anthony — Thu, 18 Sep 2025 13:47:38 +0000

Shape-shifting collisions probe secrets of early Universe

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Katarina Anthony Thu, 18/09/2025 - 15:47

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The first high-energy collisions between light nuclei at the Large Hadron Collider confirm the unusual “bowling-pin” shape of neon nuclei and offer up a new tool to study the extreme state of matter produced in the aftermath of the Big Bang.

Artistic rendering of quark–gluon plasma (Image: CERN)

This summer, the Large Hadron Collider (LHC) took a breath of fresh air. Normally filled with beams of protons, the 27-km ring was reconfigured to enable its first oxygen–oxygen and neon–neon collisions. First results from the new data, recorded over a period of six days by the ALICE, ATLAS, CMS and LHCb experiments, were presented during the Initial Stages conference held in Taipei, Taiwan, on 7–12 September.

Smashing atomic nuclei into one another allows physicists to study the quark–gluon plasma (QGP), an extreme state of matter that mimics the conditions of the Universe during its first microseconds, before atoms formed. Until now, exploration of this hot and dense state of free particles at the LHC relied on collisions between heavy ions (like lead or xenon), which maximise the size of the plasma droplet created.

Collisions between lighter ions, such as oxygen, open a new window on the QGP to better understand its characteristics and evolution. Not only are they smaller than lead or xenon, allowing a better investigation of the minimum size of nuclei needed to create the QGP, but they are less regular in shape. A neon nucleus, for example, is predicted to be elongated like a bowling pin – a picture that has now been brought into sharper focus thanks to the new LHC results.

The experiments focused on measurements of subtle patterns in the angles and directions of the particles flying outward as the QGP droplet expands and cools, which are caused by small distortions in the original collision zone. Remarkably, these “flow” patterns can be described using the same fluid-dynamics calculations that are used to model everyday fluids, allowing researchers to probe both the properties of the QGP and the geometry of the colliding nuclei. Accurate model predictions enable a more precise exploration of flow in oxygen–oxygen and neon–neon collisions than in proton–proton and proton–lead collisions.

ALICE, which specialises in the study of the QGP, as well as the general-purpose experiments ATLAS and CMS, have measured sizeable elliptic and triangular flow in oxygen–oxygen and neon–neon collisions, and found that these depend strongly on whether the collisions are glancing or head-on. The level of agreement between theory and data is comparable to that obtained for collisions of heavier xenon and lead ions, despite the much smaller system size. This provides strong evidence that flow in oxygen–oxygen and neon–neon collisions is driven by nuclear geometry, supporting the bowling-pin structure of the neon nucleus and demonstrating that hydrodynamic flow emerges robustly across collision systems at the LHC.

Complementary results presented last week by the LHCb collaboration confirm the bowling-pin shape of the neon nucleus. The results are based on lead–argon and lead–neon collisions in a fixed-target configuration, using data recorded in 2024 with its SMOG apparatus. The LHCb collaboration has also started to analyse the oxygen–oxygen and neon–neon collision data.

“Taken together, these results bring fresh perspectives on nuclear structure and how matter emerged after the Big Bang,” says CERN Director for Research and Computing Joachim Mnich.

This media update was originally published on the CERN Press website (English).

Learn more

Bowling balls vs. bowling pins? ATLAS studies the unique shape of neon ions, ATLAS Physics Briefing, 8 July 2025
Hunting for jet quenching in collisions between oxygen nuclei, ATLAS Physics Briefing, 8 July 2025

ATLAS takes a closer look at the all-charm tetraquark

Katarina Anthony — Thu, 18 Sep 2025 12:47:34 +0000

ATLAS takes a closer look at the all-charm tetraquark

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Katarina Anthony Thu, 18/09/2025 - 14:47

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The strong force, unrivaled in strength yet confined to minuscule distances, produces unique bound states of quarks known as quarkonia. These are the strong-force equivalent of the hydrogen atom, consisting of a quark and its antiquark in place of a proton and an electron, with new states corresponding to a new energy level. A particularly well-known example is charmonium, formed from a charm quark and its antiquark. The J/ψ charmonium is a ground state, and the ψ(2S) is a radial excitation of J/ψ.

The discovery of the J/ψ in 1974 sparked the “November Revolution” in particle physics, revealing the existence of more than three types of quarks and laying the groundwork for the Standard Model of particle physics as we know it today. Decades later, LHC experiments continue to study the J/ψ in detail.

But what happens when not two, but four charm quarks bind together? This results in an exotic state known as an all-charm tetraquark. The four charm quarks can be in either a "molecular" or a compact state. One such tetraquark, called X(6900), has been observed by the LHCb, ATLAS and CMS Collaborations. When produced, it appears as two charmonia in ground (J/ψ) or excited (ψ(2S)) states. Physicists have now set out to precisely measure the mass, decay width and decay modes of the all-charm tetraquark — all essential steps toward understanding the nature and formation of these exotic states.

In 2022, the ATLAS Collaboration reported a hint of excesses in the J/ψ+ψ(2S)→4𝜇 spectrum around 6.9 and 7.2 GeV. This led to several open questions. Were these excesses due to data fluctuations? Were there other excesses? Could the di-J/ψ and J/ψ+ψ(2S) channels be jointly analysed to extract the partial decay width of the all-charm tetraquark?

ATLAS has confirmed the presence of the all-charm tetraquark candidate X(6900) with a combined significance of 8.9σ — a key step forward in understanding exotic bound states of quarks.

To answer these questions, the ATLAS Collaboration used the full LHC Run-2 dataset (collected 2015–2018) to analyse all-charm tetraquark decays into two channels: J/ψ+ψ(2S) producing four muons (4𝜇) and, for the first time, producing four muons with two pions (4𝜇+2𝜋). This second channel is especially promising due to its higher decay probability (branching fraction).

One of the challenges in this analysis was in estimating the background originating from the di-J/ψ channel, where four muons are combined with two random pions, thus mimicking the 4𝜇+2𝜋 channel. ATLAS physicists trained a boosted decision tree (BDT) to distinguish signal from background and used a control region with two same-sign pions to estimate the remaining background contribution after a requirement on the BDT. They performed a simultaneous fit on the 4𝜇 and 4𝜇+2𝜋 mass spectra to extract the signal parameters from the data.

Figure 1: Fits to the 𝐽/𝜓+𝜓(2S) mass spectra of the 4𝜇 (left) and 4𝜇+2𝜋 (right) channels. The purple dash-dotted line represents the signal resonance in the respective channel. (Image: ATLAS Collaboration/CERN)

As shown in Figure 1, a resonance appears near 6.9 GeV in both channels. The combined significance of the signal is 8.9𝜎, confirming the presence of an all-charm tetraquark candidate X(6900) decaying into J/ψ+ψ(2S). Physicists also considered models where the X(6900) observed in the di-J/ψ channel is assumed to also decay into J/ψ+ψ(2S). In this case, the X(6900) in both the di-J/ψ and J/ψ+ψ(2S) channels are considered the same resonance, although current data does not exclude the possibility that the resonance in the J/ψ + ψ(2S) channel is a separate state. They performed a combined fit of the di-J/ψ and J/ψ+ψ(2S) channels, with statistical input for the di-J/ψ channel from the previous ATLAS analysis.

ATLAS researchers then measured the ratio of branching fractions for X(6900) → J/ψ+ψ(2S) and X(6900) → di-J/ψ. Surprisingly, the ratio was found to be close to 1. This was unexpected, as the di-J/ψ channel has a larger available phase space, and thus the J/ψ+ψ(2S) decay mode was expected to be relatively suppressed.

They also looked for the presence of a second resonance, X(7200), also previously hinted at in LHCb and CMS data. The ratio of signal yields for X(7200) to X(6900) was measured to be 0.12 ± 0.11, with an upper limit of 0.41 at a 95% confidence level. The existence of a potential resonance near 7.2 GeV is not supported by the current data. The event display above shows a candidate for an all-charm tetraquark, produced in a proton–proton collision within the ATLAS detector.

With LHC Run 3 data taking underway, the accumulation of additional data is expected to clarify the nature of this excess, constraining its underlying model and quantum numbers, and probing the existence of further resonances.

About the banner image: Event display of an all-charm tetraquark candidate, which consists of two charm quarks and two charm antiquarks in a bound state. It has a mass around 6.84 GeV and decays into two charmonia: J/ψ and ψ(2S). The ψ(2S) meson decays into another J/ψ and two oppositely charged pions (visualized in the images as light blue curves). Each of the two J/ψ mesons decays into two oppositely charged muons (red lines), resulting in four muons in the final state. Energy deposited in the electromagnetic and hadronic calorimeters is shown by green and yellow bars, respectively. (Image: ATLAS Collaboration/CERN)

Learn more

Observation of structures in the J/ψ+ψ(2S) mass spectrum with the ATLAS detector (Submitted to PRL, arXiv:2509.13101, see figures)
Observation of an Excess of Dicharmonium Events in the Four-Muon Final State with the ATLAS Detector (Phys. Rev. Lett. 131 (2023) 151902, arXiv:2304.08962, see figures)
LHCb Collaboration: Observation of structure in the J/𝜓-pair mass spectrum (Science Bulletin 65 (2020) 1983, arXiv:2006.16957)
CMS Collaboration: New Structures in the J/𝜓 J/𝜓 Mass Spectrum in Proton–Proton Collisions at 13 TeV (Phys. Rev. Lett. 132 (2024) 111901, arXiv:2306.07164)
CMS Collaboration: Determination of the spin and parity of all-charm tetraquark (arXiv:2506.07944)

Hunting for jet quenching in collisions between oxygen nuclei

Katarina Anthony — Mon, 08 Sep 2025 12:23:11 +0000

Hunting for jet quenching in collisions between oxygen nuclei

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In summer of 2025, the ATLAS Experiment recorded its first-ever oxygen–oxygen collisions, opening a new window onto the study of quark-gluon plasma (QGP). This extreme state of matter mimics the conditions of the early Universe during the first microseconds after the Big Bang. Traditionally, researchers have focused on studying QGP formed in the collisions of heavy ions (like lead or xenon), which maximize the size of the plasma droplet created. But in recent years, interest has grown in exploring the QGP using smaller ions (such as oxygen) to better understand the QGP across different system sizes (see Figure 1).

The ATLAS Collaboration has just released its first search for jet quenching in oxygen–oxygen collisions. This phenomenon occurs when high-energy particle jets lose energy as they traverse the QGP. Physicists use these jets rather like a dipstick, probing the nature and properties of the QGP.

Figure 1: Transverse images of the size and shape of typical collision regions for lead-lead, xenon-xenon, oxygen-oxygen, and proton-lead collisions (from left to right). In each case the filled circles represent the region where the QGP will be formed and the shaded circles show protons and neutrons in the nuclei which miss the oncoming nucleus and do not participate in the collision. (Image: ATLAS Collaboration/CERN)

The ATLAS Collaboration has measured jet quenching in oxygen–oxygen collisions – the smallest system yet to show this phenomenon.

Lead–lead and xenon–xenon collisions form large plasma droplets, allowing jets to travel long distances through the QGP. In both systems, the ATLAS Collaboration has observed clear signs of jet quenching. By contrast, proton–lead collisions produce tiny droplets of QGP, and no evidence of jet quenching was found. Oxygen–oxygen collisions offer a middle ground. Could jet quenching occur in this smaller system? If so, what would it look like?

Figure 2: Schematic drawing of a dijet system in proton-proton collisions where the jets experience no QGP (left) and nucleus-nucleus collisions (right) where the jets each travel through some amount of the QGP. (Image: ATLAS Collaboration/CERN)

To answer these questions, ATLAS physicists turned to one of the most sensitive tools for studying the QGP: dijet momentum balance. In proton-proton collisions, pairs of jets (dijet) are typically produced back-to-back with nearly equal momentum. But in lead-lead collisions, an increase in imbalanced dijets has been observed, arising either from differences in the jet’s path length through the QGP (see Figure 2) or from fluctuations in the energy-loss process itself. Researchers quantify this imbalance using the ratio (x_J), which is the ratio of the lower jet transverse momentum to the higher one. In head-on (central) lead-lead collisions, they found a strong reduction in balanced jets (with x_J~1) and an increase in imbalanced jets (with x_J~0.5-0.6) when compared to proton–proton collisions.

In their new result, ATLAS physicists found the same trend in central oxygen-oxygen collisions – but at a smaller scale. Instead of a nearly 50% reduction in the number of balanced dijets (those with x_J~1, as seen with lead) the reduction is much smaller (see Figure 3). This result marks the first measurement of jet quenching in oxygen–oxygen collisions, and the smallest system scale in which this phenomenon has been observed.

This milestone, achieved just nine weeks after data-taking, reflects a remarkable effort made by members across the ATLAS Collaboration. The result provides a vital new piece in the jet-quenching puzzle, helping researchers pinpoint the system size at which QGP begins to affect high-energy jets. Future studies of the oxygen-oxygen dataset are set to explore this phenomenon in greater detail, offering new insights into this exotic state of matter.

Figure 3: Distributions of xJ, the ratio of the lower jet transverse momentum to the higher one in lead-lead collisions (left) and oxygen-oxygen collisions (right) compared to that of proton-proton collisions. The 0-10% points in each panel are head-on (central) collisions (the 40-60% and 60-80% points in the panels are from peripheral collisions). (Image: ATLAS Collaboration/CERN)

About the event display: Collision event recorded by the ATLAS experiment on 5 July 2025, when stable beams of oxygen, colliding at a centre-of-mass energy per nucleon pair of 5.36 TeV, were delivered to ATLAS by the LHC. (Image: ATLAS Collaboration/CERN)

Learn more

Measurement of dijet transverse momentum balance in O+O and pp collisions at 5.36 TeV with the ATLAS detector (ATLAS-CONF-2025-010)
Initial Stages 2025 presentation by Brian Cole: Overview of the ATLAS experiment at LHC
Initial Stages 2025 presentation by Matthew Hoppesch: Probing Nuclear Structure and Parton Dynamics Using Jets with ATLAS
Initial Stages 2025 poster by Matthew Hoppesch: First jet results from the ATLAS Oxygen-Oxygen program
Measurements of the suppression and correlations of dijets in Pb+Pb collisions at 5.02 TeV (Phys. Rev. C 107 (2023) 054908, arXiv:2205.00682, see figures)
Measurement of jet pT correlations in Pb+Pb and pp collisions at 2.76 TeV with the ATLAS detector (Phys. Lett. B 774 (2017) 379, arXiv:1706.09363, see figures)
Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at 2.76 TeV with the ATLAS Detector at the LHC (Phys. Rev. Lett. 105 (2010) 252303, arXiv:1011.6182, see figures)
The unexpected uses of a bowling pin: exploiting 20Ne isotopes for precision characterizations of collectivity in small systems (Giacalone et al., arXiv:2402.05995)
ATLAS takes a breath of oxygen (ATLAS News, July 2025)
ATLAS gives new insight into dijet suppression in heavy-ion collisions (ATLAS Physics Briefing, April 2022)
Looking inside trillion degree matter with ATLAS at the LHC (ATLAS Feature, May 2022)

Bowling balls vs. bowling pins? ATLAS studies the unique shape of neon ions

Katarina Anthony — Mon, 08 Sep 2025 11:54:54 +0000

Bowling balls vs. bowling pins? ATLAS studies the unique shape of neon ions

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The Large Hadron Collider (LHC) smashes together atomic nuclei to create a tiny, short-lived fireball of matter called quark-gluon plasma (QGP). The QGP is thought to have filled the Universe immediately after the Big Bang. Physicists at the ATLAS experiment are now studying the collisions of oxygen and neon nuclei, to understand how their shape impacts the dynamics of the QGP.

Figure 1: The elliptic flow (v2) measured in head-on collisions of neon, oxygen, and lead nuclei at the LHC. Larger v2 means particles are emitted more along one axis than another, reflecting the shape of the colliding nuclei. Neon (bowling-pin–like) shows the strongest ellipticity, oxygen lies in between, and spherical lead (bowling-ball–like) shows the smallest. (Renderings of neon and oxygen: Giacalone et al., arXiv:2402.05995)

As the QGP expands and cools inside the ATLAS experiment, small distortions in the original collision zone transform into subtle patterns in the angles and directions of the particles flying outward. This transformation relies on momentary interactions of the particles, which is often modeled with hydrodynamics — the theory of how fluids behave. Hence, these patterns are known as flow harmonics, and allow researchers to probe both the properties of the QGP and the geometry of the colliding nuclei. The elliptic flow (v₂) is especially sensitive to whether the collision zone was elongated (oval-shaped) or more circular.

In summer 2025, the ATLAS experiment recorded collisions of oxygen–oxygen and neon–neon as part of a dedicated light-ion programme at the LHC. Oxygen and neon have similar masses, which makes them especially useful to compare as other size-related effects are minimized. Thus, any differences in their v₂ measurements would strongly hint that the two nuclei have different shapes.

The ATLAS Collaboration today presented their first study of oxygen and neon collisions at the Initial Stages conference, including first measurements of elliptic flow. This result comes just nine weeks after data-taking, reflecting the significant efforts of members across the ATLAS Collaboration, in particular its heavy-ion community.

Today’s result from the ATLAS Collaboration experimentally confirms that neon nuclei have an elongated, "bowling pin" shape.

When studying the most head-on collisions (i.e. most central), researchers found that neon’s elliptic flow was the largest, followed by oxygen, with lead’s v₂ substantially smaller (see Figure 1). These flow measurements map well with physicists’ understanding of the nuclei. Lead-208 is a “doubly-magic” nucleus. It has full “shells” of both protons and neutrons, leading to an especially tightly bound and nearly spherical ground state – a true bowling ball. When such spherical nuclei are collided head-on, they produce little intrinsic ellipticity, and thus a small v₂ measurement. By contrast, many nuclear-structure calculations predict that neon is noticeably elongated (a prolate shape) – akin to a bowling pin!

This built-in elongation produces a larger elliptic overlap, even in the most head-on collisions. This geometric bias translates into a larger v₂ measurement, as observed by the ATLAS Collaboration. Oxygen’s measured v₂ sits between neon and lead, consistent with models that describe it as having a roughly spherical or weakly clustered structure.

Figure 2: The elliptic (v2) and triangular (v3) flow ratio of neon-neon collisions over oxygen-oxygen collisions. This ratio is measured as a function of centrality, which is a measure of how head-on the collision is. The most head-on collisions have the smallest centrality percentage, as indicated by the shaded disks above the figure. The enhancement of v2 in 0-5% neon-neon collisions is evidence of its bowling pin shape. (Image: ATLAS Collaboration/CERN)

When comparing systems of near-equal mass, many detector effects and global features cancel out in ratios (see Figure 2). This makes the remaining difference a much cleaner fingerprint of initial geometry rather than, say, different amounts of energy deposited in the collision. In this sense, the pattern of particles emitted by the QGP fireball serves as an ultrafast snapshot of the nuclei’s shapes at the instant of overlap.

The impact of today’s results extends to both nuclear structure and QGP theory. These measurements provide new constraints on models that predict clustering and deformation in light nuclei, and they test how those initial-state geometries are translated into observable flow by the same hydrodynamic physics that underpins heavy-ion collisions. Future studies of more complex observables can reveal more about how a nuclei’s geometry impacts these collisions, including whether more energetic collisions (larger average momentum) yield more elliptic particle production.

About the event display: Event display of a neon-neon collision recorded by the ATLAS experiment on 8 July 2025, with a centre-of-mass energy per nucleon pair of 5.36 TeV. (Image: ATLAS Collaboration/CERN)

Learn more

Measurement of the azimuthal anisotropy of charged particles in 5.36 TeV ¹⁶O+¹⁶O and ²⁰Ne+²⁰Ne collisions with the ATLAS detector (arXiv:2509.05171)
Initial Stages 2025 presentation by Brian Cole: Overview of the ATLAS experiment at LHC
Initial Stages 2025 presentation by Aman Dimri: Measurements of particle correlations in O+O and Ne+Ne collisions with the ATLAS detector
The unexpected uses of a bowling pin: exploiting 20Ne isotopes for precision characterizations of collectivity in small systems (Giacalone et al., arXiv:2402.05995)
ATLAS takes a breath of oxygen (ATLAS News, July 2025)
Looking inside trillion degree matter with ATLAS at the LHC (ATLAS Feature, May 2022)

Turning the LHC into a Lepton-Proton Collider in the search for leptoquarks

Katarina Anthony — Thu, 28 Aug 2025 12:33:02 +0000

Turning the LHC into a Lepton-Proton Collider in the search for leptoquarks

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Katarina Anthony Thu, 28/08/2025 - 14:33

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All matter in the universe is made up of just two fundamental building blocks: quarks and leptons. Intriguingly, both have three distinct families or “generations”. For example, the electron and its associated neutrino form the first generation of leptons, while the up and down quarks form the first generation of quarks. Where does this striking similarity between the particle families come from?

This question could be answered by leptoquarks – hypothetical particles that bridge the gap between leptons and quarks by allowing them to transform into each other. If they exist, the LHC might be able to produce them directly through collisions between quarks and leptons, as shown in Figure 1. At first glance, this might seem impossible, since the LHC is a proton collider, not a lepton collider. But protons are complex particles, made of quarks and gluons, and – thanks to quantum fluctuations – they can briefly, and very rarely, contain lepton–antilepton pairs. As shown in Figure 2, a quark inside a proton can emit a photon, which then transforms into a lepton–antilepton pair. Thus, on rare occasions, the LHC can effectively turn into a lepton–proton collider – opening a new door in the search for leptoquarks!

In a newly released result, the ATLAS Collaboration conducted its first direct search for the “resonant” production of leptoquarks. While the idea of lepton–proton collisions at the LHC dates back to the 90s, its experimental exploration remained out of reach. Thanks to new, cutting-edge calculations of the proton's lepton content and the resulting leptoquark production rates from the theory community, LHC researchers are now able to probe these processes experimentally.

Figure 1: Feynman diagram for the direct production and subsequent decay of a leptoquark, where the initial lepton (ℓ) originates from one of the colliding protons. (Image: ATLAS Collaboration/CERN)

Figure 2: Illustration of quantum fluctuations leading to the small, but non-zero lepton content within the proton. A quark within the proton emits a photon that splits into a lepton-antilepton pair. (Image: ATLAS Collaboration/CERN)

While the idea of lepton–proton collisions at the LHC dates back to the 90s, its experimental exploration remained out of reach – until now.

ATLAS physicists combined data collected in LHC Run 2 (2015-2018) and LHC Run 3 (2022-2023) to maximise their chances of detecting a leptoquark signal. Although expected to be extremely rare, leptoquarks would leave a distinct signature in the ATLAS experiment that sets them apart from background Standard Model processes. Leptoquarks would decay promptly into a lepton and a quark, with the quark appearing in the detector as a narrow spray of particles (or “jet”). A leptoquark signal would thus emerge in the data as a “peak” in the mass spectrum of lepton-jet events.

Physicists focused their search on four lepton-quark combinations: where the leptoquark interacts with the electron and down quark, the electron and bottom quark, the muon and strange quark, and the muon and bottom quark. While in principle a leptoquark could interact with any lepton-quark pair, these combinations leave distinctive experimental signatures – a jet plus an electron or muon – that could be more easily identified.

No significant excess beyond the Standard Model predictions was found (see Figure 3, which shows the results for electron+jet events). Physicists set strong limits on the possible mass of the leptoquark, ruling out masses up to 4.3 TeV. For models where leptoquarks interact strongly with leptons and quarks, they improved constraints on leptoquark masses by approximately a factor of two compared to previous results as shown in Figure 4.

With the ever-growing LHC dataset, researchers will continue to explore new and creative ways to look for physics beyond the Standard Model. With each step, their understanding evolves, sharpening the picture of how these particles might appear in nature.

Figure 3: Distribution of the invariant mass of the electron-jet system in the signal region. Standard-Model predictions with uncertainties are shown as shaded bands. The bottom panel shows the ratio of observed yields to the total Standard-Model prediction. Dashed lines indicate hypothetical leptoquark signals, producing peaks at 2 TeV (red) and 3 TeV (blue), corresponding to the leptoquark masses. (Image: ATLAS Collaboration/CERN)

Figure 4: The 95% confidence level exclusion limits as a function of the leptoquark mass (x-axis) and the strength of the coupling between muon and strange quark (y-axis) mediated by the leptoquark. The region above the red line is considered as excluded. (Image: ATLAS Collaboration/CERN)

About the event display: Visualisation of a candidate event in the signal region selecting a high-energetic electron-jet pair in a back-to-back topology using the Run-3 dataset. Charged-particle trajectories in the inner detector are shown as orange lines, and the yellow/orange and green/cyan boxes represent the energy deposited in the hadronic and electromagnetic calorimeters, respectively. The green line represents the reconstructed electron, while the yellow cone illustrates the jet with an invariant mass of the electron-jet system of about 2.9 TeV. (Image: ATLAS Collaboration/CERN)

Extending the ARM of ATLAS computing

Katarina Anthony — Wed, 16 Jul 2025 12:25:12 +0000

Extending the ARM of ATLAS computing

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Experiment Briefing

ATLAS Collaboration

computing

With data from the ongoing LHC Run 3 continuing to mount, and the High-Luminosity LHC (HL-LHC) expected to deliver ten times more, the ATLAS experiment is being pushed to its computing limits. Traditional central processing units (CPUs) based on the widely used x86_64 architecture – common in servers and desktop computers – are struggling to meet these growing demands in the resource projections. To keep pace, the ATLAS Collaboration is looking beyond traditional computing solutions to meet its growing needs.

Figure 1: The HepScore23 (HS23) benchmark score (a proxy for throughput) per Watt as a function of the processor frequency for seven CPU models. The first two listed models are ARM CPUs. (Image: ATLAS Collaboration/CERN)

One promising alternative is ARM CPU architecture, which offers competitive performance with significantly lower power consumption. Predominantly used in mobile devices like smartphones and tablets, ARM chips are increasingly being adopted in cloud data centres. As of 2024, ARM CPUs held around 15% of the commercial cloud computing market – a number expected to rise significantly, driven by the boom of artificial intelligence where these CPUs are often used as host chips.

In 2022 and 2023, the ATLAS Collaboration successfully ported its entire software ecosystem, consisting of several million lines of code in C++ and Python, to run on ARM CPUs. As ATLAS software runs on the Linux operating system, already available for ARM CPUs, the porting effort primarily focused on adapting ATLAS-specific and high-energy-physics-specific code. For some packages this entailed updating the individual software build instructions and configuration, as well as applying small changes to how floating point exceptions are handled. The porting effort was accomplished with the support of the CERN EP/SFT group. All major ATLAS workflows for offline and online data processing, together with all external software packages, can now be used on ARM CPUs. Enabling ATLAS code on different platforms was found to make it more robust and numerically stable.

All major ATLAS workflows for offline and online data processing can now be used on ARM CPU architecture – which offers competitive performance and significantly lower power consumption.

Figure 2: Number of concurrently running ATLAS production jobs in the past 12 months on ARM CPUs at various WLCG sites. (Image: ATLAS Collaboration/CERN)

After integrating ARM support into the automated software build system, ATLAS teams conducted a large physics validation campaign. This series of tests ensured that results obtained from ARM-CPU-processed simulation samples matched those from traditionally-processed x86_64 samples. During this validation campaign, access to ARM CPUs from the Worldwide LHC Computing Grid (WLCG) resources was scarce. Thus, the ATLAS computing grid workflow management system was enabled to run on cloud computing resources from Amazon and Google, using several hundred ARM CPUs for a limited period of time.

ATLAS production workflows have also been integrated into HepScore23, a benchmarking suite used to measure the processing power of WLCG resources. This allowed for direct comparisons between x86_64 and ARM processors, confirming that ARM CPUs offer comparable processing performance while consuming less energy – an important consideration, particularly in view of the HL-LHC’s demanding data processing requirements. A performance comparison of several ARM and x86_64 CPUs is shown in Figure 1 where the first two listed models are ARM CPUs.

ATLAS was the first experiment at the LHC to accept pledged WLCG computing resources based on ARM CPUs. As of now, several WLCG sites are each delivering between a few hundred and a thousand ARM CPUs each, as part of their contributions to ATLAS grid computing. ATLAS routinely processes simulation and reconstruction workflows on approximately 10,000 ARM CPU cores running concurrently (see Figure 2). While this represents a small fraction of the approximately 500,000 CPU cores routinely running ATLAS grid jobs, it’s an excellent start towards more energy efficient processing. Critically, it also positions ATLAS to tap into future large-scale computing facilities where ARM CPUs are expected to play a central role.

Learn more

The ATLAS experiment software on ARM (EPJ Web Conf. 295 (2024) 05019)
Using the ATLAS experiment software on heterogeneous resources (ATL-SOFT-PROC-2025-004)
Operational experience and R&D results using the Google Cloud for High-Energy Physics in the ATLAS experiment (Int. J. Mod. Phys. A 39 (2024) 2450054, arXiv:2403.15873)
HEPScore: A new CPU benchmark for the WLCG (EPJ Web Conf. 295 (2024) 07024, arXiv:2306.08118)
The environmental impact, carbon emissions and sustainability of computing in the ATLAS experiment (arXiv:2505.08530, see figures)

Updates Feed

ATLAS turns into a cosmic-ray laboratory with proton-oxygen collisions

These results establish a novel way to use the LHC as a cosmic-ray laboratory, opening the path for detailed experimental studies of proton-oxygen interactions.

Explore the interactive event display

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ATLAS spots rare high-energy Higgs bosons for the first time

The new ATLAS result finds the first evidence of Higgs bosons at high transverse momentum from the analysis of 301 fb-1 of data, collected during LHC operation from 2015 to 2024.

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How “odd” are Higgs boson interactions?

The ATLAS Collaboration has released two new searches for new sources of CP violation linked to Higgs boson interactions with W and Z bosons.

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ATLAS extends Higgs boson studies via vector boson fusion into beauty and charm

Looking for a light

The ATLAS Collaboration finds evidence of the Higgs boson in one of the “noisiest” environments in which to study the particle.

Searching for subtle patterns

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Summary of new ATLAS results from Moriond 2026

Latest Physics Briefings and News

New results presented at the 2026 Moriond Conferences

ATLAS surpasses LEP limits in search for compressed higgsinos

The ATLAS Collaboration has now established constraints across the full range of higgsino mass splittings, closing gaps left by previous LHC results.

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The curious case of the disappearing track

The ATLAS Collaboration has now set the most stringent limits to date on the masses and lifetimes of charginos.

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

Getting to the top of jet calibration

Physicists have pioneered a novel technique to calibrate jets – collimated sprays of particles that make up the most common "footprints" left in the ATLAS experiment.

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ATLAS maps the top quark–Higgs boson interaction with multileptons

Multilepton needles in the haystack

Testing predictions

At the ATLAS experiment, scientists are exploring the relationship between the heavyweights of the subatomic world: the top quark and the Higgs boson.

The Higgs boson in motion

Testing the "symmetry" of the Higgs boson

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ATLAS expands its reach into the high-rate frontier

ATLAS researchers are delving into an extraordinary dataset – 60 billion trigger-level events collected during LHC Run 2 – to search for new particles.

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Transforming sensitivity to new physics with single-top-quark events

A window into new physics

Mapping angular information with Fourier precision

The new result represents the most complete study of single-top-quark production and decay carried out by the ATLAS Collaboration.

Tightening the bounds on new interactions

A framework for the future

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Searching for flavourful violations of the Standard Model

Nothing is truly forbidden in the quantum world. Even "forbidden" transitions are possible so long as they follow several intermediate, rule-following steps.

The Standard Model, its flavours and the search for new physics

The search for FCNC interactions in top-quark processes

If ATLAS were to observe these flavour-changing processes, it would be indisputable evidence of new physics phenomena.

The search for lepton flavour violation in Z-boson decays

The ATLAS Collaboration’s state-of-the-art searches for FCNC and LFV interactions have produced powerful probes of new physics phenomena.

Outlook

About the Authors

Further reading

Scientific articles

Shining light on the Weak force: ATLAS observes WWγ production

Among the most sensitive tests of the Standard Model are multi-boson production processes, where two or more force carriers are produced together.

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Shape-shifting collisions probe secrets of early Universe

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ATLAS takes a closer look at the all-charm tetraquark

ATLAS has confirmed the presence of the all-charm tetraquark candidate X(6900) with a combined significance of 8.9σ — a key step forward in understanding exotic bound states of quarks.

Learn more

Hunting for jet quenching in collisions between oxygen nuclei

The ATLAS Collaboration has measured jet quenching in oxygen–oxygen collisions – the smallest system yet to show this phenomenon.

Learn more

Bowling balls vs. bowling pins? ATLAS studies the unique shape of neon ions

Today’s result from the ATLAS Collaboration experimentally confirms that neon nuclei have an elongated, "bowling pin" shape.

Learn more

Turning the LHC into a Lepton-Proton Collider in the search for leptoquarks

While the idea of lepton–proton collisions at the LHC dates back to the 90s, its experimental exploration remained out of reach – until now.

Links

Extending the ARM of ATLAS computing

All major ATLAS workflows for offline and online data processing can now be used on ARM CPU architecture – which offers competitive performance and significantly lower power consumption.

Learn more

The new ATLAS result finds the first evidence of Higgs bosons at high transverse momentum from the analysis of 301 fb^-1 of data, collected during LHC operation from 2015 to 2024.