Physics Briefings

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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Katarina Anthony Wed, 15/04/2026 - 16:53

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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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Katarina Anthony Fri, 27/03/2026 - 15:13

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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)

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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

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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Katarina Anthony Tue, 10/03/2026 - 08:35

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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

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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Katarina Anthony Mon, 16/02/2026 - 03:33

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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)

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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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Katarina Anthony Thu, 04/12/2025 - 10:23

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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)

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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Katarina Anthony Wed, 08/10/2025 - 13:11

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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

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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Katarina Anthony Mon, 08/09/2025 - 14:23

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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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Katarina Anthony Wed, 16/07/2025 - 14:25

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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)

ATLAS enters a new era of jet flavour tagging – powered by AI

Katarina Anthony — Mon, 07 Jul 2025 11:40:15 +0000

ATLAS enters a new era of jet flavour tagging – powered by AI

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Modern AI techniques are unlocking new ways to analyse particle collisions, and ATLAS is embracing the possibilities.

The most common signals produced in proton-proton collisions come from particle jets – collimated sprays of particles created when quarks or gluons transition into hadrons. Jets dominate the data collected by the ATLAS experiment, yet identifying what type, or flavour, of quark initiated a jet (jet flavour tagging) is highly challenging. This information is crucial for precise Standard Model measurements and searches for new physics phenomena.

Jets arising from a bottom or charm quark (b-jets and c-jets) have unique signatures compared to jets initiated by up, down or strange quarks or gluons (“light-jets"). They contain a high number of tracks, a fraction of which are measurably displaced from the proton-proton collision point. Jet flavour tagging takes advantage of these subtle but distinctive features. Traditionally, ATLAS physicists used algorithms aimed to reconstruct various features associated with displaced decays to then feed them into a machine-learning model that would determine the jet flavour. Now, physicists are using major advancements in artificial intelligence to upend this paradigm.

Figure 1: Illustration of the GN2 algorithm with jet and track input variables, discriminating between jet flavours by exploiting secondary vertices and other properties stemming from the displaced decays of hadrons. The jet features are copied for each track associated with the jet. The combined vectors are then fed into a per-track initialisation network, followed by a transformer encoder and a global representation of the jet. njf (ntf) corresponds to the number of jet (track) features. The pooled jet representation and the resulting track embeddings serve as inputs to three task-specific networks, which predict the jet's origin, determine the origin of each track within the jet, and assess the compatibility between pairs of tracks. (Image: ATLAS Collaboration/CERN)

The ATLAS Collaboration has deployed GN2, a new jet flavour tagging algorithm that’s shaking up the way physicists tackle this crucial task. GN2 is powered by a Transformer, a type of neural network that allows the model to directly analyse information about the tracks and jets, thereby eliminating the need for an intermediate stage of hand-crafted algorithms for reconstructing displaced vertices (see Figure 1). While the primary goal of GN2 is to predict the flavour of a jet, it goes beyond this single task. It incorporates auxiliary training objectives aimed at identifying the underlying process that generated each track within the jet, as well as determining whether pairs of tracks originate from a common spatial point. These physics-inspired auxiliary tasks not only enhance the model’s performance in flavour tagging but also provide physicists with deeper insight into the specific physics signatures the model has learned.

GN2 brings a huge performance leap compared to its predecessor, DL1d. When compared in simulated top-quark-pair events, GN2 is three times better at rejecting c-jets and 1.6 times better at rejecting light jets – all while keeping the same level of b‑jet identification performance. More importantly, this performance improvement holds up across different jet energies and works just as well in real collision data as it does in simulated data (see Figure 2).

The ATLAS Collaboration has deployed a new jet flavour tagging algorithm – GN2 – that’s shaking up the way physicists identify particles.

Building AI into the ATLAS toolkit

Since the start of data-taking in 2010, ATLAS physicists have continuously improved the performance of jet flavour tagging algorithms, often by integrating the latest machine-learning techniques. Thanks to high-precision calibration procedures, the ATLAS flavour tagging community was able to accurately measure the performance of these algorithms in data and compare it with the expected performance in simulation, revealing a remarkable level of consistency (see Figure 3). This agreement was made possible by the vital contributions of the detector operations team, who ensure stable data-taking conditions, and the simulation team, who work continuously to make these complex simulations as realistic as possible

Figure 2: The light-jet rejection (left) and c-jet rejection (right) as a function of the b-jet tagging efficiency for GN2 (light blue) and DL1d (dark orange), directly obtained in simulation (hollowed circle) and rescaled to match those in collision data (solid point). The horizontal error bands correspond to the uncertainties associated with the jet tagging efficiency measurement, while the vertical error bands indicate the uncertainties associated with the rejection measurements. (Image: ATLAS Collaboration/CERN)

To keep pace with rapid advances in AI, the ATLAS flavour tagging community has now reimagined how their improvements are developed and deployed. During GN2’s development, they established flexible and robust pipelines to manage the complex integration and training of AI algorithms. These pipelines were seamlessly integrated within the ATLAS software ecosystem, dramatically speeding up the transition from development to full deployment in physics analyses.

Figure 3: Evolution of the improvement of flavour tagging algorithms. Data points show the rejections (defined as the inverse of the mis-tagging rates of c- and light-jets) measured by the calibration analyses with their uncertainties, using top-pair events for b-jets and c-jets, and Z + jets events for light-jets. Measured b-jet efficiencies are also outlined in the plot. (Image: ATLAS Collaboration/CERN)

A game-changer for ATLAS physics

GN2’s powerful new approach is already making a real difference in how ATLAS measures and explores the building blocks of matter. In the most recent di-Higgs analysis, GN2 improved the measurement of the crucial signal strength parameter by 20%. GN2 is also expected to significantly improve the sensitivity of precision studies of rare Higgs boson interactions, such as its coupling to charm quarks, which demand highly accurate identification of b-jets and c-jets. These extremely challenging measurements lie at the heart of the ATLAS Collaboration's future physics programme. Moreover, GN2 will play a key role in the search for new particles that interact preferentially with bottom and charm quarks.

A new era of jet flavour tagging has arrived, and it promises to be even more beautiful and charming than ever before!

Learn more

Transforming jet flavour tagging at ATLAS (arXiv:2505.19689, see figures)
Highlights of the HL-LHC physics projections by ATLAS and CMS (arXiv:2504.00672)
Evolution of the ATLAS flavour tagging performance measured with full Run 2 dataset (FTAG-2023-07)
Barr et al., (2024). Umami: A Python toolkit for jet flavour tagging (Journal of Open Source Software, 9(102), 5833)
Salt, a general-purpose framework for training multi-modal, multi-task models for high energy physics
The beauty and the charm of the Higgs boson, ATLAS Physics Briefing, 22 July 2024
ATLAS sets record limits on Higgs self-interaction using Run 3 data, ATLAS Physics Briefing, 22 May 2025

ATLAS closes in on rare Higgs decays

Katarina Anthony — Mon, 07 Jul 2025 11:38:00 +0000

ATLAS closes in on rare Higgs decays

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Katarina Anthony Mon, 07/07/2025 - 13:38

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The ATLAS Collaboration finds evidence of Higgs-boson decays to muons and improves sensitivity to Higgs-boson decays to a Z boson and a photon.

Since the discovery of the Higgs boson in 2012, physicists have made major strides in exploring its properties. Does that mean the subject is done and dusted? Far from it! In new results presented at the 2025 European Physical Society Conference on High Energy Physics (EPS-HEP), the ATLAS Collaboration narrowed in on two exceptionally rare Higgs-boson decays using data collected in Run 3 of the Large Hadron Collider (LHC). These studies offer deep insights into how closely the Higgs boson’s behaviour aligns with the Standard Model.

The first process under study was the Higgs-boson decay into a pair of muons (H→μμ). Despite its scarceness – occurring in just 1 out of every 5000 Higgs decays – this process provides the best opportunity to study the Higgs interaction with second-generation fermions and shed light on the origin of mass across different generations. The second investigated process was the Higgs-boson decay into a Z boson and a photon (H→Zγ), where the Z boson subsequently decays into electron or muon pairs. This rare decay is especially intriguing, as it proceeds via an intermediate "loop" of virtual particles. If new particles contribute to this loop, the process could offer hints of physics beyond the Standard Model.

Looking for needles in a haystack

Identifying these rare decays is quite the challenge. For H→μμ, researchers looked for a small excess of events clustering near a muon-pair mass of 125 GeV (the mass of the Higgs boson). This signal can be easily hidden behind the thousands of muon pairs produced through other processes (“background”).

The H→Zγ decay is even harder to isolate, as the chances of spotting its signal are complicated by the fact that the Z boson only decays into detectable leptons about 6% of the time. Compounding the challenge are the operation conditions of LHC Run 3, which features more overlapping collisions, making it easier for particle jets to mimic real photons.

To boost the sensitivity of their searches, ATLAS physicists combined the first three years of Run-3 data (165 fb^-1, collected between 2022-2024) with the full Run-2 dataset (140 fb^-1, from 2015-2018). They also developed a sophisticated method to better model background processes, categorised recorded events by the specific Higgs-production modes, and made further improvements to their event-selection techniques in order to maximize the likelihood of spotting genuine signals. Figure 1 shows the resulting muon-pair mass distribution obtained with the data collected in 2022 to 2024 and combined over all the categories. Figure 2 displays the observed distribution of the Zγ system in the combined data sample from Run 2 and Run 3.

Figure 1: Dimuon invariant mass spectrum observed in Run-3 data, for all combined analysis categories. The background and signal probability density functions (pdf) are obtained from the combined fit of all categories to the Run-3 data, corresponding to a signal strength of μ = 1.6 ±0.6. The lower panel shows the fitted signal pdf, normalized to the signal best-fit value, and the difference between the observed data and the background model. Error bars represent the data’s statistical uncertainties. (Image: ATLAS Collaboration/CERN)

Figure 2: Weighted m_Zγ distributions of data events across all categories of the combined Run 2 and Run 3 dataset. The black points represent the data, with statistical uncertainties shown as vertical error bars. Each event is weighted by ln(1 + S/B), where S and B are the signal and background yields extracted from fits to the data in the mass window 120 ≤ m_Zγ ≤ 130 GeV. The red curve corresponds to the result of a simultaneous signal-plus-background fit across all categories, while the blue curve shows the background-only component of the fit. (Image: ATLAS Collaboration/CERN)

Finding evidence and enhancing sensitivity

In the previous search for H→μμ using the full Run-2 dataset, the ATLAS Collaboration saw its first hint of this process at the level of 2 standard deviations. The comparable CMS result reached an observed (expected) significance of 3 (2.5) standard deviations. Now, with the combined Run-2 and Run-3 datasets, the ATLAS Collaboration has found evidence for H→μμ with an observed (expected) significance over the background-only hypothesis of 3.4 (2.5) standard deviations. This means that the chance that the result is a statistical fluctuation is less than one in 3000!

As for the H→Zγ process, a previous ATLAS and CMS combined analysis used Run-2 data to find evidence of this decay mode. It reported an observed (expected) excess over the background-only hypothesis of 3.4 (1.6) standard deviations. The latest ATLAS result, combining Run-2 and Run-3 data, reported an observed (expected) excess over the background-only hypothesis of 2.5 (1.9) standard deviations. This outcome provides the most stringent expected sensitivity to date for measuring the decay probability (“branching fraction”) of H→Zγ.

These achievements were made possible by the large, excellent dataset provided by the LHC, the outstanding efficiency and performance of the ATLAS experiment, and the use of novel analysis techniques. With more data on the horizon, the journey of exploration continues!

About the event displays: Left: Candidate Higgs boson decaying to two muons (H→μμ). The event features two muons (red tracks with their associated muon chambers shown as blue boxes with green measurement bars) with invariant mass 125.324 GeV, consistent with H → μμ, and two forward jets (yellow cones). 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. Right: Candidate Higgs boson decaying to a photon and a Z boson, with the Z subsequently decaying to an electron-positron (H → Zγ → eeγ). The photon is depicted by a transverse purple cone, the electron-positron by green lines, consistent with H → Zγ, and two forward jets (the yellow cones). Charged-particle trajectories in the inner detector are depicted as orange lines. The yellow/orange and green/cyan boxes represent the energy deposited in the hadronic and electromagnetic calorimeters, respectively. (Image: ATLAS Collaboration/CERN)

Learn more

Evidence for the dimuon decay of the Higgs boson in proton-proton collisions with the ATLAS detector (Phys. Rev. Lett. 135 (2025) 231802, arXiv:2507.03595, see figures)
Search for the Higgs boson decay to a Z boson and a photon in proton-proton collisions at 13 TeV and 13.6 TeV with the ATLAS detector (arXiv:2507.12598, see figures)
EPS 2025 presentation by Fabio Cerutti: ATLAS Highlights
EPS 2025 presentation by Tamar Zakareishvili: Search for rare processes and lepton-flavor-violating decays of Higgs boson at the ATLAS experiment
A search for the dimuon decay of the Standard Model Higgs boson with the ATLAS detector (Phys. Lett. B 812 (2021) 135980, arXiv: 2007.07830, see figures)
ATLAS and CMS Collaborations: Evidence for the Higgs boson decay to a Z boson and a photon at the LHC (Phys. Rev. Lett. 132 (2024) 021803, arXiv:2309.03501, see figures)
CMS Collaboration: Evidence for Higgs boson decay to a pair of muons (JHEP 01 (2021) 148)

Bound to be discovered? ATLAS explores top-quark interactions near threshold

Katarina Anthony — Mon, 07 Jul 2025 08:29:27 +0000

Bound to be discovered? ATLAS explores top-quark interactions near threshold

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Katarina Anthony Mon, 07/07/2025 - 10:29

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The top quark is a bit of a loner. While other quarks can join together to form bound states called hadrons, the top quark’s extremely short lifetime means it decays almost instantly – disappearing before it can form a long-lasting bond.

But not all relationships have to last forever. Under certain conditions, physicists predict that a top quark and an anti-top quark could exchange gluons for a short time before decaying independently. This fleeting interaction, called a quasi-bound state, can only form when both top quarks are produced almost at rest (i.e. non-relativistically) and the collision energy is close to twice the top quark’s mass. This is the production threshold, and the mass of this quasi-bound state would be slightly below it.

For a long time, it was considered experimentally unfeasible to measure this subtle effect at the LHC, since events close to the production threshold make up only a small fraction of the top-pairs produced and are difficult to spot in the data. However, thanks to the wealth of proton-proton data recorded during Run 2 of the LHC and advances in analysis techniques, this long-held assumption is now being overturned.

The production threshold came under increased scrutiny in the ATLAS Collaboration’s recent, first-ever observation of quantum entanglement in top-pair events. In that result, physicists examined the angular patterns of the top-quark decay products, which can differ when the top-pair energy is close to the production threshold. Could these patterns be used to measure the quasi-bound top-pair state?

The new ATLAS result revealed an excess of events when compared to predictions that ignore the quasi-bound state

Figure 1: Invariant mass of the top-pair system close to the production threshold as a function of angular variables after the fit to data. The orange line in the second panel shows the contribution of the non-relativistic effects compared to the prediction without this process. (Image: ATLAS Collaboration/CERN)

In a new result presented today at the European Physical Society Conference on High Energy Physics (EPS-HEP) in Marseille, ATLAS physicists measured how often top-quark pairs are produced near the production threshold. They examined the full LHC Run-2 dataset, looking for events with a low invariant mass in the top-pair system. Due to limited experimental resolution, a pseudo-bound state would not appear as a clear peak in the spectrum. To improve sensitivity, physicists divided the dataset into nine subsets (see Figure 1), based on the same angular information used to discover entanglement in top-pair events. Physicists also used new, sophisticated simulations that incorporate a more complete modelling of the non-relativistic effects of the strong force.

The new ATLAS result revealed an excess of events when compared to predictions that ignore the quasi-bound state. The model without non-relativistic effects was rejected with an observed significance of 7.7𝜎 and the cross section of the pseudo-bound state was measured to be 9.0 ± 1.3 pb, with the precision being dominated by the statistical uncertainty. A similar measurement was recently performed by the CMS Collaboration.

With the ongoing Run 3 of the LHC on track to deliver significantly more data, ATLAS physicists are set to deepen their exploration of the strong force in this special kinematic regime. Crucially, today’s measurement underscores the need for precise theoretical modeling of top-pair production, both at and above the production threshold, and reinforces the vital interplay between theory and experiment in precision studies. Continued improvements to these models will be essential to fully understand this intriguing corner of the Standard Model.

About the banner image: Display of a candidate event consistent with the formation of a top-pair quasi-bound-state. (Image: ATLAS Collaboration/CERN)

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Observation of a cross-section enhancement near the tt production threshold in 13 TeV proton-proton collisions with the ATLAS detector (ATLAS-CONF-2025-008)
EPS 2025 presentation by Fabio Cerutti: ATLAS Highlights
EPS 2025 presentation by Haifeng Li: Measurements at the ttbar threshold with the ATLAS detector
Observation of quantum entanglement in top-quark pairs using the ATLAS detector (Nature 633 (2024) 542, arXiv:2311.07288, see figures)
CMS Collaboration: Observation of a pseudoscalar excess at the top quark pair production threshold (arXiv:2503.22382)
Fuks, B., Hagiwara, K., Ma, K. et al. Simulating toponium formation signals at the LHC (Eur. Phys. J. C 85, 157 (2025), arXiv:2411.18962)
ATLAS achieves highest-energy detection of quantum entanglement, ATLAS Physics Briefing, September 2023

ATLAS sets record limits on Higgs self-interaction using Run 3 data

Katarina Anthony — Thu, 22 May 2025 11:10:06 +0000

ATLAS sets record limits on Higgs self-interaction using Run 3 data

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Katarina Anthony Thu, 22/05/2025 - 13:10

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One of the biggest open questions in particle physics today is how the Higgs boson interacts with itself. This “self-coupling” plays a pivotal role in determining the shape of the energy potential of the Higgs field, influencing both the early Universe evolution and the mechanism that gives mass to elementary particles. Physicists at the ATLAS Collaboration are shedding light on this fundamental interaction by searching for the rare pair production of Higgs bosons.

In a newly released result, the ATLAS Collaboration studied one of the “golden” decay channels of the Higgs pairs: HH → bbγγ, where one Higgs boson decays into two photons and the other into a pair of b-quarks. By combining the entire LHC Run 2 (2015–2018) and early Run 3 (2022–2024) dataset, physicists have significantly enhanced the statistical power of the analysis. This result marks the first ATLAS measurement based on over 300 fb⁻¹ of proton–proton collision data, reaching an expected sensitivity comparable to that of the full Run 2 combination across all Higgs boson pair production channels.

Searches in this channel can be particularly challenging due the extremely rare nature of Higgs pair production – predicted to occur once in a trillion proton-proton collisions – and the significant background from Standard Model processes that mimic the bbγγ signature. To overcome these challenges, ATLAS physicists used advanced multivariate techniques, including machine learning classifiers and precise b-jet and photon identification techniques. The team also developed an optimized event selection strategy to fully benefit from improved detector calibration and event reconstruction methods developed in both Run 2 and 3.

Figure 1: a) The observed (black marker), the expected μ = 0 (black dotted marker) and expected μ = 1 (red marker) upper limits on the signal strength with uncertainty 68% (95%) in the turquoise (yellow) color. The combined expected upper limit (2.6) is almost twice as good as the expected upper limit from the Run 2 Legacy (5.0). b) The observed (turquoise) and expected (yellow) statistical significance of the Higgs boson pair signal are shown separately for Run 2, Run 3, and their combination. (Image: ATLAS Collaboration/CERN)

This new result is the first ATLAS measurement based on over 300 fb⁻¹ of proton–proton collision data, combining the full LHC Run 2 (2015–2018) and current Run 3 (2022–2024) datasets.

Figure 2: A weighted distribution of the di-photon invariant mass summed over all analysis categories for both Run 2 and Run 3. Each event is weighted by log(1+S/B), where S represents the expected Higgs pair signal and B is the expected background. The curves represent different components of the fit: the turquoise line shows the continuum background only, the black line includes the single Higgs background, and the red line shows the complete fit including the Higgs pair signal. (Image: ATLAS Collaboration/CERN)

As a result of these advancements and the expanded dataset, physicists measured the signal strength defined as the observed signal normalized to its Standard Model prediction, by μ_HH= 0.9 ^+1.4_-1.1 where μ_HH= 1.0 ^+1.3_-1.0 was expected. The expected (observed) upper limit on the signal strength is 2.6 (3.8) times the Standard Model – nearly a 100% improvement over the previous best expected limit achieved by the ATLAS Collaboration in this same channel (Figure 1a). The observed statistical significance of the Standard Model signal is 0.84σ, with an expected significance close to 1σ (Figure 1b), indicating no significant excess but full consistency with Standard Model expectations (Figure 2).

The analysis also places constraints on two key parameters: the magnitude of the Higgs boson self-coupling normalised to its Standard Model prediction (κ_λ) (Figure 3a), limited to be between −1.6 and 6.6 at 95% confidence level, and the coupling modifier of two Higgs bosons and two vector bosons normalised to its Standard Model prediction (κ_2V) (Figure 3b), limited to between −0.5 and 2.6 at 95% confidence level. This level of sensitivity was achieved through the combination of a larger dataset, a more performant analysis strategy, and the dedicated efforts of experts across the ATLAS Collaboration, who rapidly analysed new data while implementing major updates to algorithms and analysis strategies.

Figure 3: a) Feynman diagram for the dominant gluon-gluon fusion production with the Higgs self-interaction (κλ) b) Feynman diagram for the vector boson fusion production with the interaction strength between two Higgs bosons and two vector bosons (κ2V). (Image: ATLAS Collaboration/CERN)

This result underscores the ATLAS Collaboration’s growing ability to explore Higgs-boson-pair production in the bbγγ channel. The significant boost in sensitivity also lays the foundation for future measurements of the Higgs self-coupling – a key to understanding the shape of the Higgs potential and the evolution of the Universe after the Big Bang. With the full Run 3 dataset soon available and High-Luminosity LHC on the horizon, ATLAS is well positioned to push these studies even further – sharpening our understanding of the Higgs boson and exploring potential signs of physics beyond the Standard Model.

About the event display: A candidate HH → bb̄γγ collision event. The two jets originating from b-quarks are represented by turquoise cones and the two photons by yellow towers. (Image: ATLAS Collaboration/CERN)

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Study of Higgs boson pair production in the HH→bbγγ final state with 308 fb−1 of data collected at 13 TeV and 13.6 TeV by the ATLAS experiment (ATLAS-CONF-2025-005)
Studies of new Higgs boson interactions through nonresonant HH production in the bbγγ final state in proton–proton collisions at 13 TeV with the ATLAS detector (JHEP 01 (2024) 066, arXiv:2310.12301, see figures)
Shedding light on Higgs-boson self-interactions, ATLAS Physics Briefing, 21 September 2023

Physics Briefings

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ATLAS maps the top quark–Higgs boson interaction with multileptons

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The Higgs boson in motion

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ATLAS expands its reach into the high-rate frontier

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Tightening the bounds on new interactions

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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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ATLAS takes a closer look at the all-charm tetraquark

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

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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.

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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.

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ATLAS enters a new era of jet flavour tagging – powered by AI

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Building AI into the ATLAS toolkit

A game-changer for ATLAS physics

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ATLAS closes in on rare Higgs decays

Looking for needles in a haystack

Finding evidence and enhancing sensitivity

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Bound to be discovered? ATLAS explores top-quark interactions near threshold

The new ATLAS result revealed an excess of events when compared to predictions that ignore the quasi-bound state

Learn more

ATLAS sets record limits on Higgs self-interaction using Run 3 data

This new result is the first ATLAS measurement based on over 300 fb⁻¹ of proton–proton collision data, combining the full LHC Run 2 (2015–2018) and current Run 3 (2022–2024) datasets.

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.