In preparation for the High-Luminsity LHC (HL-LHC) [1], the ATLAS detector will undergo major detector upgrades, including the replacement of the current Inner Detector with the new all-silicon Inner Tracker (ITk) [2]. The ITk consists of a pixel detector close to the beamline surrounded by a large-area strip detector. During detector production, the electrical properties of silicon sensors and readout electronics must be characterized through a series of quality control (QC) and quality assurance tests. These tests ensure any defect is captured at the earliest possible stage. One such defect, callled a pinhole, occurs when the strip implant and the metal readout electrode are shorted through the intermediary dielectric layer. Notably, the introduction of pinholes during module assembly and pinhole effects on completed modules, especially on leakage current measurement circuitry, have never been studied. In this paper, we investigate the effect of such connections on the sensor leakage current measurements of completed modules and introduce new ways to locate pinholed strips. With minor modifications to testing procedures, such defects are shown not to impede module testing or performance.

The LUCID-2 detector serves as the primary luminometer of the ATLAS experiment and is the only one capable of delivering reliable luminosity measurements across all beam configurations, luminosity regimes, and at the bunch-crossing level. During LHC Run-2, ATLAS achieved a luminosity precision of 0.8%, the most accurate determination ever obtained at a hadron collider. LUCID-2 continues to provide luminosity measurements for ATLAS in LHC Run-3, and preliminary results based on the collected datasets will be shown. The ATLAS physics program at the High-Luminosity LHC demands a luminosity precision of 1%. Meeting this requirement in an environment with up to 140 simultaneous interactions per bunch crossing (and up to 200 in the ultimate scenario) necessitates the use of several dedicated luminosity detectors. LUCID-3, the planned upgrade of LUCID-2, is among these. This contribution presents two LUCID-3 design options currently under investigation: one relying on photomultiplier tubes (PMTs), similar to LUCID-2 but positioned farther from the beam pipe to reduce acceptance and prevent detector saturation; and a second detector that uses optical fibers as Cherenkov radiators, read out by PMTs placed in a low-radiation region. All PMTs will be calibrated and monitored with a 207Bi radioactive source to ensure long-term stability better than 1%. The main results obtained with both type of detector in the 2024 data taking period will be discussed, with emphasis on the implications for the final LUCID-3 design.

The ATLAS collaboration at the LHC has published inclusive cross-section measurements for the single-top and t\overline{t} t t ¯ production modes at center-of-mass energies of \sqrt{s} = 5.02, 8.16 s = 5.02 , 8.16 , 13 13 , and 13.6 13.6 TeV. Single-top measurements are conducted in the t t -channel and tW t W channel. In addition to the nominal cross-section measurements, various measurements of other interesting observables such as the V_{tb} V t b element of the Cabibbo Kobayashi Maskawa (CKM) quark-mixing matrix, the ratio of the inclusive cross-sections between tq t q and t\overline{q} t q ¯ , the ratio of inclusive cross-sections between t\overline{t} t t ¯ and Z→ \ell\ell Z → ℓ ℓ , and the nuclear modification factor (defined as the ratio of the inclusive t\overline{t} t t ¯ cross section in heavy-ion collisions to the inclusive t\overline{t} t t ¯ cross-section in pp p p collisions) are also reported. These results are compared to their corresponding SM predictions, calculated at (N)NLO in QCD. All results are in good agreement with SM predictions.

A bstract In this work, we explore a conversion-driven freeze-out scenario, where the next-to-lightest stable particle (NLSP) sets the dark matter (DM) abundance through the process “NLSP SM ↔ DM SM”. Although DM is produced via a freeze-out mechanism, its interaction strength with the visible sector can range from weak to feeble couplings. This results in a vast, largely unexplored parameter space that evades current direct, indirect, and collider bounds, while remaining testable in the near future. We study this mechanism in the context of an alternative U(1) B−L model, where four chiral fermions are required to cancel gauge anomalies, unlike the usual case with three right-handed neutrinos. The observed relic abundance is successfully reproduced within this framework. The viable parameter space can be probed by future direct detection experiments, while remaining inaccessible to indirect searches. Our results show that the DM relic density is highly sensitive to the NLSP-SM interaction strength and the mass difference between the NLSP and DM, but not to the DM-SM direct interaction. For certain parameter choices, the NLSP decays to DM via two or three body processes involving an extra gauge boson and SM particles, leading to long-lived decays outside the CMS or ATLAS detectors at the LHC. In contrast, if the decay proceeds via a CP-odd Higgs, it occurs promptly within the detector. We investigate prospects for detecting such long-lived NLSPs at the proposed MATHUSLA detector, with similar expectations for the ongoing FASER experiment. Finally, we find that choosing arbitrarily small values of the gauge coupling or BSM fermionic mixing angle can violate successful BBN predictions.

Anisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system's radial expansion, has received less attention. Notably, direct experimental evidence for the global and collective nature of radial flow fluctuations has been lacking. This Letter presents the first measurement of transverse momentum (p_{T}) dependence of radial flow fluctuations (v_{0}(p_{T})) over 0.5<p_{T}<10 GeV and demonstrates its collective nature using a two-particle correlation method in Pb+Pb collisions at sqrt[s_{NN}]=5.02 TeV. The data reveal three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in p_{T}, and centrality-independent shape in p_{T}. The comparison with a hydrodynamic model demonstrates the sensitivity of v_{0}(p_{T}) to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP.

Abstract The field of high energy physics (HEPs) has seen a marked increase in the use of machine learning (ML) techniques in recent years. The proliferation of applications has revolutionised many aspects of the data processing pipeline at collider experiments including the Large Hadron Collider (LHC). In this whitepaper, we discuss the increasingly crucial role that ML plays in real-time analysis (RTA) at the LHC, namely in the context of the unique challenges posed by the trigger systems of the large LHC experiments. We describe a small selection of the ML applications in use at the large LHC experiments to demonstrate the breadth of use-cases. We continue by emphasising the importance of collaboration and engagement between the HEP community and industry, highlighting commonalities and synergies between the two. The mutual benefits are showcased in several interdisciplinary examples of RTA from industrial contexts. This whitepaper, compiled by the SMARTHEP network, does not provide an exhaustive review of ML at the LHC but rather offers a high-level overview of specific real-time use cases.

A bstract The top-quark Yukawa coupling is extracted from the distribution of the top-quark pair ( $$ t\overline{t} $$ t t ¯ ) invariant mass in proton-proton collisions using 140 fb − 1 of data at $$ \sqrt{s}=13 $$ s = 13 TeV collected in 2015–2018 by the ATLAS experiment at the Large Hadron Collider. In the region near the production threshold, the $$ t\overline{t} $$ t t ¯ invariant mass spectrum is sensitive to electroweak virtual corrections, including contributions from Higgs boson exchange, thereby providing sensitivity to the top-quark Yukawa coupling. This is the first measurement in ATLAS that aims to obtain this coupling exploiting this approach. The $$ t\overline{t} $$ t t ¯ system is reconstructed in the single-lepton final state, requiring exactly one isolated electron or muon and at least four jets with at least two identified as originating from b -quarks. The measured Yukawa coupling is found to be in good agreement with the Standard Model prediction. An upper limit on the top-quark Yukawa coupling strength of Y t &lt; 2 . 1 relative to the Standard Model prediction is observed at 95% confidence level, consistent with the expected sensitivity.

Jet flavour tagging enables the identification of jets originating from heavy-flavour quarks in proton–proton collisions at the Large Hadron Collider, playing a critical role in its physics programmes. This paper presents GN2, a transformer-based flavour tagging algorithm deployed by the ATLAS Collaboration that represents a different methodology compared to previous approaches. Designed to classify jets based on the flavour of their constituent particles, GN2 processes low-level tracking information in an end-to-end architecture and incorporates physics-informed auxiliary training objectives to enhance both interpretability and performance. Its performance is validated in both simulation and collision data. The measured c-jet (light-jet) rejection in data is improved by a factor of 3.5 (1.8) for a 70% b-jet tagging efficiency, compared to the previous algorithm. GN2 provides substantial benefits for physics analyses involving heavy-flavour jets, such as measurements of Higgs boson pair production and the couplings of bottom and charm quarks to the Higgs boson, and demonstrates the impact of advanced machine learning methods in experimental particle physics.

Abstract The investigation of quartic gauge couplings provides a crucial test of the Standard Model and serves as a potential window into new physics at higher energy scales. Within the framework of Effective Field Theory, deviations from the SM can be parameterized through dimension-8 operators. In this study, we analyze the process $pp \rightarrow \gamma\gamma jj$ at the High-Luminosity Large Hadron Collider (HL-LHC) and the Future Circular Collider in hadron mode (FCC-hh) to probe the sensitivity to anomalous quartic gauge couplings (aQGCs), particularly $f_{T8}/\Lambda^4$ and $f_{T9}/\Lambda^4$. Monte Carlo simulations of signal and relevant backgrounds are performed using MadGraph for event generation, Pythia for parton showering and hadronization, and Delphes for detector simulation. A multivariate analysis based on Boosted Decision Trees is employed to optimize the signal-to-background discrimination, incorporating a comprehensive set of kinematic and reconstructed variables of the final state particles. Additionally, we evaluate unitarity-violating effects associated with dimension-8 operators by imposing energy cutoffs on the di-photon invariant mass. The expected exclusion and discovery significances are computed, accounting for systematic uncertainties to ensure a realistic assessment of collider reach. Our findings indicate that the FCC-hh offers significantly improved sensitivity compared to the HL-LHC and current experimental results by LHC, reinforcing its potential for probing aQGCs. Notably, even under a 10\% systematic uncertainty, our projected limits for FCC-hh at 95\% confidence level surpass the current best constraints reported by the ATLAS and CMS collaborations, highlighting the enhanced discovery prospects at future high-energy colliders.

Abstract Processes with $$\tau $$ τ -leptons in the final state are important for Standard Model measurements and searches for physics beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider observes $$\tau $$ τ -leptons produced in proton–proton collisions only through their decay products. Data analyses involving hadronically decaying $$\tau $$ τ -leptons face challenges due to backgrounds from jets misidentified as $$\tau $$ τ -leptons that are not modelled reliably by Monte Carlo simulations. Data-driven methods such as the fake-factor method allow such misidentified backgrounds to be predicted by measuring transfer factors, known as fake factors, in data from dedicated regions. This paper describes a refined technique for determining the fake factors, the Universal Fake Factor method. It evaluates the fake factors for a signal region by using fake factors from samples enriched in different sources of jets misidentified as $$\tau $$ τ -leptons (light-quark, gluon, b -quark, and pile-up jets). Each fake factor is calculated as a linear combination of fake factors measured in these different enriched samples. For the full Run 2 data set, the systematic uncertainty of the calculated fake factors, evaluated using $$W(\mu \nu )$$ W ( μ ν ) enriched event sample, ranges from 15 to 35% depending on the $$\tau $$ τ -lepton’s transverse momentum and charged-particle decay multiplicity.

The neutral Standard Model Higgs boson was discovered in 2012 at CERN, and the search for further particles of extended models continues, in particular, the search for an axion-like particle (ALP). Using machine learning technologies, this analysis addresses the separation of ALP production from unwanted background reactions. In this project, the Run 2 data from the ATLAS detector are used and the efficiency as well as the significance of the machine learning algorithm are optimized as a function of the theoretical ALP mass. Abstract Published by the Jagiellonian University 2025 authors

This article presents a recent QCD measurement of the jet cross-section ratios from the ATLAS experiment at CERN’s Large Hadron Collider, using proton–proton collisions at a center-of-mass energy of 13 TeV. The jet cross-section ratios are derived from multi-differential particle-level cross sections for several inclusive jet multiplicity bins for at least 2, 3, 4, and 5 jets. These ratios improve sensitivity to the strong coupling parameter while reduce sensitivity to uncorrelated systematic uncertainties and parton distribution functions. The three-to-two jet cross-section ratio is reported for the first time at 13 TeV center-of-mass energy. Additionally, higher jet multiplicity ratios are measured experimentally for the first time, providing a crucial reference for future theoretical developments in high-precision QCD predictions involving multiple jets. Abstract Published by the Jagiellonian University 2025 authors

Erratum: Search for boosted diphoton resonances in the 10 to 70 GeV mass range using 138 fb−1 of 13 TeV pp collisions with the ATLAS detector

The ATLAS experiment at the Large Hadron Collider explores the use of modern neural networks for a multi-dimensional calibration of its calorimeter signal defined by clusters of topologically connected cells (topo-clusters). The Bayesian neural network (BNN) approach not only yields a continuous and smooth calibration function that improves performance relative to the standard calibration but also provides uncertainties on the calibrated energies for each topo-cluster. The results obtained by using a trained BNN are compared to the standard local hadronic calibration and to a calibration provided by training a deep neural network. The uncertainties predicted by the BNN are interpreted in the context of a fractional contribution to the systematic uncertainties of the trained calibration. They are also compared to uncertainty predictions obtained from an alternative estimator employing repulsive ensembles.

Abstract Higgs boson production cross-sections via gluon–gluon fusion and vector-boson fusion in proton–proton collisions are measured in the $$H\rightarrow WW^*\rightarrow \ell \nu \ell \nu $$ H → W W ∗ → ℓ ν ℓ ν decay channel. The Large Hadron Collider delivered proton–proton collisions at a centre-of-mass energy of $$13\,\text {TeV}$$ 13 TeV between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of $$140\,\text {fb}^{-1}.$$ 140 fb - 1 . The total cross-sections for Higgs boson production by gluon–gluon fusion and vector-boson fusion times the $$H\rightarrow WW^*$$ H → W W ∗ branching ratio are measured to be $$12.4^{+1.3}_{-1.2}\,\text {pb}$$ 12 . 4 - 1.2 + 1.3 pb and $$0.79^{+0.18}_{-0.16}\,\text {pb},$$ 0 . 79 - 0.16 + 0.18 pb , respectively, in agreement with the Standard Model predictions. Higgs boson production is further characterised through measurements of Simplified Template Cross-Sections in a total of fifteen kinematic fiducial regions. A new scheme of kinematic fiducial regions has been introduced to enhance the sensitivity to CP-violating effects in Higgs boson interactions. Both schemes are used to constrain CP-even and CP-odd dimension-six operators in the Standard Model effective field theory

Abstract ATLAS, a general-purpose experiment at the Large Hadron Collider (LHC), makes use of a large internationally-distributed computing infrastructure, including over $$10^6$$ 10 6 TB of managed data on disk and tape and almost one million simultaneously running CPU cores. Upgrades for the High-Luminosity LHC (HL-LHC) will increase the required computing resources by a factor of 3–4 by the beginning of the 2030s, and by an order of magnitude before the conclusion of data taking at the beginning of the 2040s. These resources are spread over around 100 computing sites worldwide. Efforts are underway within the experiment to evaluate and mitigate various aspects of the environmental impact of the sites, with the additional long-term goal of making recommendations to the sites that will significantly reduce the total expected environmental impact in the HL-LHC era. These efforts take several forms: building awareness in the experiment community, adjusting aspects of the computing policy, and modifications of data center configurations, either in ways that take advantage of particular features of ATLAS workloads or in generic ways that reduce the environmental impact of the computing resources. This paper describes the ongoing investigations and approaches that have already provided useful and actionable outcomes.

A search for the dimuon decay of the Higgs boson is presented based on pp collision data recorded by ATLAS during Run 3 of the Large Hadron Collider, corresponding to an integrated luminosity of 165 fb^{-1} at sqrt[s]=13.6 TeV. To enhance the sensitivity, the results are combined with those from Run 2. An excess of events over the background is observed with a significance of 3.4σ (2.5σ expected). The best-fit signal strength is μ=1.4±0.4. This result provides evidence for the H→μμ decay with ATLAS data and offers a direct probe of the Higgs-boson Yukawa coupling to second-generation fermions.

A bstract A measurement of the top-quark pole mass $$ {m}_t^{\textrm{pole}} $$ m t pole is presented in $$ t\overline{t} $$ t t ¯ events with an additional jet, $$ t\overline{t} $$ t t ¯ + 1-jet, produced in pp collisions at $$ \sqrt{s}=13 $$ s = 13 TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of 140 fb − 1 . Events with one electron and one muon of opposite electric charge in the final state are selected to measure the $$ t\overline{t} $$ t t ¯ + 1-jet differential cross-section as a function of the inverse of the invariant mass of the $$ t\overline{t} $$ t t ¯ + 1-jet system. Iterative Bayesian Unfolding is used to correct the data to enable comparison with fixed-order calculations at next-to-leading-order accuracy in the strong coupling. The process $$ pp\to t\overline{t}j\left(2\to 3\right) $$ pp → t t ¯ j 2 → 3 , where top quarks are taken as stable particles, and the process $$ pp\to b\overline{b}{l}^{+}\nu {l}^{-}\overline{\nu}j\left(2\to 7\right) $$ pp → b b ¯ l + ν l − ν ¯ j 2 → 7 , which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using a χ 2 fit of the unfolded normalized differential cross-section distribution. The results obtained with the 2 → 3 and 2 → 7 calculations are compatible within theoretical uncertainties, providing an important consistency check. The more precise determination is obtained for the 2 → 3 measurement: $$ {m}_t^{\textrm{pole}}=170.7\pm 0.3\left(\textrm{stat}.\right)\pm 1.4\left(\textrm{syst}.\right)\pm 0.3\left(\textrm{scale}\right)\pm 0.2\left(\textrm{PDF}\oplus {\alpha}_{\textrm{S}}\right) $$ m t pole = 170.7 ± 0.3 stat . ± 1.4 syst . ± 0.3 scale ± 0.2 PDF ⊕ α S GeV, which is in good agreement with other top-quark mass results.

The trigger and readout electronics for the ATLAS Thin Gap Chamber (TGC) are being upgraded to meet the requirements of the HL-LHC ATLAS trigger and data acquisition system. We have completed the production of frontend electronics and implemented key functionalities, including Hit Bunch Crossing IDentification, Hit Bitmap formation for each bunch crossing across all channels, serialization of the Hit Bitmap, timing signal distribution, clock phase adjustment, slow control, and FPGA programming. During the year end technical stop of the LHC between 2024 and 2025, we deployed a full-chain slice of the upgraded electronics in an ATLAS detector sector and successfully demonstrated its operation in the cavern. This included verification of control path functionality via optical links from the System-on-Chip device on the backend, an automated configuration scheme, timing distribution with adjustment, and readout of all channels in a sector using test pulse injections with accurate Hit Bunch Crossing IDentification. The setup also incorporated the full infrastructure — fiber network, power supply, and signal cables — identical to the final Phase-2 system. We will present the system design, detailed performance results, and insights gained from the full-system commissioning in the cavern.

Erratum: Measurement of the total and differential cross-sections of $$ t\overline{t}W $$ production in pp collisions at $$ \sqrt{s}=13 $$ TeV with the ATLAS detector