Search

This Letter presents a search for highly ionizing magnetic monopoles in 262  μb−1 of ultraperipheral Pb+Pb collision data at sNN=5.36  TeV collected by the ATLAS detector at the LHC. A new methodology that exploits the properties of clusters of hits reconstructed in the innermost silicon detector layers is introduced to study highly ionizing particles in heavy-ion data. No significant excess above the background, which is estimated using a data-driven technique, is observed. Using a nonperturbative semiclassical model, upper limits at 95% confidence level are set on the cross section for pair production of monopoles with a single Dirac magnetic charge in the mass range of 20–150 GeV. Depending on the model, monopoles with a single Dirac magnetic charge and mass below 80–120 GeV are excluded. © 2025 CERN, for the ATLAS Collaboration 2025 CERN

Abstract: A search for a light charged Higgs boson produced in decays of the top quark, $$t \rightarrow H^{\pm } b$$ t → H ± b with $$H^{\pm } \rightarrow cs$$ H ± → c s , is presented. This search targets the production of top-quark pairs $$t\bar{t} \rightarrow Wb H^{\pm } b$$ t t ¯ → W b H ± b , with $$W \rightarrow \ell u $$ W → ℓ ν ( $$\ell = e, \mu $$ ℓ = e , μ ), resulting in a lepton-plus-jets final state characterised by an isolated electron or muon and at least four jets. The search exploits b-quark and c-quark identification techniques as well as multivariate methods to suppress the dominant $$t\bar{t}$$ t t ¯ background. The data analysed correspond to $$140\hbox { fb}^{-1}$$ 140 fb - 1 of $$pp$$ pp collisions at $$\sqrt{s} = 13\hbox { TeV}$$ s = 13 TeV recorded with the ATLAS detector at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction $$\mathscr {B}(t\rightarrow H^{\pm } b)$$ B ( t → H ± b ) , assuming $$\mathscr {B}(t\rightarrow Wb) + \mathscr {B}(t \rightarrow H^{\pm } (\rightarrow cs)b)=1.0$$ B ( t → W b ) + B ( t → H ± ( → c s ) b ) = 1.0 , are set between 0.066% (0.077%) and 3.6% (2.3%) for a charged Higgs boson with a mass between 60 and 168 GeV.

A search for the production of three Higgs bosons (HHH) in the bb¯bb¯bb¯ final state is presented. The search uses 126  fb−1 of proton-proton collision data at s=13  TeV collected with the ATLAS detector at the Large Hadron Collider. The analysis targets both nonresonant and resonant production of HHH. The resonant interpretations primarily consider a cascade decay topology of X→SH→HHH with masses of the new scalars X and S up to 1.5 and 1 TeV, respectively. In addition to scenarios where S is off-shell, the nonresonant interpretation includes a search for Standard Model HHH production, with limits on the trilinear and quartic Higgs self-coupling set. No evidence for HHH production is observed. An upper limit of 59 fb is set, at the 95% confidence level, on the cross section for Standard Model HHH production. © 2025 CERN, for the ATLAS Collaboration 2025 CERN

Abstract: The high-luminosity phase of LHC operations (HL-LHC), will feature a large increase in simultaneous proton-proton interactions per bunch crossing up to 200, compared with a typical leveling target of 64 in Run 3. Such an increase will create a very challenging environment in which to perform charged particle trajectory reconstruction, a task crucial for the success of the ATLAS physics program, and will exceed the capabilities of the current ATLAS Inner Detector (ID). A new all-silicon Inner Tracker (ITk) will replace the current ID in time for the start of the HL-LHC. To ensure successful use of the ITk capabilities in Run 4 and beyond, the ATLAS tracking software has been successfully adapted to achieve state-of-the-art track reconstruction in challenging high-luminosity conditions with the ITk detector. This paper presents the expected tracking performance of the ATLAS ITk based on the latest available developments since the ITk technical design reports.

This Letter presents a constraint on the total width of the Higgs boson (ΓH) using a combined measurement of on-shell Higgs boson production and the production of four top quarks, which involves contributions from off-shell Higgs boson-mediated processes. This method relies on the assumption that the tree-level Higgs-top Yukawa coupling strength is the same for on-shell and off-shell Higgs boson production processes, thereby avoiding any assumptions about the relationship between on-shell and off-shell gluon fusion Higgs production rates, which were central to previous measurements. The result is based on up to 140 fb−1 of proton–proton collisions at a centre-of-mass energy of s = 13 TeV collected with the ATLAS detector at the Large Hadron Collider. The observed (expected) 95% confidence level upper limit on ΓH is 450 MeV (75 MeV). Additionally, considering the constraint on the Higgs-top Yukawa coupling from loop-induced Higgs boson production and decay processes further yields an observed (expected) upper limit of 160 MeV (55 MeV).

Abstract : This paper presents measurements of top-antitop quark pair ( $$ t\overline{t} $$ t t ¯ ) production in association with additional b-jets. The analysis utilises 140 fb −1 of proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Fiducial cross-sections are extracted in a final state featuring one electron and one muon, with at least three or four b-jets. Results are presented at the particle level for both integrated cross-sections and normalised differential cross-sections, as functions of global event properties, jet kinematics, and b-jet pair properties. Observable quantities characterising b-jets originating from the top quark decay and additional b-jets are also measured at the particle level, after correcting for detector effects. The measured integrated fiducial cross-sections are consistent with $$ t\overline{t}b\overline{b} $$ t t ¯ b b ¯ predictions from various next-to-leading-order matrix element calculations matched to a parton shower within the uncertainties of the predictions. State-of-the-art theoretical predictions are compared with the differential measurements; none of them simultaneously describes all observables. Differences between any two predictions are smaller than the measurement uncertainties for most observables.

Abstract : A search is performed for light, spin-0 bosons decaying into two photons in the 66 to 110 GeV mass range, using 140 fb −1 of proton-proton collisions at $$ \sqrt{s} $$ s = 13 TeV produced by the Large Hadron Collider and collected by the ATLAS detector. Multivariate analysis techniques are used to define event categories that improve the sensitivity to new resonances beyond the Standard Model. A model-independent search for a generic spin-0 particle and a model-dependent search for an additional low-mass Higgs boson are performed in the diphoton invariant mass spectrum. No significant excess is observed in either search. Mass-dependent upper limits at the 95% confidence level are set in the model-independent scenario on the fiducial cross-section times branching ratio into two photons in the range of 8 fb to 53 fb. Similarly, in the model-dependent scenario upper limits are set on the total cross-section times branching ratio into two photons as a function of the Higgs boson mass in the range of 19 fb to 102 fb.

Inclusive cross-sections for top-quark pair production in association with charm quarks are measured with proton–proton collision data at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 140 fb−1, collected with the ATLAS experiment at the LHC between 2015 and 2018. The measurements are performed by requiring one or two charged leptons (electrons and muons), two b-tagged jets, and at least one additional jet in the final state. A custom flavor-tagging algorithm is employed for the simultaneous identification of b-jets and c-jets. In a fiducial phase space that replicates the acceptance of the ATLAS detector, the cross-sections for tt¯+≥2c and tt¯+1c production are measured to be 1.28−0.24+0.27pb and 6.4−0.9+1.0pb, respectively. The measurements are primarily limited by uncertainties in the modeling of inclusive tt¯ and tt¯+bb¯ production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various tt¯ simulations are largely consistent with the measured cross-section values, though all underpredict the observed values by 0.5 to 2.0 standard deviations. In a phase-space volume without requirements on the tt¯ decay products and the jet multiplicity, the cross-section ratios of tt¯+≥2c and tt¯+1c to total tt¯+jets production are determined to be (1.23±0.25)% and (8.8±1.3)%.

A combination of searches for the single production of vectorlike top quarks (T) is presented. These analyses are based on proton-proton collisions at s=13  TeV recorded in 2015–2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 139  fb−1. The T decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a Z boson (T→Ht and T→Zt). The individual searches used in the combination are differentiated by the number of leptons (e, μ) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vectorlike top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vectorlike top quark SU(2) singlet with electroweak coupling, κ, of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date. © 2025 CERN, for the ATLAS Collaboration 2025 CERN

A combination of searches for singly and doubly charged Higgs bosons, H± and H±±, produced via vector-boson fusion is performed using 140 fb−1 of proton–proton collisions at a centre-of-mass energy of 13 TeV, collected with the ATLAS detector during Run 2 of the Large Hadron Collider. Searches targeting decays to massive vector bosons in leptonic final states (electrons or muons) are considered. New constraints are reported on the production cross-section times branching fraction for charged Higgs boson masses between 200 GeV and 3000 GeV. The results are interpreted in the context of the Georgi-Machacek model for which the most stringent constraints to date are set for the masses considered in the combination.

Keeping in consideration the expected need of node densification of heterogeneous cellular networks in near future, it is imperative that more accurate path loss models be used when analyzing heterogeneous cellular networks performance in terms of user equipment association probability, coverage probability and spectral efficiency. In this paper, we have derived the generalized user equipment association probabilitiesâ expressions of a two-tier Dense Heterogeneous Cellular Network incorporating Downlink Uplink Decoupled technique using multi-slope path loss model, which incorporates the effect of physical environment on the path loss based on separation between transmitter and receiver. For analyzing network performance, we have considered dual-slope and tri-slope path loss models as special cases. The derived analytical expressions have been validated through network simulations and found in good agreement. The results have also been compared with conventional single-slope path loss model and it has been found that the decoupled uplink downlink association probability is higher when incorporating multi-slope path loss model as compared to single-slope path loss model.

Z boson events at the Large Hadron Collider can be selected with high purity and are sensitive to a diverse range of QCD phenomena. As a result, these events are often used to probe the nature of the strong force, improve Monte Carlo event generators, and search for deviations from standard model predictions. All previous measurements of Z boson production characterize the event properties using a small number of observables and present the results as differential cross sections in predetermined bins. In this analysis, a machine learning method called omnifold is used to produce a simultaneous measurement of twenty-four Z+jets observables using 139  fb^{-1} of proton-proton collisions at sqrt[s]=13  TeV collected with the ATLAS detector. Unlike any previous fiducial differential cross-section measurement, this result is presented unbinned as a dataset of particle-level events, allowing for flexible reuse in a variety of contexts and for new observables to be constructed from the twenty-four measured observables.

Introduction: Emergency physicians have the highest rates of burnout among our physician peers, with prior literature suggesting clinician schedules can play a significant role in burnout. We assessed our transition from a tenure- and age-based paradigm to an egalitarian, night shift buy-out program that allows schedule flexibility for physicians at all stages of their careers.Methods: The night shift buy-out program was implemented in the emergency department (ED) of an academic, quaternary-care center that treats approximately 100,000 adult patients annually with 56 faculty emergency physicians. We sought to create a cost-neutral program, carefully balancing incentives between nocturnists and those wanting to reduce allotted night shifts. Ultimately, the program was designed to allow all faculty to buy out of any number of nights for $500 per night shift, with the funds generated used to increase nocturnist salaries. We analyzed two years of the program (July 2022–June 2024) to assess trends in night shift buy-outs, the primary outcome. We also conducted an all-faculty survey after the program’s first year to gauge sentiments about the program.Results: Over two years, 22 faculty (42%) fully bought out of nights; an additional 10 (15%) bought out of some nights. By year two, the program could grant all faculty their preferred night-shift allotment. Faculty who bought out fully had worked longer in EM on average, worked fewer clinical hours per year, were more likely to be associate/full professors, and were less likely to be women. Nocturnists had the highest mean clinical hours of the four groups, had the lowest average tenure, and were least likely to be associate/full professors. A total of 86% of faculty responded to the survey, to which more than 80% of those buying out reported that reducing the night-shift burden was either “very important” or “critical for continuing in this job.”Conclusion: Our academic ED transitioned from a tenure- and age-based, overnight shift paradigm to an egalitarian buy-out program that allows physicians flexibility at all career stages. This approach could improve career satisfaction and reduce burnout among emergency physicians.

Abstract: Properties of the underlying-event in pp interactions are investigated primarily via the strange hadrons $$K_{S}^{0}$$ K S 0 , $$\Lambda $$ Λ and $$\bar{\Lambda }$$ Λ ¯ , as reconstructed using the ATLAS detector at the LHC in minimum-bias pp collision data at $$\sqrt{s} = 13$$ s = 13 TeV. The hadrons are reconstructed via the identification of the displaced two-particle vertices corresponding to the decay modes "Equation missing", $$\Lambda \rightarrow \pi ^-p$$ Λ → π - p and $$\bar{\Lambda }\rightarrow \pi ^+\bar{p}$$ Λ ¯ → π + p ¯ . These are used in the construction of underlying-event observables in azimuthal regions computed relative to the leading charged-particle jet in the event. None of the hadronisation and underlying-event physics models considered can describe the data over the full kinematic range considered. Events with a leading charged-particle jet in the range of $$10 < p_T \le 40$$ 10 < p T ≤ 40 GeV are studied using the number of prompt charged particles in the transverse region. The ratio $$N(\Lambda + \bar{\Lambda })/N(K_{S}^{0})$$ N ( Λ + Λ ¯ ) / N ( K S 0 ) as a function of the number of such charged particles varies only slightly over this range. This disagrees with the expectations of some of the considered Monte Carlo models.

Abstract: The ATLAS tile calorimeter (TileCal) is the hadronic sampling calorimeter covering the central region of the ATLAS detector at the Large Hadron Collider (LHC). This paper gives an overview of the calorimeter’s operation and performance during the years 2015–2018 (Run 2). In this period, ATLAS collected proton–proton collision data at a centre-of-mass energy of 13 TeV and the TileCal was 99.65% efficient for data-taking. The signal reconstruction, the calibration procedures, and the detector operational status are presented. The performance of two ATLAS trigger systems making use of TileCal information, the minimum-bias trigger scintillators and the tile muon trigger, is discussed. Studies of radiation effects allow the degradation of the output signals at the end of the LHC and HL-LHC operations to be estimated. Finally, the TileCal response to isolated muons, hadrons and jets from proton–proton collisions is presented. The energy and time calibration methods performed excellently, resulting in good stability and uniformity of the calorimeter response during Run 2. The setting of the energy scale was performed with an uncertainty of 2%. The results demonstrate that the performance is in accordance with specifications defined in the Technical Design Report.

Abstract: Proton–proton collision data recorded by the ATLAS detector in 2011, at a centre-of-mass energy of 7 TeV, have been used for an improved determination of the W-boson mass and a first measurement of the W-boson width at the LHC. Recent fits to the proton parton distribution functions are incorporated in the measurement procedure and an improved statistical method is used to increase the measurement precision. The measurement of the W-boson mass yields a value of $$m_W = 80{,}366.5 \pm 9.8~(\text {stat.}) \pm 12.5~(\text {syst.})$$ m W = 80 , 366.5 ± 9.8 ( stat. ) ± 12.5 ( syst. )  MeV  $$= 80{,}366.5 \pm 15.9$$ = 80 , 366.5 ± 15.9  MeV, and the width is measured as $$\Gamma _W = 2202 \pm 32~(\text {stat.}) \pm 34~(\text {syst.})$$ Γ W = 2202 ± 32 ( stat. ) ± 34 ( syst. )  MeV  $$= 2202 \pm 47$$ = 2202 ± 47  MeV. The first uncertainty components are statistical and the second correspond to the experimental and physics-modelling systematic uncertainties. Both results are consistent with the expectation from fits to electroweak precision data. The present measurement of $$m_W$$ m W is compatible with and supersedes the previous measurement performed using the same data.

High-energy nuclear collisions create a quark-gluon plasma, whose initial condition and subsequent expansion vary from event to event, impacting the distribution of the eventwise average transverse momentum [P([p_{T}])]. Disentangling the contributions from fluctuations in the nuclear overlap size (geometrical component) and other sources at a fixed size (intrinsic component) remains a challenge. This problem is addressed by measuring the mean, variance, and skewness of P([p_{T}]) in ^{208}Pb+^{208}Pb and ^{129}Xe+^{129}Xe collisions at sqrt[s_{NN}]=5.02 and 5.44 TeV, respectively, using the ATLAS detector at the LHC. All observables show distinct features in ultracentral collisions, which are explained by a suppression of the geometrical component as the overlap area reaches its maximum. These results demonstrate a new technique to separate geometrical and intrinsic fluctuations, providing constraints on initial conditions and properties of the quark-gluon plasma, such as the speed of sound.

Abstract : During the 2015–2018 data-taking period, the Large Hadron Collider delivered proton-proton bunch crossings at a centre-of-mass energy of 13 TeV to the ATLAS experiment at a rate of roughly 30 MHz, where each bunch crossing contained an average of 34 independent inelastic proton-proton collisions. The ATLAS trigger system selected roughly 1 kHz of these bunch crossings to be recorded to disk. Offline algorithms then identify one of the recorded collisions as the collision of interest for subsequent data analysis, and the remaining collisions are referred to as pile-up. Pile-up collisions represent a trigger-unbiased dataset, which is evaluated to have an integrated luminosity of 1.33 pb−1 in 2015–2018. This is small compared with the normal trigger-based ATLAS dataset, but when combined with vertex-by-vertex jet reconstruction it provides up to 50 times more dijet events than the conventional single-jet-trigger-based approach, and does so without adding any additional cost or requirements on the trigger system, readout, or storage. The pile-up dataset is validated through comparisons with a special trigger-unbiased dataset recorded by ATLAS, and its utility is demonstrated by means of a measurement of the jet energy resolution in dijet events, where the statistical uncertainty is significantly reduced for jet transverse momenta below 65 GeV.

A search for heavy right-handed Majorana neutrinos is performed with the ATLAS detector at the CERN Large Hadron Collider, using the (Formula Presented) of proton-proton collision data at (Formula Presented) collected during Run 2. This search targets (Formula Presented) production, in which both top quarks decay into a bottom quark and a (Formula Presented) boson, where one of the (Formula Presented) bosons decays hadronically and the other decays into an electron or muon and a heavy neutral lepton. The heavy neutral lepton is identified through a decay into an electron or muon and another (Formula Presented) boson, resulting in a pair of same-charge same-flavor leptons in the final state. This paper presents the first search for heavy neutral leptons in the mass range of 15-75 GeV using (Formula Presented) events. No significant excess is observed over the background expectation, and upper limits are placed on the signal cross sections. Assuming a benchmark scenario of the phenomenological type-I seesaw model, these cross section limits are then translated into upper limits on the mixing parameters of the heavy Majorana neutrino with Standard Model neutrinos.

This Letter presents a differential cross-section measurement of Lund subjet multiplicities, suitable for testing current and future parton shower Monte Carlo algorithms. This measurement is made in dijet events in 140 fb−1 of s=13 TeV proton–proton collision data collected with the ATLAS detector at CERN's Large Hadron Collider. The data are unfolded to account for acceptance and detector-related effects, and are then compared with several Monte Carlo models and to recent resummed analytical calculations. The experimental precision achieved in the measurement allows tests of higher-order effects in QCD predictions. Most predictions fail to accurately describe the measured data, particularly at large values of jet transverse momentum accessible at the Large Hadron Collider, indicating the measurement's utility as an input to future parton shower developments and other studies probing fundamental properties of QCD and the production of hadronic final states up to the TeV-scale.

Charged particle reconstruction in the presence of many simultaneous proton–proton (pp) collisions in the LHC is a challenging task for the ATLAS experiment’s reconstruction software due to the combinatorial complexity. This paper describes the major changes made to adapt the software to reconstruct high-activity collisions with an average of 50 or more simultaneous pp interactions per bunch crossing (pile-up) promptly using the available computing resources. The performance of the key components of the track reconstruction chain and its dependence on pile-up are evaluated, and the improvement achieved compared to the previous software version is quantified. For events with an average of 60pp collisions per bunch crossing, the updated track reconstruction is twice as fast as the previous version, without significant reduction in reconstruction efficiency and while reducing the rate of combinatorial fake tracks by more than a factor two.

The need for large-scale production of highly accurate simulated event samples for the extensive physics programme of the ATLAS experiment at the Large Hadron Collider motivates the development of new simulation techniques. Building on the recent success of deep learning algorithms, variational autoencoders and generative adversarial networks are investigated for modelling the response of the central region of the ATLAS electromagnetic calorimeter to photons of various energies. The properties of synthesised showers are compared with showers from a full detector simulation using geant4. Both variational autoencoders and generative adversarial networks are capable of quickly simulating electromagnetic showers with correct total energies and stochasticity, though the modelling of some shower shape distributions requires more refinement. This feasibility study demonstrates the potential of using such algorithms for ATLAS fast calorimeter simulation in the future and shows a possible way to complement current simulation techniques.

Abstract : A search is presented for new particles produced in proton-proton collisions at a centre-of-mass energy of 13 TeV that result in final states comprising a massive vector (W or Z) boson that decays hadronically and large missing transverse momentum. The data sample was collected with the ATLAS experiment at the Large Hadron Collider from 2015 to 2018 and corresponds to an integrated luminosity of 140 fb−1. No significant excess over the Standard Model expectation is observed. Model-independent 95% confidence-level limits on the visible cross-section that range from 0.3 fb to 79.5 fb are obtained for non-Standard-Model processes. Exclusion limits are also presented for models with axion-like particles, for two-Higgs-doublet models with a pseudo-scalar mediator between the Standard Model and the dark sector, for the invisible decay of the Higgs boson and for pair-produced weakly interacting dark matter candidates.

This paper describes a measurement of the jet radius dependence of the dijet momentum balance between leading back-to-back jets in Formula Presented of Formula Presented collisions collected in 2018 and Formula Presented of Formula Presented collisions collected in 2017 by the ATLAS detector at the LHC. Both datasets were collected at Formula Presented TeV. Jets are reconstructed using the anti-Formula Presented algorithm with jet radius parameters Formula Presented, 0.3, 0.4, 0.5, and 0.6. The dijet momentum balance distributions are constructed for leading jets with transverse momentum Formula Presented from 100 to 562 GeV for Formula Presented, 0.3, and 0.4 jets, and from 158 to 562 GeV for Formula Presented and 0.6 jets. The absolutely normalized dijet momentum balance distributions are constructed to compare measurements of the dijet yields in Formula Presented collisions directly to the dijet cross sections in Formula Presented collisions. For all jet radii considered here, there is a suppression of more balanced dijets in Formula Presented collisions compared with Formula Presented collisions, while for more imbalanced dijets there is an enhancement. There is a jet radius dependence to the dijet yields, being stronger for more imbalanced dijets than for more balanced dijets. Additionally, jet pair nuclear modification factors are measured. The subleading jet yields are found to be more suppressed than leading jet yields in dijets. A jet radius dependence of the pair nuclear modification factors is observed, with the suppression decreasing with increasing jet radius. These measurements provide new constraints on jet quenching scenarios in the quark-gluon plasma.

A search is presented for direct pair production of the stop, the supersymmetric partner of the top quark, in a decay through an Formula Presented-parity violating coupling to a charged lepton and a Formula Presented-quark. The dataset corresponds to an integrated luminosity of Formula Presented of proton-proton collisions at a center-of-mass energy of Formula Presented collected between 2015 and 2018 by the ATLAS detector at the LHC. The final state has two charged leptons (electrons or muons) and two Formula Presented-jets. The results of the search are interpreted in the context of a Minimal Supersymmetric Standard Model with an additional Formula Presented gauge symmetry that is spontaneously broken. No significant excess is observed over the Standard Model background, and exclusion limits on stop pair production are set at 95% confidence level. The corresponding lower limits on the stop mass for 100% branching ratios to a Formula Presented-quark and an electron, muon, or tau-lepton are 1.9 TeV, 1.8 TeV and 800 GeV, respectively, extending the reach of previous LHC searches.