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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 145 records · Page 8Linked to original sources

arXiv2025

Search for long-lived charged particles using large specific ionisation loss and time of flight in 140 $fb^{-1}$ of $pp$ collisions at $\sqrt{s}\ = 13$ TeV with the ATLAS detector

This paper presents a search for massive, charged, long-lived particles with the ATLAS detector at the Large Hadron Collider using an integrated luminosity of 140 $fb^{-1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV. These particles are expected to move significantly slower than the speed of light. In this paper, two signal regions provide complementary sensitivity. In one region, events are selected with at least one charged-particle track with high transverse momentum, large specific ionisation measured in the pixel detector, and time of flight to the hadronic calorimeter inconsistent with the speed of light. In the other region, events are selected with at least two tracks of opposite charge which both have a high transverse momentum and an anomalously large specific ionisation. The search is sensitive to particles with lifetimes greater than about 3 ns with masses ranging from 200 GeV to 3 TeV. The results are interpreted to set constraints on the supersymmetric pair production of long-lived R-hadrons, charginos and staus, with mass limits extending beyond those from previous searches in broad ranges of lifetime.

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arXiv2025

Search for the production of a Higgs boson in association with a single top quark in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for the production of a Higgs boson in association with a single top quark, $tH$, is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of $140\mathrm{fb}^{-1}$ at a centre-of-mass energy of $13$ TeV, collected by the ATLAS detector at the LHC. The search targets Higgs-boson decays into $b\bar{b}$, $WW^{*}$, $ZZ^{*}$, and $ττ$, accompanied by an isolated lepton (electron or muon) from the top-quark decay. Multivariate techniques are employed to enhance the separation between signal and background processes. The observed signal strength, $μ_{tH}$, defined as the ratio between the measured cross-section and the predicted Standard Model value, is $μ_{tH}~=~8.1~\pm~2.6~\mathrm{(stat.)}~\pm~2.0~\mathrm{(syst.)}$. The significance of the observed (expected) signal above the background-only expectation is $2.8$ ($0.4$) standard deviations. The corresponding observed (expected) upper limit at the 95% confidence level on the $tH$ cross-section is found to be $13.9$ ($6.1$) times the value predicted by the Standard Model. An interpretation with an inverted sign of the top-quark Yukawa coupling is performed, and the signal strength and corresponding limit are reported.

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arXiv2025

Measurements of $W^+W^-$ production cross-sections in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Measurements of $W^+W^-\rightarrow e^\pm νμ^\mp ν$ production cross-sections are presented, providing a test of the predictions of perturbative quantum chromodynamics and the electroweak theory. The measurements are based on data from $pp$ collisions at $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the Large Hadron Collider in 2015-2018, corresponding to an integrated luminosity of 140 fb$^{-1}$. The number of events due to top-quark pair production, the largest background, is reduced by rejecting events containing jets with $b$-hadron decays. An improved methodology for estimating the remaining top-quark background enables a precise measurement of $W^+W^-$ cross-sections with no additional requirements on jets. The fiducial $W^+W^-$ cross-section is determined in a maximum-likelihood fit with an uncertainty of 3.1%. The measurement is extrapolated to the full phase space, resulting in a total $W^+W^-$ cross-section of $127\pm4$ pb. Differential cross-sections are measured as a function of twelve observables that comprehensively describe the kinematics of $W^+W^-$ events. The measurements are compared with state-of-the-art theory calculations and excellent agreement with predictions is observed. A charge asymmetry in the lepton rapidity is observed as a function of the dilepton invariant mass, in agreement with the Standard Model expectation. A CP-odd observable is measured to be consistent with no CP violation. Limits on Standard Model effective field theory Wilson coefficients in the Warsaw basis are obtained from the differential cross-sections.

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arXiv2025

Search for $t\bar{t}H/A \rightarrow t\bar{t}t\bar{t}$ production in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search is presented for a heavy scalar ($H$) or pseudo-scalar ($A$) predicted by the two-Higgs-doublet models, where the $H/A$ is produced in association with a top-quark pair ($t\bar{t}H/A$), and with the $H/A$ decaying into a $t\bar{t}$ pair. Events are selected requiring exactly one or two opposite-charge electrons or muons. Data-driven corrections are applied to improve the modelling of the $t\bar{t}$+jets background in the regime with high jet and $b$-jet multiplicities. These include a novel multi-dimensional kinematic reweighting based on a neural network trained using data and simulations. An $H/A$-mass parameterised graph neural network is trained to optimise the signal-to-background discrimination. In combination with the previous search performed by the ATLAS Collaboration in the multilepton final state, the observed upper limits on the $t\bar{t}H/A \rightarrow t\bar{t}t\bar{t}$ production cross-section at 95% confidence level range between 14 fb and 5.0 fb for an $H/A$ with mass between 400 GeV and 1000 GeV, respectively. Assuming that both the $H$ and $A$ contribute to the $t\bar{t}t\bar{t}$ cross-section, $\tanβ$ values below 1.7 or 0.7 are excluded for a mass of 400 GeV or 1000 GeV, respectively. The results are also used to constrain a model predicting the pair production of a colour-octet scalar, with the scalar decaying into a $t\bar{t}$ pair.

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arXiv2025

Search for cascade decays of charged sleptons and sneutrinos in final states with three leptons and missing transverse momentum in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for cascade decays of charged sleptons and sneutrinos using final states characterized by three leptons (electrons or muons) and missing transverse momentum is presented. The analysis is based on a dataset with 140 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of $\sqrt{s}$=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. This paper focuses on a supersymmetric scenario that is motivated by the muon anomalous magnetic moment observation, dark mattter relic density abundance, and electroweak naturalness. A mass spectrum involving light higgsinos and heavier sleptons with a bino at intermediate mass is targeted. No significant deviation from the Standard Model expectation is observed. This search enables to place stringent constraints on this model, excluding at the 95% confidence level charged slepton and sneutrino masses up to 450 GeV when assuming a lightest neutralino mass of 100 GeV and mass-degenerate selectrons, smuons and sneutrinos.

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arXiv2025

A search for dark matter produced in association with a dark Higgs boson decaying into a Higgs boson pair in 3$b$ or 4$b$ final states using $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search is performed for dark matter particles produced in association with a resonant pair of Higgs bosons using 140 fb$^{-1}$ of proton--proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. This signature is expected in some extensions of the Standard Model predicting the production of dark matter particles, and is interpreted in terms of a dark Higgs model containing a $Z'$ mediator in which the dark Higgs boson $s$ decays into a pair of Higgs bosons. The dark Higgs boson is reconstructed through final states with at least three $b$-tagged jets, produced by the pair of Higgs boson decays, in events with significant missing transverse momentum consistent with the presence of dark matter. The observed data are found to be in good agreement with Standard Model predictions, constraining scenarios with dark Higgs boson masses within the range of 250 to 400 GeV and $Z'$ mediators up to 2.3 TeV.

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arXiv2025

A search for electroweak $t\bar{t}Wj$ production in multileptonic final states at $\sqrt{s} = 13$ TeV with the ATLAS detector and bounds on effective field theory operators

A search is presented for the electroweak production of a top quark pair in association with a $W$ boson and at least one additional jet, known as the ${t\bar{t}W\kern-0.2em j_{\mathrm{EW}}}$ process. This process has embedded within it a $tW$-scattering vertex, which is probed directly for the first time. The collision data were collected with the ATLAS detector during Run 2 of the LHC and correspond to an integrated luminosity of 140 $\mathrm{fb^{-1}}$ at $\sqrt{s}=13$~TeV. The search uses same-charge pairs of electrons and muons together with jets, of which at least one is $b$-tagged. The properties of the most forward jet relative to the rest of the event are used to discriminate the electroweak ${t\bar{t}W\kern-0.2em j_{\text{EW}}}$ production process from its strong $t\bar{t}W$ production counterpart. A measured (expected) 95% CL upper limit on the cross section is set at $σ_{{t\bar{t}W\kern-0.2em j_{\mathrm{EW}}}} < 251$~fb (230 fb), to be compared with the expected SM cross section of 47.7 fb. Limits are set on the SM Effective Field Theory (EFT) operators $O_{Ht}$ and $O^{(1)}_{HQ}$, which modify the electroweak couplings of the top quark through contributions to the $tW$-scattering vertex. The interpretation acts as a case study to emphasize the importance of energy-dependent sensitivity, multi-process and multi-operator EFT contributions.

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arXiv2025

Measurement of $t$-channel single-top-quark production in $pp$ collisions at $\sqrt{s} = 5.02$ TeV with the ATLAS detector

The observation of the electroweak production of single-top-quarks is made using 255 $\text{pb}^{-1}$ of proton-proton collision data recorded at $\sqrt{s} = 5.02$ TeV with the ATLAS detector at the Large Hadron Collider. An event selection is used to identify single-top-quark candidates arising from $t$-channel production with the top quark decaying semi-leptonically. Events passing the selection are then used to measure the inclusive cross-section for the combined production of single-top-quarks and antiquarks, $σ({tq + \bar{t}q})$, and the ratio $R_t$ between these two. They are measured to be $σ({tq + \bar{t}q}) = 27.1^{+4.4}_{-4.1} (\text{stat.}) ^{+4.4}_{-3.7} (\text{syst.})$ pb and $R_t = 2.73^{+1.43}_{-0.82} (\text{stat.}) ^{+1.01}_{-0.29} (\text{syst.})$. The individual single-top-quark ($t q$) and single-top-antiquark ($\bar{t} q$) production cross-sections are measured to be $σ(t q) = 19.8^{+3.9}_{-3.1} (\text{stat.}) ^{+2.9}_{-2.2} (\text{syst.})$ pb and $σ(\bar{t}q) = 7.3^{+3.2}_{-2.1} (\text{stat.}) ^{+2.8}_{-1.5} (\text{syst.})$ pb. All measurements are in good agreement with the Standard Model predictions.

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arXiv2025

Azimuthal anisotropies of charged particles with high transverse momentum in Pb+Pb collisions at $\sqrt{s_{_\text{NN}}} = 5.02$ TeV with the ATLAS detector

A measurement is presented of elliptic ($v_2$) and triangular ($v_3$) azimuthal anisotropy coefficients for charged particles produced in Pb+Pb collisions at $\sqrt{s_{_\text{NN}}} = 5.02$ TeV using a dataset corresponding to an integrated luminosity of $0.44$ nb$^{-1}$ collected with the ATLAS detector at the LHC in 2018. The values of $v_2$ and $v_3$ are measured for charged particles over a wide range of transverse momentum ($p_\text{T}$), 1-400 GeV, and Pb+Pb collision centrality, 0-60%, using the scalar product and multiparticle cumulant methods. These methods are sensitive to event-by-event fluctuations and nonflow effects in the measurements of azimuthal anisotropies. Positive values of $v_2$ are observed up to a $p_{\text{T}}$ of approximately 100 GeV from both methods across all centrality intervals. Positive values of $v_3$ are observed up to approximately 25 GeV using both methods, though the application of the three-subevent technique to the multiparticle cumulant method leads to significant changes at the highest $p_{\text{T}}$. At high $p_{\text{T}}$ ($p_{\text{T}} \gtrapprox 10$ GeV), charged particles are dominantly from jet fragmentation. These jets, and hence the measurements presented here, are sensitive to the path-length dependence of parton energy loss in the quark-gluon plasma produced in Pb+Pb collisions.

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arXiv2025

A precise measurement of the jet energy scale derived from single-particle measurements and in situ techniques in proton-proton collisions at $\sqrt{s}=$ 13 TeV with the ATLAS detector

The jet energy calibration and its uncertainties are derived from measurements of the calorimeter response to single particles in both data and Monte Carlo simulation using proton-proton collisions at $\sqrt{s} = 13$ TeV collected with the ATLAS detector during Run 2 at the Large Hadron Collider. The jet calibration uncertainty for anti-k$_t$ jets with a jet radius parameter of $R = 0.4$ and in the central jet rapidity region is about 2.5% for transverse momenta ($p_T$) of 20 GeV, about 0.5% for $p_T = 300$ GeV and 0.7% for $p_T = 4$ TeV. Excellent agreement is found with earlier determinations obtained from $p_T$-balance based in situ methods ($Z/γ$+jets). The combination of these two independent methods results in the most precise jet energy measurement achieved so far with the ATLAS detector with a relative uncertainty of 0.3% at $p_T = 300$ GeV and 0.6% at $4$ TeV. The jet energy calibration is also derived with the single-particle calorimeter response measurements separately for quark- and gluon-induced jets and furthermore for jets with $R$ varying from 0.2 to 1.0 retaining the correlations between these measurements. Differences between inclusive jets and jets from boosted top-quark decays, with and without grooming the soft jet constituents, are also studied.

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arXiv2025

Search for emerging jets in $pp$ collisions at $\sqrt{s} = 13.6$ TeV with the ATLAS experiment

A search for emerging jets is presented using 51.8 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 13.6$ TeV, collected by the ATLAS experiment during 2022 and 2023. The search explores a hypothetical dark sector featuring 'dark quarks', which are charged under a confining gauge group and couple to the Standard Model via a new mediator particle. These dark quarks undergo showering and hadronization within the dark sector, forming long-lived dark mesons that decay back into Standard Model particles. This results in jets which contain multiple displaced vertices known as emerging jets. The analysis targets events with pairs of emerging jets, produced either through a vector mediator, $Z'$, in the $s$-channel, or a scalar mediator, $Φ$, in the $t$-channel. No significant excess over the Standard Model background is observed. Assuming a dark pion proper decay length between 5 and 50 mm, $Z'$ mediator masses between 600 and 2550 GeV are excluded for quark and dark quark coupling values of 0.01 and 0.1, respectively. For a quark-dark quark coupling of $0.1$, $Φ$ mediator masses between 600 and 1375 GeV are excluded. These results provide the first direct constraints on emerging jet pair production via a $Z'$ mediator, and represent the first search to investigate emerging jet production via $t$-channel exchange of a scalar mediator.

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arXiv2025

Search for vector-like leptons coupling to first- and second-generation Standard Model leptons in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for pair production of vector-like leptons coupling to first- and second-generation Standard Model leptons is presented. The search is based on a dataset of proton-proton collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb$^{-1}$. Events are categorised depending on the flavour and multiplicity of leptons (electrons or muons), as well as on the scores of a deep neural network targeting particular signal topologies according to the decay modes of the vector-like leptons. In each of the signal regions, the scalar sum of the transverse momentum of the leptons and the missing transverse momentum is analysed. The main background processes are estimated using dedicated control regions in a simultaneous fit with the signal regions to data. No significant excess above the Standard Model background expectation is observed and limits are set at 95% confidence level on the production cross-sections of vector-like electrons and muons as a function of the vector-like lepton mass, separately for SU(2) doublet and singlet scenarios. The resulting mass lower limits are 1220 GeV (1270 GeV) and 320 GeV (400 GeV) for vector-like electrons (muons) in the doublet and singlet scenarios, respectively.

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arXiv2025

Search for new physics in final states with semi-visible jets or anomalous signatures using the ATLAS detector

A search is presented for hadronic signatures of beyond the Standard Model (BSM) physics, with an emphasis on signatures of a strongly-coupled hidden dark sector accessed via resonant production of a $Z'$ mediator. The ATLAS experiment dataset collected at the Large Hadron Collider from 2015 to 2018 is used, consisting of proton-proton collisions at $\sqrt{s}$ = 13 TeV and corresponding to an integrated luminosity of 140 fb$^{-1}$. The $Z'$ mediator is considered to decay to two dark quarks, which each hadronize and decay to showers containing both dark and Standard Model particles, producing a topology of interacting and non-interacting particles within a jet known as ``semi-visible". Machine learning methods are used to select these dark showers and reject the dominant background of mismeasured multijet events, including an anomaly detection approach to preserve broad sensitivity to a variety of BSM topologies. A resonance search is performed by fitting the transverse mass spectrum based on a functional form background estimation. No significant excess over the expected background is observed. Results are presented as limits on the production cross section of semi-visible jet signals, parameterized by the fraction of invisible particles in the decay and the $Z'$ mass, and by quantifying the significance of any generic Gaussian-shaped mass peak in the anomaly region.

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arXiv2025

Searches for direct slepton production in the compressed-mass corridor in $\sqrt{s}=13$ TeV $pp$ collisions with the ATLAS detector

This paper presents searches for the direct pair production of charged light-flavour sleptons, each decaying into a stable neutralino and an associated Standard Model lepton. The analyses focus on the challenging ``corridor'' region, where the mass difference, $Δm$, between the slepton ($\tilde{e}$ or $\tildeμ$) and the lightest neutralino ($\tildeχ^{0}_{1}$) is less or similar to the mass of the $W$ boson, $m(W)$, with the aim to close a persistent gap in sensitivity to models with $Δm \lesssim m(W)$. Events are required to contain a high-energy jet, significant missing transverse momentum, and two same-flavour opposite-sign leptons ($e$ or $μ$). The analysis uses $pp$ collision data at $\sqrt{s} = 13$ TeV recorded by the ATLAS detector, corresponding to an integrated luminosity of 140 fb$^{-1}$. Several kinematic selections are applied, including a set of boosted decision trees. These are each optimised for different $Δm$ to provide expected sensitivity for the first time across the full $Δm$ corridor. The results are generally consistent with the Standard Model, with the most significant deviations observed with a local significance of 2.0 $σ$ in the selectron search, and 2.4 $σ$ in the smuon search. While these deviations weaken the observed exclusion reach in some parts of the signal parameter space, the previously present sensitivity gap to this corridor is largely reduced. Constraints at the 95% confidence level are set on simplified models of selectron and smuon pair production, where selectrons (smuons) with masses up to 300 (350) GeV can be excluded for $Δm$ between 2 GeV and 100 GeV.

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arXiv2025

Measurements of the production cross-sections of a Higgs boson in association with a vector boson and decaying into $WW^\ast$ with the ATLAS detector at $\sqrt{s} = 13$ TeV

Measurements of the total and differential Higgs boson production cross-sections, via $WH$ and $ZH$ associated production using $H\rightarrow WW^\ast\rightarrow\ellν\ellν$ and $H\rightarrow WW^\ast\rightarrow\ellνjj$ decays, are presented. The analysis uses proton-proton events delivered by the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector between 2015 and 2018. The data correspond to an integrated luminosity of 140 fb$^{-1}$. The sum of the $WH$ and $ZH$ cross-sections times the $H\rightarrow WW^\ast$ branching fraction is measured to be $0.44^{+0.10}_{-0.09}$ (stat.) $^{+0.06}_{-0.05}$ (syst.) pb, in agreement with the Standard Model prediction. Higgs boson production is further characterised through measurements of the differential cross-section as a function of the transverse momentum of the vector boson and in the framework of Simplified Template Cross-Sections.

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arXiv2025

Energy scale and resolution for anti-$k_t$ jets with radius parameters $R=0.2$ and 0.6 measured in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Jets with different radius parameters $R$ are an important tool to probe quantum chromodynamics processes at different angular scales. Jets with small $R=0.2$ are instrumental in measurements of the substructure of large-$R$ jets resulting from collimated hadronic decays of energetic $W$, $Z$, and Higgs bosons, top quarks, and of potential new resonances. Measurements are presented of the energy scale and resolution of jets with radius parameters $R=0.2$ and 0.6 obtained with the ATLAS detector using $37 \text{fb}^{-1}$ of proton--proton collisions from the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13$ TeV, with their associated uncertainties. A new in situ method to measure jet energy scale differences between data and Monte Carlo simulations is presented. The systematic uncertainties in the jet energy scale for central jets ($|η| < 1.2$) typically vary from 1% to about 5% as a function of $|η|$ at very low transverse momentum, $p_{\text{T}}$, of around 20 GeV for both $R=0.2$ and 0.6 jets. The relative energy resolution ranges from $(35 \pm 6)$% at $p_{\text{T}} = 20$ GeV to $(6 \pm 0.5)$% at $p_{\text{T}} = 300$ GeV for central $R=0.2$ jets, and is found to be slightly worse for $R=0.6$ jets. Finally, the effect of close-by hadronic activity on the jet energy scale is investigated and is found to be well modelled by the ATLAS Monte Carlo simulations.

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arXiv2025

Measurement of double-differential charged-current Drell-Yan cross-sections at high transverse masses in $pp$ collisions at $\sqrt{s} =$ 13 TeV with the ATLAS detector

This paper presents a first measurement of the cross-section for the charged-current Drell-Yan process $pp\rightarrow W^{\pm} \rightarrow \ell^{\pm} ν$ above the resonance region, where $\ell$ is an electron or muon. The measurement is performed for transverse masses, $m_{\text{T}}^{\text{W}}$, between 200 GeV and 5000 GeV, using a sample of 140 fb$^{-1}$ of $pp$ collision data at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV collected by the ATLAS detector at the LHC during 2015-2018. The data are presented single differentially in transverse mass and double differentially in transverse mass and absolute lepton pseudorapidity. A test of lepton flavour universality shows no significant deviations from the Standard Model. The electron and muon channel measurements are combined to achieve a total experimental precision of 3% at low $m_{\text{T}}^{\text{W}}$. The single- and double differential $W$-boson charge asymmetries are evaluated from the measurements. A comparison to next-to-next-to-leading-order perturbative QCD predictions using several recent parton distribution functions and including next-to-leading-order electroweak effects indicates the potential of the data to constrain parton distribution functions. The data are also used to constrain four fermion operators in the Standard Model Effective Field Theory formalism, in particular the lepton-quark operator Wilson coefficient $c_{\ell q}^{(3)}.$

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arXiv2025

Search for displaced leptons in $\sqrt{s}=13$ TeV and $13.6$ TeV $pp$ collisions with the ATLAS detector

A search for leptons displaced from the primary vertex is performed with the ATLAS detector at the Large Hadron Collider. The search includes the full proton-proton collision dataset collected during Run 2 at $\sqrt{s}=13$ TeV and a partial dataset collected during Run 3 in 2022-2023 at $\sqrt{s}=13.6$ TeV, corresponding to integrated luminosities of 140 fb$^{-1}$ and 56.3 fb$^{-1}$, respectively. Final states with displaced electrons or muons are considered, and novel triggers introduced in Run 3 are employed that use large impact parameter tracking to reconstruct displaced tracks with low momentum. In addition, photon reconstruction and multivariate techniques are employed to broaden the sensitivity to channels with large background rates or highly displaced electrons, respectively. The results are consistent with the Standard Model background expectations and are used to set model-independent limits on the production of displaced electrons and muons. The analysis is also interpreted in the context of a gauge-mediated supersymmetry breaking model with pair-produced long-lived sleptons, and a dark sector model with pair-produced chargino-like states. The results include 95% CL exclusions of selectrons with lifetimes from 4 ps to 60 ns and a mass of 150 GeV, and exclusions of selectrons, smuons, and staus with a lifetime of 0.3 ns for masses up to 740 GeV, 830 GeV, and 440 GeV, respectively. Dark charginos with masses up to 380 GeV are excluded for a mass difference with the neutral state of 40 GeV, and mass differences down to 17 GeV are excluded for dark charginos with a 100 GeV mass.

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