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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 127 records · Page 7Linked to original sources

arXiv2025

Search for light long-lived particles in $pp$ collisions at $\sqrt{s}=13$ TeV using displaced vertices in the ATLAS inner detector

A search for long-lived particles (LLPs) using 140 fb$^{-1}$ of $pp$ collision data with $\sqrt{s} = 13$ TeV recorded by the ATLAS experiment at the LHC is presented. The search targets LLPs with masses between $5$ and $55$ GeV that decay hadronically in the ATLAS inner detector. Benchmark models with LLP pair production from exotic decays of the Higgs boson and models featuring long-lived axion-like particles (ALPs) are considered. No significant excess above the expected background is observed. Upper limits are placed on the branching ratio of the Higgs boson to pairs of LLPs, the cross-section for ALPs produced in association with a vector boson, and, for the first time, on the branching ratio of the top quark to an ALP and a $u/c$ quark.

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arXiv2025

Observation of $VVZ$ production at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for the production of three massive vector bosons, $VVZ (V=W, Z)$, in proton-proton collisions at $\sqrt{s} = 13$ TeV is performed using data with an integrated luminosity of $140$ fb$^{-1}$ recorded by the ATLAS detector at the Large Hadron Collider. Events produced in the leptonic final states $WWZ \to \ellν\ellν\ell \ell$ ($\ell=e, μ$), $WZZ \to \ellν\ell\ell \ell\ell$, $ZZZ \to \ell\ell \ell\ell \ell\ell$, and the semileptonic final states $WWZ \to qq \ellν\ell \ell$ and $WZZ \to \ellνqq \ell \ell$, are analysed. The measured cross section for the $pp \rightarrow VVZ$ process is $660^{+93}_{-90}(\text{stat.})^{+88}_{-81}(\text{syst.})$ fb, and the observed (expected) significance is 6.4 (4.7) standard deviations, representing the observation of $VVZ$ production. In addition, the measured cross section for the $pp \rightarrow WWZ$ process is $442 \pm 94 (\text{stat.})^{+60}_{-52}(\text{syst.})$ fb, and the observed (expected) significance is 4.4 (3.6) standard deviations, representing evidence of $WWZ$ production. The measured cross sections are consistent with the Standard Model predictions. Constraints on physics beyond the Standard Model are also derived in the effective field theory framework by setting limits on Wilson coefficients for dimension-8 operators describing anomalous quartic gauge boson couplings.

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arXiv2025

Probing the Higgs boson CP properties in vector-boson fusion production in the $H\rightarrowτ^+τ^-$ channel with the ATLAS detector

The CP properties of the Higgs boson are studied in the vector-boson fusion production mode. The analysis exploits the decay mode of the Higgs boson into two $τ$-leptons using 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS experiment at the Large Hadron Collider. Results are obtained using the Optimal Observable method. CP-violating interactions between the Higgs boson and electroweak gauge bosons are considered in the effective field theory framework, with the interaction strength described in the HISZ basis by $\tilde{d}$, and in the Warsaw basis by $c_{H\tilde{W}}$, $c_{H\tilde{B}}$, and $c_{H\tilde{W}B}$. No deviations relative to the Standard Model are observed, and limits are obtained on the strength parameters. The $\tilde{d}$ parameter is constrained to the interval [$-0.012, 0.044$] at the 95% confidence level while $c_{H\tilde{W}}$ is constrained to [$-0.24, 0.83$], when considering both linear and quadratic effects of physics beyond the Standard Model.

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arXiv2025

Measurements of Higgs boson production via gluon-gluon fusion and vector-boson fusion using $H\rightarrow WW^\ast \rightarrow \ellν\ellν$ decays in $pp$ collisions with the ATLAS detector and their effective field theory interpretations

Higgs boson production cross-sections via gluon-gluon fusion and vector-boson fusion in proton-proton collisions are measured in the $H\rightarrow WW^\ast \rightarrow \ellν\ellν$ decay channel. The Large Hadron Collider delivered proton-proton collisions at a centre-of-mass energy of $13\,\textrm{TeV}$ between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of $140\,\textrm{fb}^{-1}$. The total cross-sections for Higgs boson production by gluon-gluon fusion and vector-boson fusion times the $H\rightarrow WW^\ast$ branching ratio are measured to be $12.4^{+1.3}_{-1.2}\,\textrm{pb}$ and $0.79^{+0.18}_{-0.16}\,\textrm{pb}$, 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.

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arXiv2025

Measurement of the top-quark pole mass in dileptonic $t\bar{t}+ 1\text{-jet}$ events at $\sqrt{s}=13$ TeV with the ATLAS experiment

A measurement of the top-quark pole mass $m_{t}^\text{pole}$ is presented in $t\bar{t}$ events with an additional jet, $t\bar{t}+1\text{-jet}$, produced in $pp$ collisions at $\sqrt{s}=13$ TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of $140~\text{fb}^{-1}$. Events with one electron and one muon of opposite electric charge in the final state are selected to measure the $t\bar{t}+1\text{-jet}$ differential cross-section as a function of the inverse of the invariant mass of the $t\bar{t}+1\text{-jet}$ 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 $pp \to t\bar{t}j$ ($2 \rightarrow 3$), where top quarks are taken as stable particles, and the process $pp \to b\bar{b}l^+νl^- \barν j$ ($2 \to 7$), 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 $2 \to 3$ and $2 \to 7$ calculations are compatible within theoretical uncertainties, providing an important consistency check. The more precise determination is obtained for the $2 \to 3 $ measurement: $m_{t}^\text{pole}=170.7\pm0.3~(\text{stat.})\pm1.4~(\text{syst.})~\pm 0.3~(\text{scale})~\pm 0.2~(\text{PDF}\oplusα_\text{S})~\text{GeV},$ which is in good agreement with other top-quark mass results.

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arXiv2025

Measurement of substructure-dependent suppression of large-radius jets with charged particles in Pb+Pb collisions with ATLAS

Measurements of jet substructure in Pb+Pb collisions provide key insights into the mechanism of jet quenching in the hot and dense QCD medium created in these collisions. This Letter presents a measurement of the suppression of large-radius jets with a radius parameter of $R = 1.0$ and its dependence on the jet substructure. The measurement uses 1.72 nb$^{-1}$ of Pb+Pb data and 255 pb$^{-1}$ of $pp$ data, both at $\sqrt{s_{_\mathrm{NN}}} = 5.02$ TeV, recorded with the ATLAS detector at the Large Hadron Collider. Large-radius jets are reconstructed by reclustering $R = 0.2$ calorimetric jets and are measured for transverse momentum above $200$ GeV. Jet substructure is evaluated using charged-particle tracks, and the overall level of jet suppression is quantified using the jet nuclear modification factor ($R_\mathrm{AA}$). The jet $R_\mathrm{AA}$ is measured as a function of jet $p_{\mathrm{T}}$, the charged $k_t$ splitting scale ($\sqrt{d_{12}}$), and the angular separation ($ΔR_{12}$) of two leading sub-jets. The jet $R_\mathrm{AA}$ gradually decreases with increasing $\sqrt{d_{12}}$, implying significantly stronger suppression of large-radius jets with larger $k_t$ splitting scale. The jet $R_\mathrm{AA}$ gradually decreases for $ΔR_{12}$ in the range $0.01{-}0.2$ and then remains consistent with a constant for $ΔR_{12} \gtrsim 0.2$. The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.

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arXiv2025

Evidence for the dimuon decay of the Higgs boson in pp collisions with the ATLAS detector

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 $3.4 σ$ ($2.5 σ$ expected). The best-fit signal strength is $μ= 1.4\pm0.4$. This result provides evidence for the $H \to μμ$ decay with ATLAS data and offers a direct probe of the Higgs-boson Yukawa coupling to second-generation fermions.

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arXiv2025

Search for electroweak production of vector-like leptons in $τ$-lepton and $b$-jet final states in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

A search for pair-production of vector-like leptons is presented, considering their decays into a third-generation Standard Model (SM) quark and a vector leptoquark ($U_1$) as predicted by an ultraviolet-complete extension of the SM, referred to as the '4321' model. Given the assumed decay of $U_1$ into third-generation SM fermions, the final state can contain multiple $τ$-leptons and $b$-quarks. This search is based on a dataset of $pp$ 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 up to 140 fb$^{-1}$. No significant excess above the SM background prediction is observed, and 95% confidence level limits on the cross-section times branching ratio are derived as a function of the vector-like lepton mass. A lower observed (expected) limit of 910 GeV (970 GeV) is set on the vector-like lepton mass. Additionally, the results are interpreted for a supersymmetric model with an $R$-parity violating coupling to the third-generation quarks and leptons. Lower observed (expected) limits are obtained on the higgsino mass at 880 GeV (940 GeV) and on the wino mass at 1170 GeV (1170 GeV).

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arXiv2025

Search for electroweak-scale dijet resonances using trigger-level analysis with the ATLAS detector in $132$ fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV

This article reports on a search for dijet resonances using $132$ fb$^{-1}$ of $pp$ collision data recorded at $\sqrt{s} = 13$ TeV by the ATLAS detector at the Large Hadron Collider. The search is performed solely on jets reconstructed within the ATLAS trigger to overcome bandwidth limitations imposed on conventional single-jet triggers, which would otherwise reject data from decays of sub-TeV dijet resonances. Collision events with two jets satisfying transverse momentum thresholds of $p_{\textrm{T}} \ge 85$ GeV and jet rapidity separation of $|y^{*}|<0.6$ are analysed for dijet resonances with invariant masses from $375$ to $1800$ GeV. A data-driven background estimate is used to model the dijet mass distribution from multijet processes. No significant excess above the expected background is observed. Upper limits are set at $95\%$ confidence level on coupling values for a benchmark leptophobic axial-vector $Z^{\prime}$ model and on the production cross-section for a new resonance contributing a Gaussian-distributed line-shape to the dijet mass distribution.

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arXiv2025

Performance and efficiency of a transformer-based quark/gluon jet tagger in the ATLAS experiment

A deep-learning approach based on the transformer architecture is developed to distinguish between jets originating from quarks and gluons. The algorithm operates on jets with transverse momentum $p_{\text{T}} > 20$ and pseudorapidity $|η| < 4.5$ and takes as input several properties derived from the jet constituents, using information from the ATLAS detector's tracker and calorimeter. The algorithm's performance is evaluated by analyzing dijet data events from proton-proton collisions at $\sqrt{s} = 13$ and $13.6$ TeV during Run 2 and Run 3 of the Large Hadron Collider. Two methods are used to obtain distributions from quark- or gluon-initiated jets in data: a matrix method fully based on Monte Carlo simulation and a new approach named `jet topics' which has less dependence on the modelling of the physics process under study. The quark and gluon identification efficiencies measured in data for the 50% quark-identification-efficiency working point vary from the simulated ones for quark-initiated (gluon-initiated) jets by factors of 0.88-1.30 (0.61-1.05) with uncertainties of 10%-70% (10%-95%). The uncertainties estimated with the jet topics method are smaller than those estimated with the matrix method, with up to 20% less systematic uncertainty in some phase-space regions. The advances in jet identification reported here provide a robust tool for precision Standard Model measurements and searches for new physics at the LHC.

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arXiv2025

Search for neutral long-lived particles that decay into displaced jets in the ATLAS calorimeter in association with leptons or jets using $pp$ collisions at $\sqrt{s}=13$ TeV

A search for neutral long-lived particles (LLPs) decaying in the ATLAS hadronic calorimeter using 140 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV delivered by the LHC is presented. The analysis is composed of three channels. The first targets pair-produced LLPs, where at least one LLP is produced with sufficiently low boost that its decay products can be resolved as separate jets. The second and third channels target LLPs respectively produced in association with a $W$ or $Z$ boson that decays leptonically. In each channel, different search regions target different kinematic regimes, to cover a broad range of LLP mass hypotheses and models. No excesses of events relative to the background predictions are observed. Higgs boson branching fractions to pairs of hadronically decaying neutral LLPs larger than 1% are excluded at 95% confidence level for proper decay lengths in the range of 30 cm to 4.5 m depending on the LLP mass, a factor of three improvement on previous searches in the hadronic calorimeter. The production of long-lived dark photons in association with a $Z$ boson with cross-sections above 0.1 pb is excluded for dark photon mean proper decay lengths in the range of 20 cm to 50 m, improving previous ATLAS results by an order of magnitude. Finally, long-lived photo-phobic axion-like particle models are probed for the first time by ATLAS, with production cross-sections above 0.1 pb excluded in the 0.1 mm to 10 m range.

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arXiv2025

Constraining off-shell Higgs boson production and the Higgs boson total width using $WW\to \ellν\ellν$ final states with the ATLAS detector

A measurement of off-shell Higgs boson production is performed in the $H^{*} \rightarrow WW$ channel. The measurement uses a proton-proton collision dataset with an integrated luminosity of 140 fb$^{-1}$ collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. Final states in which both $W$ bosons decay leptonically are targeted, and events are categorised based on the flavour of the final-state leptons, the jet multiplicity, and the output of neural-network-based classifiers. The data are found to be compatible with the Standard Model expectation. An observed (expected) upper bound on the 95% symmetric confidence level interval is set on the rate of off-shell Higgs boson production at a value of 3.4 (4.4) times the Standard Model prediction. These results are combined with the results from the measurement of on-shell Higgs boson production in the same final states to obtain an observed (expected) upper bound at 95% confidence level on the Higgs boson total width of 13.1 (17.3) MeV.

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arXiv2025

Search for $H\rightarrow c\bar{c}$ and measurement of $H\rightarrow b\bar{b}$ in vector-boson fusion production with the ATLAS Detector

A search for Standard Model (SM) Higgs bosons produced via vector-boson fusion at the Large Hadron Collider and decaying into a charm quark-antiquark pair ($H\rightarrow c\bar{c}$) is presented. The datasets used correspond to integrated luminosities of 37.5 fb$^{-1}$ and 51.5 fb$^{-1}$ and were collected by the ATLAS detector from proton-proton collisions at $\sqrt{s}$=13 and 13.6 TeV, respectively. The observed (expected) upper limit on the $H\rightarrow c\bar{c}$ production cross-section times branching ratio is 41 (28) times the SM prediction at 95% confidence level. Combining this search with the previous $H\rightarrow c\bar{c}$ search in associated production with a $W$ or $Z$ boson yields an observed (expected) limit on the Higgs-charm Yukawa coupling modifier of $|κ_{c}| $ < 4.7 (3.9). Higgs bosons decaying into a bottom quark-antiquark pair ($H\rightarrow b\bar{b}$) are measured simultaneously using the 51.5 fb$^{-1}$ dataset at $\sqrt{s}$=13.6 TeV, with an observed signal strength of 0.97$^{+0.57}_{-0.50}$ relative to the SM expectation. When combined with previous $H\rightarrow b\bar{b}$ results at 13 TeV, the observed (expected) significance reaches 3.2 (3.6) standard deviations, providing evidence for $H\rightarrow b\bar{b}$ events from vector-boson fusion.

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arXiv2025

Search for diphoton resonances in the 66 to 110 GeV mass range using $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

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 $\sqrt{s}=13$ 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.

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arXiv2025

Measurements of $ZZ \rightarrow \ell\ellνν$ and $ZZjj \rightarrow \ell\ellννjj$ productions in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This article presents measurements of $Z$ boson pair production cross-sections at a center-of-mass energy of $\sqrt{s} = 13$ TeV, using 140 fb$^{-1}$ of proton-proton data collected with the ATLAS detector at the Large Hadron Collider. The analysis includes both inclusive $ZZ$ production and $ZZ$ production in association with two jets ($ZZjj$), where one $Z$ boson decays into a pair of charged leptons $\ell^+\ell^-$ ($\ell = e, μ$), and the other decays into a neutrino pair. The measured fiducial cross-section of the inclusive $ZZ$ production is $21.0\pm1.0$ fb, while for the $ZZjj$ production it is $0.96^{+0.18}_{-0.16}$ fb. Extrapolation of the $ZZ$ cross-section to the full phase space, for $Z$ bosons within the mass range $66-116$~GeV, yields $σ(pp \rightarrow ZZ) = 15.38\pm0.81$ pb. Differential cross-sections are presented for an extended set of kinematic observables. The results are found to be in good agreement with Standard Model predictions. Constraints on anomalous gauge boson self-interactions are derived using both the vertex function formalism and the effective field theory framework.

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arXiv2025

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

Measurements of integrated and differential cross-sections for $W^{\pm}Z$ production in proton-proton collisions are presented. The data collected by the ATLAS detector at the Large Hadron Collider from 2015 to 2018 at a centre-of-mass energy of $\sqrt{s} =$ 13 TeV are used, corresponding to an integrated luminosity of 140 fb$^{-1}$. The $W^{\pm}Z$ candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons or muons. The integrated cross-section per lepton flavour for the production of $W^{\pm}Z$ is measured in the detector fiducial region with a relative precision of 4%. The measured value is compared with the Standard Model prediction at a precision of up to next-to-next-to-leading-order in QCD and next-to-leading-order in electroweak. Cross-sections for $W^+Z$ and $W^-Z$ production and their ratio are presented. The $W^{\pm}Z$ production is also measured differentially as functions of various kinematic variables, including new observables sensitive to CP-violation effects. All measurements are compared with state-of-the-art Standard Model predictions from fixed-order calculations or Monte Carlo generators based on next-to-leading-order matrix elements interfaced with parton showers. An effective field theory interpretation of the measurements is performed, considering both CP-conserving and CP-violating dimension-6 operators modifying the $W^{\pm}Z$ production. In the absence of observed deviations from the Standard Model, limits on CP-conserving Wilson coefficients are extracted using the transverse mass of the $W^{\pm}Z$ system. For CP-violating coefficients a machine learning approach is used to construct an observable with enhanced sensitivity to CP-violation effects.

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arXiv2025

Observation of double parton scattering in same-sign W boson pair production in pp collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This letter reports the measurement of double parton scattering in same-sign W boson pair production with the ATLAS detector. The data set used corresponds to an integrated luminosity of 140 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV, collected during Run 2 of the Large Hadron Collider. The study is performed in final states including two same-charge leptons, electron or muon, missing transverse momentum, and up to one jet. An excess of events is observed over the expected background contributions with a significance of 8.8 standard deviations. The measured fiducial cross section times leptonic branching fraction is $4.59 \pm 0.64$ fb. The measurement corresponds to a double parton scattering effective cross section of $10.6 \pm 1.8$ mb.

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arXiv2025

A continuous calibration of the ATLAS flavour-tagging classifiers via optimal transportation maps

A calibration of the ATLAS flavour-tagging algorithms using a new calibration procedure based on optimal transportation maps is presented. Simultaneous, continuous corrections to the $b$-jet, $c$-jet, and light-flavour jet classification probabilities from jet-tagging algorithms in simulation are derived for $b$-jets using $t\bar t \to eμννbb$ data. After application of the derived calibration maps, closure between simulation and observation is achieved for jet flavour observables used in ATLAS analyses of Large Hadron Collider (LHC) Run 2 proton-proton collision data. This continuous calibration opens up new possibilities for the future use of jet flavour information in LHC analyses and also serves as a guide for deriving high-dimensional corrections to simulation via transportation maps, an important development for a broad range of inference tasks.

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