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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 271 records · Page 15Linked to original sources

arXiv2024

Measurement of the $VH, H\rightarrow ττ$ process with the ATLAS detector at 13 TeV

A measurement of the Standard Model Higgs boson produced in association with a $W$ or $Z$ boson and decaying into a pair of $τ$-leptons is presented. This search is based on proton-proton collision data collected at $\sqrt{s}=13$ TeV by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 140 fb$^{-1}$. For the Higgs boson candidate, only final states with at least one $τ$-lepton decaying hadronically ($τ\rightarrow \mathrm{hadrons} + ν_τ$) are considered. For the vector bosons, only leptonic decay channels are considered: $Z \rightarrow \ell\ell$ and $W\rightarrow \ellν_\ell$, with $\ell=e,μ$. An excess of events over the expected background is found with an observed (expected) significance of 4.2 (3.6) standard deviations, providing evidence of the Higgs boson produced in association with a vector boson and decaying into a pair of $τ$-leptons. The ratio of the measured cross-section to the Standard Model prediction is $μ_{\text{VH}}^{ττ} = 1.28\ ^{+0.30}_{-0.29}\ (\mathrm{stat.})\ ^{+0.25}_{-0.21}\ (\mathrm{syst.})$. This result represents the most accurate measurement of the \vh process achieved to date.

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arXiv2024

Combination of searches for Higgs boson pair production in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This Letter presents results from a combination of searches for Higgs boson pair production using 126$-$140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV recorded with the ATLAS detector. At 95% confidence level (CL), the upper limit on the production rate is 2.9 times the standard model (SM) prediction, with an expected limit of 2.4 assuming no Higgs boson pair production. Constraints on the Higgs boson self-coupling modifier $κ_λ=λ_{HHH}/λ_{HHH}^\mathrm{SM}$, and the quartic $HHVV$ coupling modifier $κ_{2V}=g_{HHVV}/g_{HHVV}^\mathrm{SM}$, are derived individually, fixing the other parameter to its SM value. The observed 95% CL intervals are $-1.2 < κ_λ < 7.2$ and $0.6 < κ_{2V} < 1.5$, respectively, while the expected intervals are $-1.6 < κ_λ < 7.2$ and $0.4 < κ_{2V} < 1.6$ in the SM case. Constraints obtained for several interaction parameters within Higgs effective field theory are the strongest to date, offering insights into potential deviations from SM predictions.

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arXiv2024

Simultaneous energy and mass calibration of large-radius jets with the ATLAS detector using a deep neural network

The energy and mass measurements of jets are crucial tasks for the Large Hadron Collider experiments. This paper presents a new calibration method to simultaneously calibrate these quantities for large-radius jets measured with the ATLAS detector using a deep neural network (DNN). To address the specificities of the calibration problem, special loss functions and training procedures are employed, and a complex network architecture, which includes feature annotation and residual connection layers, is used. The DNN-based calibration is compared to the standard numerical approach in an extensive series of tests. The DNN approach is found to perform significantly better in almost all of the tests and over most of the relevant kinematic phase space. In particular, it consistently improves the energy and mass resolutions, with a 30% better energy resolution obtained for transverse momenta $p_{\text{T}}>500$ GeV.

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arXiv2024

Differential cross-sections for events with missing transverse momentum and jets measured with the ATLAS detector in 13 TeV proton-proton collisions

Measurements of inclusive, differential cross-sections for the production of events with missing transverse momentum in association with jets in proton-proton collisions at $\sqrt{s}=13~$TeV are presented. The measurements are made with the ATLAS detector using an integrated luminosity of $140~$fb$^{-1}$ and include measurements of dijet distributions in a region in which vector-boson fusion processes are enhanced. They are unfolded to correct for detector resolution and efficiency within the fiducial acceptance, and are designed to allow robust comparisons with a wide range of theoretical predictions. A measurement of differential cross sections for the $Z~\to νν$ process is made. The measurements are generally well-described by Standard Model predictions except for the dijet invariant mass distribution. Auxiliary measurements of the hadronic system recoiling against isolated leptons, and photons, are also made in the same phase space. Ratios between the measured distributions are then derived, to take advantage of cancellations in modelling effects and some of the major systematic uncertainties. These measurements are sensitive to new phenomena, and provide a mechanism to easily set constraints on phenomenological models. To illustrate the robustness of the approach, these ratios are compared with two common Dark Matter models, where the constraints derived from the measurement are comparable to those set by dedicated detector-level searches.

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arXiv2024

Measurement of differential cross-sections in $t\bar t$ and $t\bar t+$jets production in the lepton+jets final state in $pp$ collisions at $\sqrt s=13$ TeV using 140 fb$^{-1}$ of ATLAS data

Differential cross-sections for top-quark pair production, inclusively and in association with jets, are measured in $pp$ collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC using an integrated luminosity of 140 fb$^{-1}$. The events are selected with one charged lepton (electron or muon) and at least four jets. The differential cross-sections are presented at particle level as functions of several jet observables, including angular correlations, jet transverse momenta and invariant masses of the jets in the final state, which characterise the kinematics and dynamics of the top-antitop system and the hard QCD radiation in the system with associated jets. The typical precision is 5%-15% for the absolute differential cross-sections and 2%-4% for the normalised differential cross-sections. Next-to-leading-order and next-to-next-to-leading-order QCD predictions are found to provide an adequate description of the rate and shape of the jet-angular observables. The description of the transverse momentum and invariant mass observables is improved when next-to-next-to-leading-order QCD corrections are included.

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arXiv2024

Measurement of jet substructure in boosted $t\bar{t}$ events with the ATLAS detector using 140 fb$^{-1}$ of 13 TeV $pp$ collisions

Measurements of the substructure of top-quark jets are presented, using 140 fb$^{-1}$ of 13 TeV $pp$ collision data recorded with the ATLAS detector at the LHC. Top-quark jets reconstructed with the anti-$k_{t}$ algorithm with a radius parameter $R=1.0$ are selected in top-quark pair ($t\bar{t}$) events where one top quark decays semileptonically and the other hadronically, or where both top quarks decay hadronically. The top-quark jets are required to have transverse momentum $p_\mathrm{T} > 350$ GeV, yielding large samples of data events with jet $p_\mathrm{T}$ values between 350 and 600 GeV. One- and two-dimensional differential cross-sections for eight substructure variables, defined using only the charged components of the jets, are measured in a particle-level phase space by correcting for the smearing and acceptance effects induced by the detector. The differential cross-sections are compared with the predictions of several Monte Carlo simulations in which top-quark pair-production quantum chromodynamic matrix-element calculations at next-to-leading-order precision in the strong coupling constant $α_\mathrm{S}$ are passed to leading-order parton shower and hadronization generators. The Monte Carlo predictions for measures of the broadness, and also the two-body structure, of the top-quark jets are found to be in good agreement with the measurements, while variables sensitive to the three-body structure of the top-quark jets exhibit some tension with the measured distributions.

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arXiv2024

Differential cross-section measurements of the production of four charged leptons in association with two jets using the ATLAS detector

Differential cross-sections are measured for the production of four charged leptons in association with two jets. These measurements are sensitive to final states in which the jets are produced via the strong interaction as well as to the purely-electroweak vector boson scattering process. The analysis is performed using proton-proton collision data collected by ATLAS at $\sqrt{s}=13$ TeV and with an integrated luminosity of 140 fb$^{-1}$. The data are corrected for the effects of detector inefficiency and resolution and are compared to state-of-the-art Monte Carlo event generator predictions. The differential cross-sections are used to search for anomalous weak-boson self-interactions that are induced by dimension-six and dimension-eight operators in Standard Model effective field theory.

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arXiv2024

Search for pair-produced higgsinos decaying via Higgs or $Z$ bosons to final states containing a pair of photons and a pair of $b$-jets with the ATLAS detector

A search is presented for the pair production of higgsinos $\tildeχ$ in gauge-mediated supersymmetry models, where the lightest neutralinos $\tildeχ_1^0$ decay into a light gravitino $\tilde{G}$ in association with either a Higgs $h$ or a $Z$ boson. The search is performed with the ATLAS detector at the Large Hadron Collider using 139 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV. It targets final states in which a Higgs boson decays into a photon pair, while the other Higgs or $Z$ boson decays into a $b\bar{b}$ pair, with missing transverse momentum associated with the two gravitinos. Search regions dependent on the amount of missing transverse momentum are defined by the requirements that the diphoton mass should be consistent with the mass of the Higgs boson, and the $b\bar{b}$ mass with the mass of the Higgs or $Z$ boson. The main backgrounds are estimated with data-driven methods using the sidebands of the diphoton mass distribution. No excesses beyond Standard Model expectations are observed and higgsinos with masses up to 320 GeV are excluded, assuming a branching fraction of 100% for $\tildeχ_1^0\rightarrow h\tilde{G}$. This analysis excludes higgsinos with masses of 130 GeV for branching fractions to $h\tilde{G}$ as low as 36%, thus providing complementarity to previous ATLAS searches in final states with multiple leptons or multiple $b$-jets, targeting different decays of the electroweak bosons.

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arXiv2024

Search for charged-lepton-flavour violating $μτqt$ interactions in top-quark production and decay in $pp$ collisions at $\sqrt{s}= 13$ TeV with the ATLAS detector at the LHC

A search for charged-lepton-flavour violating $μτqt$ ($q=u,c$) interactions is presented, considering both top-quark production and decay. The data analysed correspond to 140 $\textrm{fb}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}= $13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The analysis targets events containing two muons with the same electric charge, a hadronically decaying $τ$-lepton and at least one jet, with exactly one $b$-tagged jet, produced by a $μτqt$ interaction. Agreement with the Standard Model expectation within $1.6σ$ is observed, and limits are set at the 95% CL on the charged-lepton-flavour violation branching ratio of $\mathcal{B}(t \to μτq) < 8.7 \times 10^{-7}$. An Effective Field Theory interpretation is performed yielding 95% CL limits on Wilson coefficients, dependent on the flavour of the associated light quark and the Lorentz structure of the coupling. These range from $|c_{\mathsf{lequ}}^{3(2313)}| / Λ^{2} < 0.10\textrm{ TeV}^{-2}$ for $μτut$ to $|c_{\mathsf{ lequ}}^{1(2323)}| / Λ^{2} < 1.8\textrm{ TeV}^{-2}$ for $μτct$. An additional interpretation is performed for scalar leptoquark production inducing charged lepton flavour violation, with fixed inter-generational couplings. Upper limits on leptoquark coupling strengths are set at the 95% CL, ranging from $λ^{\textrm{LQ}} = $1.3 to $λ^{\textrm{LQ}} = $3.7 for leptoquark masses between 0.5 and 2.0 TeV.

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arXiv2024

Search for leptoquark pair production decaying into $te^- \bar{t}e^+$ or $tμ^- \bar{t}μ^+$ in multi-lepton final states in $pp$ collisions at 13 TeV with the ATLAS detector

A search for leptoquark pair production decaying into $te^- \bar{t}e^+$ or $tμ^- \bar{t}μ^+$ in final states with multiple leptons is presented. The 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 139~fb$^{-1}$. Events are selected with two or more light leptons (electron or muon) and at least two jets out of which at least one jet is identified as coming from a $b$-hadron. Four signal regions, with the requirement of at least three light leptons, are considered based on the number of leptons of a given flavour. The main background processes are estimated using dedicated control regions in a simultaneous fit with the signal regions to data. No excess above the Standard Model background prediction is observed and 95% confidence level limits on the production cross section times branching ratio are derived as a function of the leptoquark mass. Under the assumption of exclusive decays into $te^{-}$ ($tμ^{-}$), the corresponding lower limit on the scalar mixed-generation leptoquark mass $m_{\mathrm{LQ}_{\mathrm{mix}}^{\mathrm{d}}}$ is at 1.58 (1.59) TeV and on the vector leptoquark mass $m_{\tilde{U}_1}$ at 1.67 (1.67) TeV in the minimal coupling scenario and at 1.95 (1.95) TeV in the Yang-Mills scenario.

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arXiv2024

Observation of electroweak production of $W^+W^-$ in association with jets in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS Detector

A measurement of the production of $W$ bosons with opposite electric charges in association with two jets is presented based on 140 fb$^{-1}$ of data collected by the ATLAS detector in proton-proton collisions at $\sqrt{s}=13$ TeV. The analysis is sensitive to the scattering of $W$ bosons, which is of particular interest in the ATLAS physics programme as it can be used to probe the electroweak symmetry breaking mechanism of the Standard Model. This signal is observed with a significance of 7.1 standard deviations above the background expectation, while 6.2 standard deviations were expected. The measured cross-section is determined in a signal-enriched fiducial volume and is found to be $2.7\pm0.5$ fb, which is consistent with the theoretical prediction of $2.20^{+0.14}_{-0.13}$ fb.

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arXiv2024

Measurement of $ZZ$ production cross-sections in the four-lepton final state in $pp$ collisions at $\sqrt{s}=13.6$ TeV with the ATLAS experiment

This paper reports cross-section measurements of $ZZ$ production in $pp$ collisions at $\sqrt{s}=13.6$ TeV at the Large Hadron Collider. The data were collected by the ATLAS detector in 2022, and correspond to an integrated luminosity of 29 fb$^-1$. Events in the $ZZ\rightarrow4\ell$ ($\ell = e$, $μ$) final states are selected and used to measure the inclusive and differential cross-sections in a fiducial region defined close to the analysis selections. The inclusive cross-section is further extrapolated to the total phase space with a requirement of 66 $< m_Z <$ 116 GeV for both $Z$ bosons, yielding $16.8 \pm 1.1$ pb. The results are well described by the Standard Model predictions.

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arXiv2024

Calibration of a soft secondary vertex tagger using proton-proton collisions at $\sqrt{s}=$13 TeV with the ATLAS detector

Several processes studied by the ATLAS experiment at the Large Hadron Collider produce low-momentum $b$-flavored hadrons in the final state. This paper describes the calibration of a dedicated tagging algorithm that identifies $b$-flavored hadrons outside of hadronic jets by reconstructing the soft secondary vertices originating from their decays. The calibration is based on a proton-proton collision dataset at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 140 fb$^{-1}$. Scale factors used to correct the algorithm's performance in simulated events are extracted for the $b$-tagging efficiency and the mistag rate of the algorithm using a data sample enriched in $t\bar{t}$ events. Several orthogonal measurement regions are defined, binned as a function of the multiplicities of soft secondary vertices and jets containing a $b$-flavored hadron in the event. The mistag rate scale factors are estimated separately for events with low and high average number of interactions per bunch crossing. The results, which are derived from events with low missing transverse momentum, are successfully validated in a phase space characterized by high missing transverse momentum and therefore are applicable to new physics searches carried out in either phase space regimes.

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arXiv2024

A search for top-squark pair production, in final states containing a top quark, a charm quark and missing transverse momentum, using the 139 fb$^{-1}$ of $pp$ collision data collected by the ATLAS detector

This paper presents a search for top-squark pair production in final states with a top quark, a charm quark and missing transverse momentum. The data were collected with the ATLAS detector during LHC Run 2 and corresponds to an integrated luminosity of 139fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV. The analysis is motivated by an extended Minimal Supersymmetric Standard Model featuring a non-minimal flavour violation in the second- and third-generation squark sector. The top squark in this model has two possible decay modes, either $\tilde{t}_1 \rightarrow c\tildeχ_1^0$ or $\tilde{t}_1\rightarrow t\tildeχ_1^0$, where the $\tildeχ_1^0$ is undetected. The analysis is optimised assuming that both of the decay modes are equally probable, leading to the most likely final state of $tc + E_{\text{T}}^{\text{miss}}$. Good agreement is found between the Standard Model expectation and the data in the search regions. Exclusion limits at 95% CL are obtained in the $m(\tilde{t}_1)$ vs $m(\tildeχ_1^0)$ plane and, in addition, limits on the branching ratio of the $\tilde{t}_1\rightarrow t\tildeχ_1^0$ decay as a function of $m(\tilde{t}_1)$ are also produced. Top-squark masses of up to 800 GeV are excluded for scenarios with light neutralinos, and top-squark masses up to 600 GeV are excluded in scenarios where the neutralino and the top squark are almost mass degenerate.

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arXiv2024

Combination of searches for Higgs boson decays into a photon and a massless dark photon using $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A combination of searches for Higgs boson decaying into a visible photon and a massless dark photon ($H\toγγ_{\text{d}}$) is presented using 139 fb$^{-1}$ of proton--proton collision data at a centre-of-mass energy of $\sqrt{s} = 13$ TeV recorded by the ATLAS detector at the Large Hadron Collider. The observed (expected) 95\% confidence level upper limit on the Standard Model Higgs boson decay branching ratio is determined to be $\mathcal{B}(H\toγγ_{\text{d}}) < $ 1.3% (1.5)%. The search is also sensitive to higher-mass Higgs bosons decaying into the same final state. The observed (expected) 95% CL limit on the cross section times branching ratio ranges from 16 fb (26 fb) for $m_H = 400$ GeV to 1.0 fb (1.5 fb) for $m_H = 3$ TeV. Results are also interpreted in the context of a minimal simplified model.

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arXiv2024

Beam-induced backgrounds measured in the ATLAS detector during local gas injection into the LHC beam vacuum

Inelastic beam-gas collisions at the Large Hadron Collider (LHC), within a few hundred metres of the ATLAS experiment, are known to give the dominant contribution to beam backgrounds. These are monitored by ATLAS with a dedicated Beam Conditions Monitor (BCM) and with the rate of fake jets in the calorimeters. These two methods are complementary since the BCM probes backgrounds just around the beam pipe while fake jets are observed at radii of up to several metres. In order to quantify the correlation between the residual gas density in the LHC beam vacuum and the experimental backgrounds recorded by ATLAS, several dedicated tests were performed during LHC Run 2. Local pressure bumps, with a gas density several orders of magnitude higher than during normal operation, were introduced at different locations. The changes of beam-related backgrounds, seen in ATLAS, are correlated with the local pressure variation. In addition the rates of beam-gas events are estimated from the pressure measurements and pressure bump profiles obtained from calculations. Using these rates, the efficiency of the ATLAS beam background monitors to detect beam-gas events is derived as a function of distance from the interaction point. These efficiencies and characteristic distributions of fake jets from the beam backgrounds are found to be in good agreement with results of beam-gas simulations performed with the FLUKA Monte Carlo programme.

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arXiv2024

Search for heavy neutral Higgs bosons decaying into a top quark pair in 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for heavy pseudo-scalar ($A$) and scalar ($H$) Higgs bosons decaying into a top-quark pair ($t\bar{t}$) has been performed with 140 fb$^{-1}$ of proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13$ TeV. Interference effects between the signal process and Standard Model (SM) $t\bar{t}$ production are taken into account. Final states with exactly one or exactly two electrons or muons are considered. No significant deviation from the SM prediction is observed. The results of the search are interpreted in the context of a two-Higgs-doublet model (2HDM) of type II in the alignment limit with mass-degenerate pseudo-scalar and scalar Higgs bosons ($m_A = m_H$) and the hMSSM parameterisation of the minimal supersymmetric extension of the Standard Model. Ratios of the two vacuum expectation values, $\tanβ$, smaller than 3.49 (3.16) are excluded at 95% confidence level for $m_A = m_H = 400$ GeV in the 2HDM (hMSSM). Masses up to 1240 GeV are excluded for the lowest tested $\tanβ$ value of 0.4 in the 2HDM. In the hMSSM, masses up to 950 GeV are excluded for $\tanβ=1.0$. In addition, generic exclusion limits are derived separately for single scalar and pseudo-scalar states for different choices of their mass and total width.

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arXiv2024

Search for electroweak production of supersymmetric particles in final states with two $τ$-leptons in $\sqrt{s}$ = 13 TeV $pp$ collisions with the ATLAS detector

Three searches for the direct production of $τ$-sleptons or charginos and neutralinos in final states with at least two hadronically decaying $τ$-leptons are presented. For chargino and neutralino production, decays via intermediate $τ$-sleptons or $W$ and $h$ bosons are considered. The analysis uses a dataset of $pp$ collisions corresponding to an integrated luminosity of $139\,$fb$^{-1}$, recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. No significant deviation from the expected Standard Model background is observed and supersymmetric particle mass limits at 95% confidence level are obtained in simplified models. For direct production of $\tilde~χ^+_1\tilde~χ^-_1$, chargino masses are excluded up to 970 GeV, while $\tilde~χ^{\pm}_1$ and $\tilde~χ^0_2$ masses up to 1160 GeV (330 GeV) are excluded for $\tilde~χ^{\pm}_1\tilde~χ^0_2$/$\tilde~χ^+_1\tilde~χ^-_1$ production with subsequent decays via $τ$-sleptons ($W$ and $h$ bosons). Masses of $τ$-sleptons up to 500 GeV are excluded for mass degenerate $\tilde~τ_{L,R}$ scenarios and up to 425 GeV for $\tilde~τ_L$-only scenarios. Sensitivity to $\tilde~τ_R$-only scenarios from the ATLAS experiment is presented here for the first time, with $\tilde~τ_R$ masses excluded up to 350 GeV.

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