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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 217 records · Page 12Linked to original sources

arXiv2025

Search for supersymmetry using vector boson fusion signatures and missing transverse momentum in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This paper presents a search for supersymmetric particles in models with highly compressed mass spectra, in events consistent with being produced through vector boson fusion. The search uses 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS experiment at the Large Hadron Collider. Events containing at least two jets with a large gap in pseudorapidity, large missing transverse momentum, and no reconstructed leptons are selected. A boosted decision tree is used to separate events consistent with the production of supersymmetric particles from those due to Standard Model backgrounds. The data are found to be consistent with Standard Model predictions. The results are interpreted using simplified models of $R$-parity-conserving supersymmetry in which the lightest supersymmetric partner is a bino-like neutralino with a mass similar to that of the lightest chargino and second-to-lightest neutralino, both of which are wino-like. Lower limits at 95% confidence level on the masses of next-to-lightest supersymmetric partners in this simplified model are established between 117 and 120 GeV when the lightest supersymmetric partners are within 1 GeV in mass.

hep-ex↗

arXiv2025

Search for singly produced vector-like top partners in multilepton final states with 139 $\mathrm{fb}^{-1}$ of $pp$ collision data at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for the single production of a vector-like top partner ($T$) with mass greater than 1 TeV decaying into a $Z$ boson and a top quark is presented, using the full Run 2 dataset corresponding to 139 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}$ = 13 TeV, collected in 2015-2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a pair of electrons or muons with opposite-sign charges which form a $Z$-boson candidate, as well as by the presence of $b$-tagged jets and forward jets. Events with exactly two or at least three leptons are categorized into two independently optimized analysis channels. No significant excess above the background expectation is observed and the results from the two channels are statistically combined to set exclusion limits at 95% confidence level on the masses and couplings of $T$. The results are interpreted in several benchmark scenarios to set limits on the mass and universal coupling strength ($κ$) of the vector-like quark. For singlet $T$ quarks, $κ$ values between 0.22 and 0.64 are excluded for masses between 1000 and 1975 GeV. For $T$ quarks in the doublet scenario, where the production cross section is much lower, $κ$ values between 0.54 and 0.88 are excluded for masses between 1000 and 1425 GeV.

hep-ex↗

arXiv2025

Underlying-event studies with strange hadrons in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Properties of the underlying-event in $pp$ interactions are investigated primarily via the strange hadrons $K_{S}^{0}$, $Λ$ and $\barΛ$, as reconstructed using the ATLAS detector at the LHC in minimum-bias $pp$ collision data at $\sqrt{s} = 13$ TeV. The hadrons are reconstructed via the identification of the displaced two-particle vertices corresponding to the decay modes $K_{S}^{0}\rightarrowπ^+π^-$, $Λ\rightarrowπ^-p$ and $\barΛ\rightarrowπ^+\bar{p}$. 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 $10 < p_T \leq 40$ GeV are studied using the number of prompt charged particles in the transverse region. The ratio $N(Λ+ \barΛ)/N(K_{S}^{0})$ 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.

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arXiv2025

Operation and performance of the ATLAS tile calorimeter in LHC Run 2

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.

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arXiv2025

Search for a light CP-odd Higgs boson decaying into a pair of $τ$-leptons in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

This paper reports a search for a light CP-odd scalar resonance with a mass of 20 GeV to 90 GeV in 13 TeV proton-proton collision data with an integrated luminosity of 140 fb$^{-1}$ collected with the ATLAS detector at the Large Hadron Collider. The analysis assumes the resonance is produced via gluon-gluon fusion and decays into a $τ^{+}τ^{-}$ pair which subsequently decays into a fully leptonic $μ^{+}ν_μ \barν_τ e^{-} \barν_{e} ν_τ$ or $e^{+}ν_{e}\barν_τ μ^-\barν_μν_τ$ final state. No significant excess of events above the predicted Standard Model background is observed. The results are interpreted within a flavour-aligned two-Higgs-doublet model, and a model-independent cross-section interpretation is also given. Upper limits at 95$%$ confidence level between 3.0 pb and 68 pb are set on the cross-section for producing a CP-odd Higgs boson that decays into a $τ^+τ^-$ pair.

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arXiv2024

Using pile-up collisions as an abundant source of low-energy hadronic physics processes in ATLAS and an extraction of the jet energy resolution

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.

hep-ex↗

arXiv2024

Jet radius dependence of dijet momentum balance and suppression in Pb+Pb collisions at 5.02 TeV with the ATLAS detector

This paper describes a measurement of the jet radius dependence of the dijet momentum balance between leading back-to-back jets in 1.72 $nb^{-1}$ of Pb+Pb collisions collected in 2018 and 255 pb$^{-1}$ of $pp$ collisions collected in 2017 by the ATLAS detector at the LHC. Both data sets were collected at $ \sqrt{s_{\rm NN}}=$ 5.02 TeV. Jets are reconstructed using the anti-$k_t$ algorithm with jet radius parameters $R=$ 0.2, 0.3, 0.4, 0.5 and 0.6. The dijet momentum balance distributions are constructed for leading jets with transverse momentum $p_{\rm T}$ from 100 to 562 GeV for $R=$ 0.2, 0.3 and 0.4 jets, and from 158 to 562 GeV for $R=$ 0.5 and 0.6 jets. The absolutely normalized dijet momentum balance distributions are constructed to compare measurements of the dijet yields in Pb+Pb collisions directly to the dijet cross sections in $pp$ collisions. For all jet radii considered here, there is a suppression of more balanced dijets in Pb+Pb collisions compared to $pp$ 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.

nucl-ex↗

arXiv2024

Search for a resonance decaying into a scalar particle and a Higgs boson in final states with leptons and two photons in proton-proton collisions at $\sqrt s=13$ TeV with the ATLAS detector

A search for a hypothetical heavy scalar particle, $X$, decaying into a singlet scalar particle, $S$, and a Standard Model Higgs boson, $H$, using 140 fb$^{-1}$ of proton-proton collision data at the centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the LHC is presented. The explored mass range is $300 \leq m_X \leq 1000$ GeV and $170 \leq m_S \leq 500$ GeV. The signature of this search is one or two leptons ($e$ or $μ$) from the decay of vector bosons originating from the $S$ particle, $S \rightarrow W^{\pm}W^{\mp}/ZZ$, and two photons from the Higgs boson decay, $H\rightarrow γγ$. No significant excess is observed above the expected Standard Model background. The observed (expected) upper limits at the 95% confidence level on the cross-section for $gg\rightarrow X\rightarrow SH$, assuming the same $S\rightarrow WW/ZZ$ branching ratios as for a SM-like heavy Higgs boson, are between 530 (800) fb and 120 (170) fb.

hep-ex↗

arXiv2024

Sensor response and radiation damage effects for 3D pixels in the ATLAS IBL Detector

Pixel sensors in 3D technology equip the outer ends of the staves of the Insertable B Layer (IBL), the innermost layer of the ATLAS Pixel Detector, which was installed before the start of LHC Run 2 in 2015. 3D pixel sensors are expected to exhibit more tolerance to radiation damage and are the technology of choice for the innermost layer in the ATLAS tracker upgrade for the HL-LHC programme. While the LHC has delivered an integrated luminosity of $\simeq$ 235 fb$^{-1}$ since the start of Run 2, the 3D sensors have received a non-ionising energy deposition corresponding to a fluence of ${\simeq} 8.5\times10^{14}$ 1 MeV neutron-equivalent cm$^{-2}$ averaged over the sensor area. This paper presents results of measurements of the 3D pixel sensors' response during Run 2 and the first two years of Run 3, with predictions of its evolution until the end of Run 3 in 2025. Data are compared with radiation damage simulations, based on detailed maps of the electric field in the Si substrate, at various fluence levels and bias voltage values. These results illustrate the potential of 3D technology for pixel applications in high-radiation environments.

physics.ins-det↗

arXiv2024

Search for short- and long-lived axion-like particles in $H\rightarrow a a \rightarrow 4γ$ decays with the ATLAS experiment at the LHC

Presented is the search for anomalous Higgs boson decays into two axion-like particles (ALPs) using the full Run 2 data set of 140 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS experiment. The ALPs are assumed to decay into two photons, providing sensitivity to recently proposed models that could explain the $(g-2)_μ$ discrepancy. This analysis covers an ALP mass range from 100 MeV to 62 GeV and ALP-photon couplings in the range $10^{-5}\, \text{TeV}^{-1}<C_{aγγ}/Λ<1\, \text{TeV}^{-1}$, and therefore includes signatures with significantly displaced vertices and highly collinear photons. No significant excess of events above the Standard Model background is observed. Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to two ALPs in the four-photon final state, and are in the range of $ 10^{-5}$ to $3\times 10^{-2}$, depending on the hypothesized ALP mass and ALP-photon coupling strength.

hep-ex↗

arXiv2024

Search for a new $Z'$ gauge boson via the $pp \rightarrow W^{\pm(*)} \rightarrow Z' μ^{\pm} ν\rightarrow μ^{\pm}μ^{\mp}μ^{\pm}ν$ process in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for a new $Z'$ gauge boson predicted by $L_μ-L_τ$ models, based on charged-current Drell-Yan production, $pp \rightarrow W^{\pm(*)} \rightarrow Z' μ^{\pm} ν\rightarrow μ^{\pm}μ^{\mp}μ^{\pm}ν$, is presented. The data sample used corresponds to an integrated luminosity of 140 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} = 13$ TeV recorded by the ATLAS detector at the Large Hadron Collider. The search examines a final state of $3μ$ plus large missing transverse momentum. Upper limits are set on the $Z'$ production cross-section times branching ratio in the mass range of 5-81 GeV. After combining with the previous $Z'$ search using the neutral-current Drell-Yan production with a $4μ$ final state, the most stringent exclusion limits to date are achieved in the parameter space of the $Z'$ coupling strength and mass.

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arXiv2024

Accuracy versus precision in boosted top tagging with the ATLAS detector

The identification of top quark decays where the top quark has a large momentum transverse to the beam axis, known as $top$ $tagging$, is a crucial component in many measurements of Standard Model processes and searches for beyond the Standard Model physics at the Large Hadron Collider. Machine learning techniques have improved the performance of top tagging algorithms, but the size of the systematic uncertainties for all proposed algorithms has not been systematically studied. This paper presents the performance of several machine learning based top tagging algorithms on a dataset constructed from simulated proton-proton collision events measured with the ATLAS detector at $\sqrt{s} = 13$ TeV. The systematic uncertainties associated with these algorithms are estimated through an approximate procedure that is not meant to be used in a physics analysis, but is appropriate for the level of precision required for this study. The most performant algorithms are found to have the largest uncertainties, motivating the development of methods to reduce these uncertainties without compromising performance. To enable such efforts in the wider scientific community, the datasets used in this paper are made publicly available.

hep-ex↗

arXiv2024

Measurement of top-quark pair production in association with charm quarks in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

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 $t\bar{t}+ {\geq} 2c$ and $t\bar{t}+1c$ production are measured to be $1.28^{+0.27}_{-0.24}\;\text{pb}$ and $6.4^{+1.0}_{-0.9}\;\text{pb}$, respectively. The measurements are primarily limited by uncertainties in the modeling of inclusive $t\bar{t}$ and $t\bar{t}+b\bar{b}$ production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various $t\bar{t}$ 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 $t\bar{t}$ decay products and the jet multiplicity, the cross-section ratios of $t\bar{t}+ {\geq} 2c$ and $t\bar{t}+1c$ to total $t\bar{t}+\text{jets}$ production are determined to be $(1.23 \pm 0.25) \%$ and $(8.8 \pm 1.3) \%$.

hep-ex↗

arXiv2024

Measurements of Lund subjet multiplicities in 13 TeV proton-proton collisions with the ATLAS detector

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 $\sqrt{s}=13$ 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.

hep-ex↗

arXiv2024

Fiducial and differential cross-section measurements of electroweak $Wγjj$ production in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

The observation of the electroweak production of a $W$ boson and a photon in association with two jets, using $pp$ collision data at the Large Hadron Collider at a centre of mass energy of $\sqrt{s}=13$ TeV, is reported. The data were recorded by the ATLAS experiment from 2015 to 2018 and correspond to an integrated luminosity of 140 fb$^{-1}$. This process is sensitive to the quartic gauge boson couplings via the vector boson scattering mechanism and provides a stringent test of the electroweak sector of the Standard Model. Events are selected if they contain one electron or muon, missing transverse momentum, at least one photon, and two jets. Multivariate techniques are used to distinguish the electroweak $Wγjj$ process from irreducible background processes. The observed significance of the electroweak $Wγjj$ process is well above six standard deviations, compared to an expected significance of 6.3 standard deviations. Fiducial and differential cross sections are measured in a fiducial phase space close to the detector acceptance, which are in reasonable agreement with leading order Standard Model predictions from $MadGraph5+Pythia8$ and $Sherpa$. The results are used to constrain new physics effects in the context of an effective field theory.

hep-ex↗

arXiv2024

Search for a resonance decaying into a scalar particle and a Higgs boson in the final state with two bottom quarks and two photons in proton-proton collisions at a center of mass energy of 13 TeV with the ATLAS detector

A search for the resonant production of a heavy scalar $X$ decaying into a Higgs boson and a new lighter scalar $S$, through the process $X \to S(\to bb) H(\to γγ)$, where the two photons are consistent with the Higgs boson decay, is performed. The search is conducted using an integrated luminosity of 140 fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The search is performed over the mass range 170 $\leq$ $m_{X}$ $\leq$ 1000 GeV and 15 $\leq$ $m_{S}$ $\leq$ 500 GeV. Parameterised neural networks are used to enhance the signal purity and to achieve continuous sensitivity in a domain of the ($m_{X}$, $m_{S}$) plane. No significant excess above the expected background is found and 95% CL upper limits are set on the cross section times branching ratio, ranging from 39 fb to 0.09 fb. The largest deviation from the background-only expectation occurs for ($m_{X}$, $m_{S}$) = (575, 200) GeV with a local (global) significance of 3.5 (2.0) standard deviations.

hep-ex↗

arXiv2024

Combination of searches for singly and doubly charged Higgs bosons produced via vector-boson fusion in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A combination of searches for singly and doubly charged Higgs bosons, $H^{\pm}$ and $H^{\pm\pm}$, 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.

hep-ex↗

arXiv2024

A search for $R$-parity violating supersymmetric decays of the top squark to a $b$-jet and a lepton in $\sqrt{s}$ = 13 TeV $pp$ collisions with the ATLAS detector

A search is presented for direct pair production of the stop, the supersymmetric partner of the top quark, in a decay through an $R$-parity violating coupling to a charged lepton and a $b$-quark. The dataset corresponds to an integrated luminosity of 140 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of $\sqrt{s}$ = 13 TeV collected between 2015 and 2018 by the ATLAS detector at the LHC. The final state has two charged leptons (electrons or muons) and two $b$-jets. The results of the search are interpreted in the context of a Minimal Supersymmetric Standard Model with an additional $B-L$ 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 $b$-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.

hep-ex↗