arXiv subjects

ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 289 records · Page 16Linked to original sources

arXiv2024

Measurements of electroweak $W^{\pm}Z$ boson pair production in association with two jets in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Measurements of integrated and differential cross-sections for electroweak $W^{\pm}Z$ production in association with two jets ($W^{\pm}Zjj$) 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 $W^{\pm}Zjj$ candidate events are reconstructed using leptonic decay modes of the gauge bosons. Events containing three identified leptons, either electrons or muons, and two jets are selected. Processes involving pure electroweak $W^{\pm}Zjj$ production at Born level are separated from $W^{\pm}Zjj$ production involving a strong coupling. The measured integrated fiducial cross-section of electroweak $W^{\pm}Zjj$ production per lepton flavour is $σ_{WZjj\mathrm{-EW} \rightarrow \ell^{'} ν\ell \ell jj} = 0.368 \; \pm 0.037 \,(\mathrm{stat.}) \; \pm 0.059 \,(\mathrm{syst.}) \; \pm 0.003 \,(\mathrm{lumi.}) \; \mathrm{fb}$, where $\ell$ and $\ell^{'}$ are either an electron or a muon. Respective cross-sections of electroweak and strong $W^{\pm}Zjj$ production are measured separately for events with exactly two jets or with more than two jets, and in three bins of the invariant mass of the two jets. The inclusive $W^{\pm}Zjj$ production cross-section, without separating electroweak and strong production, is also measured to be $σ_{WZjj \rightarrow \ell^{'} ν\ell \ell jj} = 1.462 \; \pm 0.063 \,(\mathrm{stat.}) \; \pm 0.118 \,(\mathrm{syst.}) \; \pm 0.012 \,(\mathrm{lumi.}) \; \mathrm{fb}$, per lepton flavour. The inclusive $W^{\pm}Zjj$ production cross-section is measured differentially for several kinematic observables. Finally, the measurements are used to constrain anomalous quartic gauge couplings by extracting 95% confidence level intervals on dimension-$8$ operators.

hep-ex↗

arXiv2024

Combination of measurements of the top quark mass from data collected by the ATLAS and CMS experiments at $\sqrt{s}$ = 7 and 8 TeV

A combination of fifteen top quark mass measurements performed by the ATLAS and CMS experiments at the LHC is presented. The data sets used correspond to an integrated luminosity of up to 5 and 20$^{-1}$ of proton-proton collisions at center-of-mass energies of 7 and 8 TeV, respectively. The combination includes measurements in top quark pair events that exploit both the semileptonic and hadronic decays of the top quark, and a measurement using events enriched in single top quark production via the electroweak $t$-channel. The combination accounts for the correlations between measurements and achieves an improvement in the total uncertainty of 31% relative to the most precise input measurement. The result is $m_\mathrm{t}$ = 172.52 $\pm$ 0.14 (stat) $\pm$ 0.30 (syst) GeV, with a total uncertainty of 0.33 GeV.

hep-ex↗

arXiv2024

Inclusive and differential cross-section measurements of $t\bar{t}Z$ production in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector, including EFT and spin-correlation interpretations

Measurements of both the inclusive and differential production cross sections of a top-quark-top-antiquark pair in association with a $Z$ boson ($t\bar{t}Z$) are presented. Final states with two, three or four isolated leptons (electrons or muons) are targeted. The measurements use the data recorded by the ATLAS detector in $pp$ collisions at $\sqrt{s}=13$ TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of $140$ fb$^{-1}$. The inclusive cross section is measured to be $σ_{t\bar{t}Z}= 0.86 \pm 0.04~\mathrm{(stat.)} \pm 0.04~\mathrm{(syst.)}~$pb and found to be in agreement with the most advanced Standard Model predictions. The differential measurements are presented as a function of a number of observables that probe the kinematics of the $t\bar{t}Z$ system. Both the absolute and normalised differential cross-section measurements are performed at particle level and parton level for specific fiducial volumes, and are compared with NLO+NNLL theoretical predictions. The results are interpreted in the framework of Standard Model effective field theory and used to set limits on a large number of dimension-6 operators involving the top quark. The first measurement of spin correlations in $t\bar{t}Z$ events is presented: the results are in agreement with the Standard Model expectations, and the null hypothesis of no spin correlations is disfavoured with a significance of $1.8$ standard deviations.

hep-ex↗

arXiv2024

A statistical combination of ATLAS Run 2 searches for charginos and neutralinos at the LHC

Statistical combinations of searches for charginos and neutralinos using various decay channels are performed using $139\,$fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13\,$TeV with the ATLAS detector at the Large Hadron Collider. Searches targeting pure-wino chargino pair production, pure-wino chargino-neutralino production, or higgsino production decaying via Standard Model $W$, $Z$, or $h$ bosons are combined to extend the mass reach to the produced SUSY particles by 30-100 GeV. The depth of the sensitivity of the original searches is also improved by the combinations, lowering the 95% CL cross-section upper limits by 15%-40%.

hep-ex↗

arXiv2024

The ATLAS Trigger System for LHC Run 3 and Trigger performance in 2022

The ATLAS trigger system is a crucial component of the ATLAS experiment at the LHC. It is responsible for selecting events in line with the ATLAS physics programme. This paper presents an overview of the changes to the trigger and data acquisition system during the second long shutdown of the LHC, and shows the performance of the trigger system and its components in the proton-proton collisions during the 2022 commissioning period as well as its expected performance in proton-proton and heavy-ion collisions for the remainder of the third LHC data-taking period (2022-2025).

hep-ex↗

arXiv2024

Search for heavy Majorana neutrinos in $e^{\pm} e^{\pm}$ and $e^{\pm} μ^{\pm}$ final states via WW scattering in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for heavy Majorana neutrinos in scattering of same-sign $W$ boson pairs in proton-proton collisions at $\sqrt{s}= 13$ TeV at the LHC is reported. The dataset used corresponds to an integrated luminosity of 140 fb$^{-1}$, collected with the ATLAS detector during 2015-2018. The search is performed in final states including a same-sign $ee$ or $eμ$ pair and at least two jets with large invariant mass and a large rapidity difference. No significant excess of events with respect to the Standard Model background predictions is observed. The results are interpreted in a benchmark scenario of the Phenomenological Type-I Seesaw model. New constraints are set on the values of the $\vert V_{e N} \vert^{2}$ and $\vert V_{e N} V^{*}_{μN} \vert$ parameters for heavy Majorana neutrino masses between 50 GeV and 20 TeV, where $V_{\ell N}$ is the matrix element describing the mixing of the heavy Majorana neutrino mass eigenstate with the Standard Model neutrino of flavour $\ell = e, μ$. The sensitivity to the Weinberg operator is investigated and constraints on the effective $ee$ and $eμ$ Majorana neutrino masses are reported. The statistical combination of the $ee$ and $eμ$ channels with the previously published $μμ$ channel is performed.

hep-ex↗

arXiv2024

Search for Nearly Mass-Degenerate Higgsinos Using Low-Momentum Mildly Displaced Tracks in $pp$ Collisions at $\sqrt{s}$ = 13 TeV with the ATLAS Detector

Higgsinos with masses near the electroweak scale can solve the hierarchy problem and provide a dark matter candidate, while detecting them at the LHC remains challenging if their mass splitting is $\mathcal{O}(1 \text{GeV})$. This Letter presents a novel search for nearly mass-degenerate Higgsinos in events with an energetic jet, missing transverse momentum, and a low-momentum track with a significant transverse impact parameter using 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS experiment. For the first time since LEP, a range of mass splittings between the lightest charged and neutral Higgsinos from $0.3$ GeV to $0.9$ GeV is excluded at 95$\%$ confidence level, with a maximum reach of approximately $170$ GeV in the Higgsino mass.

hep-ex↗

arXiv2024

Combination of searches for pair-produced leptoquarks at $\sqrt{s} = 13$ TeV with the ATLAS detector

A statistical combination of various searches for pair-produced leptoquarks is presented, using the full LHC Run 2 (2015-2018) data set of $139$ fb$^{-1}$ collected with the ATLAS detector from proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV. All possible decays of the leptoquarks into quarks of the third generation and charged or neutral leptons of any generation are investigated. Since no significant deviations from the Standard Model expectation are observed in any of the individual analyses, combined exclusion limits are set on the production cross-sections for scalar and vector leptoquarks. The resulting lower bounds on leptoquark masses exceed those from the individual analyses by up to 100 GeV, depending on the signal hypothesis.

hep-ex↗

arXiv2024

Search for high-mass resonances in final states with a $τ$-lepton and missing transverse momentum with the ATLAS detector

A search for high-mass resonances decaying into a $τ$-lepton and a neutrino using proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV is presented. The full Run 2 data sample corresponding to an integrated luminosity of 139 fb$^{-1}$ recorded by the ATLAS experiment in the years 2015-2018 is analyzed. The $τ$-lepton is reconstructed in its hadronic decay modes and the total transverse momentum carried out by neutrinos is inferred from the reconstructed missing transverse momentum. The search for new physics is performed on the transverse mass between the $τ$-lepton and the missing transverse momentum. No excess of events above the Standard Model expectation is observed and upper exclusion limits are set on the $W^\prime\to τν$ production cross-section. Heavy $W^\prime$ vector bosons with masses up to 5.0 TeV are excluded at 95% confidence level, assuming that they have the same couplings as the Standard Model $W$ boson. For non-universal couplings, $W^\prime$ bosons are excluded for masses less than 3.5-5.0 TeV, depending on the model parameters. In addition, model-independent limits on the visible cross-section times branching ratio are determined as a function of the lower threshold on the transverse mass of the $τ$-lepton and missing transverse momentum.

hep-ex↗

arXiv2024

Searches for exclusive Higgs boson decays into $D^*γ$ and $Z$ boson decays into $D^0γ$ and $K^0_sγ$ in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Searches for the exclusive decays of the Higgs boson into $D^*γ$ and of the $Z$ boson into $D^0γ$ and $K^0_sγ$ can probe flavour-violating Higgs and $Z$ boson couplings to light quarks. Searches for these decays are performed with a $pp$ collision data sample corresponding to an integrated luminosity of $136.3$ fb$^{-1}$ collected at $\sqrt{s}=13$ TeV between 2016-2018 with the ATLAS detector at the CERN Large Hadron Collider. In the $D^*γ$ and $D^0γ$ channels, the observed (expected) 95$\%$ confidence-level upper limits on the respective branching fractions are ${\cal B}(H\rightarrow D^*γ)< 1.0 (1.2)\times 10^{-3}$, ${\cal B}(Z\rightarrow D^0γ)< 4.0 (3.4)\times 10^{-6}$, while the corresponding results in the $K^0_sγ$ channel are ${\cal B}(Z\rightarrow K^0_sγ)< 3.1 (3.0)\times 10^{-6}$.

hep-ex↗

arXiv2024

Search for pair production of higgsinos in events with two Higgs bosons and missing transverse momentum in $\sqrt{s}=13$ TeV $pp$ collisions at the ATLAS experiment

This paper presents a search for pair production of higgsinos, the supersymmetric partners of the Higgs bosons, in scenarios with gauge-mediated supersymmetry breaking. Each higgsino is assumed to decay into a Higgs boson and a nearly massless gravitino. The search targets events where each Higgs boson decays into $b\bar{b}$, leading to a reconstructed final state with at least three energetic $b$-jets and This paper presents a search for pair production of higgsinos, the supersymmetric partners of the Higgs bosons, in scenarios with gauge-mediated supersymmetry breaking. Each higgsino is assumed to decay into a Higgs boson and a nearly massless gravitino. The search targets events where each Higgs boson decays into $b\bar{b}$, leading to a reconstructed final state with at least three energetic $b$-jets and missing transverse momentum. Two complementary analysis channels are used, with each channel specifically targeting either low or high values of the higgsino mass. The low-mass (high-mass) channel exploits 126 (139) fb$^{-1}$ of $\sqrt{s}=13$ TeV data collected by the ATLAS detector during Run 2 of the Large Hadron Collider. No significant excess above the Standard Model prediction is found. At 95% confidence level, masses between 130 GeV and 940 GeV are excluded for higgsinos decaying exclusively into Higgs bosons and gravitinos. Exclusion limits as a function of the higgsino decay branching ratio to a Higgs boson are also reported.

hep-ex↗

arXiv2024

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

A combination of searches for a new resonance decaying into a Higgs boson pair is presented, using up to 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the LHC. The combination includes searches performed in three decay channels: $b\bar{b}b\bar{b}$, $b\bar{b}τ^+τ^-$ and $b\bar{b}γγ$. No excess above the expected Standard Model background is observed and upper limits are set at the 95% confidence level on the production cross section of Higgs boson pairs originating from the decay of a narrow scalar resonance with mass in the range 251 GeV-5 TeV. The observed (expected) limits are in the range 0.96-600 fb (1.2-390 fb). The limits are interpreted in the Type-I Two-Higgs-Doublet Model and the Minimimal Supersymmetric Standard Model, and constrain parameter space not previously excluded by other searches.

hep-ex↗

arXiv2024

The ATLAS Experiment at the CERN Large Hadron Collider: A Description of the Detector Configuration for Run 3

The ATLAS detector is installed in its experimental cavern at Point 1 of the CERN Large Hadron Collider. During Run 2 of the LHC, a luminosity of $\mathcal{L}=2\times 10^{34}\mathrm{cm}^{-2}\mathrm{s}^{-1}$ was routinely achieved at the start of fills, twice the design luminosity. For Run 3, accelerator improvements, notably luminosity levelling, allow sustained running at an instantaneous luminosity of $\mathcal{L}=2\times 10^{34}\mathrm{cm}^{-2}\mathrm{s}^{-1}$, with an average of up to 60 interactions per bunch crossing. The ATLAS detector has been upgraded to recover Run 1 single-lepton trigger thresholds while operating comfortably under Run 3 sustained pileup conditions. A fourth pixel layer 3.3 cm from the beam axis was added before Run 2 to improve vertex reconstruction and $b$-tagging performance. New Liquid Argon Calorimeter digital trigger electronics, with corresponding upgrades to the Trigger and Data Acquisition system, take advantage of a factor of 10 finer granularity to improve triggering on electrons, photons, taus, and hadronic signatures through increased pileup rejection. The inner muon endcap wheels were replaced by New Small Wheels with Micromegas and small-strip Thin Gap Chamber detectors, providing both precision tracking and Level-1 Muon trigger functionality. Tile Calorimeter scintillation counters were added to improve electron energy resolution and background rejection. Upgrades to Minimum Bias Trigger Scintillators and Forward Detectors improve luminosity monitoring and enable total proton-proton cross section, diffractive physics, and heavy ion measurements. These upgrades are all compatible with operation in the much harsher environment anticipated after the High-Luminosity upgrade of the LHC and are the first steps towards preparing ATLAS for the High-Luminosity upgrade of the LHC. This paper describes the Run 3 configuration of the ATLAS detector.

physics.ins-det↗

arXiv2024

Performance of the ATLAS forward proton Time-of-Flight detector in Run 2

We present performance studies of the Time-of-Flight (ToF) subdetector of the ATLAS Forward Proton (AFP) detector at the LHC. Efficiencies and resolutions are measured using high-statistics data samples collected at low and moderate pile-up in 2017, the first year when the detectors were installed on both sides of the interaction region. While low efficiencies are observed, of the order of a few percent, the resolutions of the two ToF detectors measured individually are 21 ps and 28 ps, yielding an expected resolution of the longitudinal position of the interaction, $z_{vtx}$, in the central ATLAS detector of $5.3 \pm 0.6$ mm. This is in agreement with the observed width of the distribution of the difference between $z_{vtx}$ measured independently by the central ATLAS tracker and by the ToF detector, of $6.0 \pm 2.0$ mm.

physics.ins-det↗

arXiv2024

ATLAS Run 2 searches for electroweak production of supersymmetric particles interpreted within the pMSSM

A summary of the constraints from searches performed by the ATLAS Collaboration for the electroweak production of charginos and neutralinos is presented. Results from eight separate ATLAS searches are considered, each using 140 fb$^{-1}$ of proton-proton data at a centre-of-mass energy of $\sqrt{s}$=13 TeV collected at the Large Hadron Collider during its second data-taking run. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, where R-parity conservation is assumed and the lightest supersymmetric particle is assumed to be the lightest neutralino. Constraints from previous electroweak, flavour and dark matter related measurements are also considered. The results are presented in terms of constraints on supersymmetric particle masses and are compared with limits from simplified models. Also shown is the impact of ATLAS searches on parameters such as the dark matter relic density and the spin-dependent and spin-independent scattering cross-sections targeted by direct dark matter detection experiments. The Higgs boson and Z boson `funnel regions', where a low-mass neutralino would not oversaturate the dark matter relic abundance, are almost completely excluded by the considered constraints. Example spectra for non-excluded supersymmetric models with light charginos and neutralinos are also presented.

hep-ex↗

arXiv2024

Search for pair-production of vector-like quarks in lepton+jets final states containing at least one $b$-tagged jet using the Run 2 data from the ATLAS experiment

A search is presented for the pair-production of heavy vector-like quarks in the lepton+jets final state using 140 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}= 13$ TeV collected with the ATLAS detector. The search is optimised for vector-like top-quarks ($T$) that decay into a $W$ boson and a $b$-quark, with one $W$ boson decaying leptonically and the other hadronically. Other vector-like quark flavours and decay modes are also considered. Events are selected with one high transverse-momentum electron or muon, large missing transverse momentum, a large-radius jet identified as a $W$ boson, and multiple small-radius jets, at least one of which is $b$-tagged. Vector-like $T$-quarks with 100% branching ratio to $Wb$ are excluded at 95% CL for masses below 1700 GeV. These limits are also applied to vector-like $Y$-quarks, which decay exclusively into a $W$ boson and a $b$-quark. Isospin singlets with $ {\cal B}(T \to Wb:Ht:Zt)={1/2}:{1/4}:{1/4}$ are excluded for masses below 1360 GeV.

hep-ex↗

arXiv2024

Test of CP-invariance of the Higgs boson in vector-boson fusion production and its decay into four leptons

A search for CP violation in the decay kinematics and vector-boson production of the Higgs boson is performed in the $H \to ZZ^{*} \to 4\ell$ ($\ell = e,μ$) decay channel. The results are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 $\mbox{fb$^{-1}$}$. Matrix element-based optimal observables are used to constrain CP-odd couplings beyond the Standard Model in the framework of Standard Model effective field theory expressed in the Warsaw and Higgs bases. Differential fiducial cross-section measurements of the optimal observables are also performed, and a new fiducial cross-section measurement for vector-boson-fusion production is provided. All measurements are in agreement with the Standard Model prediction of a CP-even Higgs boson.

hep-ex↗

arXiv2024

Measurement of vector boson production cross sections and their ratios using $pp$ collisions at $\sqrt{s}=13.6$ TeV with the ATLAS detector

Fiducial and total $W^\pm$ and $Z$ boson cross sections, their ratios and the ratio of top-antitop-quark pair and $W$-boson fiducial cross sections are measured in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13.6$ TeV, corresponding to an integrated luminosity of 29 fb$^{-1}$ of data collected in 2022 by the ATLAS experiment at the Large Hadron Collider. The measured fiducial cross-section values for $W^+\to \ell^+ν$, $W^-\to \ell^-\barν$, and $Z\to \ell^+\ell^-$ ($\ell=e$ or $μ$) boson productions are $4250\pm 150$ pb, $3310\pm 120$ pb, and $744\pm 20$ pb, respectively, where the uncertainty is the total uncertainty, including that arising from the luminosity of about 2.2%. The measurements are in agreement with Standard-Model predictions calculated at next-to-next-to-leading-order in $α_s$, next-to-next-to-leading logarithmic accuracy and next-to-leading-order electroweak accuracy.

hep-ex↗