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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 343 records · Page 19Linked to original sources
arXiv2024

Search for exclusive Higgs and $Z$ boson decays to $ϕγ$ and $ργ$ with the ATLAS detector

A search for the exclusive decays of the Higgs and $Z$ bosons to a $ϕ$ or $ρ$ meson and a photon is performed with a $pp$ collision data sample corresponding to an integrated luminosity of up to 35.6 $fb^{-1}$ collected at $\sqrt{s}=13$ TeV with the ATLAS detector at the CERN Large Hadron Collider. These decays have been suggested as a probe of the Higgs boson couplings to light quarks. No significant excess of events is observed above the background, as expected from the Standard Model. Upper limits at 95$\% $ confidence level were obtained on the branching fractions of the Higgs boson decays to $ϕγ$ and $ργ$ of $5.0\times10^{-4}$ and $10.4\times10^{-4}$, respectively. The corresponding 95$\% $ confidence level upper limits for the $Z$ boson decays are $0.7\times10^{-6}$ and $4.0\times10^{-6}$ for $ϕγ$ and $ργ$, respectively.

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arXiv2024

Search for single vector-like $B$ quark production and decay via $B\rightarrow bH(b\bar{b})$ in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search is presented for single production of a vector-like $B$ quark decaying into a Standard Model $b$-quark and a Standard Model Higgs boson, which decays into a $b\bar{b}$ pair. The search is carried out in 139 fb$^{-1}$ of $\sqrt{s} = 13$ TeV proton-proton collision data collected by the ATLAS detector at the LHC between 2015 and 2018. No significant deviation from the Standard Model background prediction is observed, and mass-dependent exclusion limits at the 95% confidence level are set on the resonance production cross-section in several theoretical scenarios determined by the couplings $c_W$, $c_Z$ and $c_H$ between the $B$ quark and the Standard Model $W$, $Z$ and Higgs bosons, respectively. For a vector-like $B$ occurring as an isospin singlet, the search excludes values of $c_W$ greater than 0.45 for a $B$ resonance mass ($m_B$) between 1.0 and 1.2 TeV. For 1.2 TeV < $m_B$ < 2.0 TeV, $c_W$ values larger than 0.50-0.65 are excluded. If the $B$ occurs as part of a $(B,Y)$ doublet, the smallest excluded $c_Z$ coupling values range between 0.3 and 0.5 across the investigated resonance mass range 1.0 TeV < $m_B$ < 2.0 TeV.

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arXiv2024

Pursuit of paired dijet resonances in the Run 2 dataset with ATLAS

New particles with large masses that decay into hadronically interacting particles are predicted by many models of physics beyond the Standard Model. A search for a massive resonance that decays into pairs of dijet resonances is performed using 140 fb$^{-1}$ of proton$-$proton collisions at $\sqrt{s}=13$ TeV recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. Resonances are searched for in the invariant mass of the tetrajet system, and in the average invariant mass of the pair of dijet systems. A data-driven background estimate is obtained by fitting the tetrajet and dijet invariant mass distributions with a four-parameter dijet function and a search for local excesses from resonant production of dijet pairs is performed. No significant excess of events beyond the Standard Model expectation is observed, and upper limits are set on the production cross-sections of new physics scenarios.

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arXiv2024

Search for exotic decays of the Higgs boson into long-lived particles in $pp$ collisions at $\sqrt{s} = 13$ TeV using displaced vertices in the ATLAS inner detector

A novel search for exotic decays of the Higgs boson into pairs of long-lived neutral particles, each decaying into a bottom quark pair, is performed using 139 fb$^{-1}$ of $\sqrt{s} = 13$ TeV proton-proton collision data collected with the ATLAS detector at the LHC. Events consistent with the production of a Higgs boson in association with a leptonically decaying $Z$ boson are analysed. Long-lived particle (LLP) decays are reconstructed from inner-detector tracks as displaced vertices with high mass and track multiplicity relative to Standard Model processes. The analysis selection requires the presence of at least two displaced vertices, effectively suppressing Standard Model backgrounds. The residual background contribution is estimated using a data-driven technique. No excess over Standard Model predictions is observed, and upper limits are set on the branching ratio of the Higgs boson to LLPs. Branching ratios above 10% are excluded at 95% confidence level for LLP mean proper lifetimes $cτ$ as small as 4 mm and as large as 100 mm. For LLP masses below 40 GeV, these results represent the most stringent constraint in this lifetime regime.

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arXiv2024

Search for non-resonant production of semi-visible jets using Run 2 data in ATLAS

Semi-visible jets, with a significant contribution to the event's missing transverse momentum, can arise in strongly interacting dark sectors. This results in an event topology where one of the jets can be aligned with the direction of the missing transverse momentum. The first search for semi-visible jets produced via a $t$-channel mediator exchange is presented. The analysis uses proton-proton collisions with an integrated luminosity of 139 fb$^{-1}$ and a centre-of-mass energy of 13 TeV, collected with the ATLAS detector during the Run 2 of the LHC. No excess over Standard Model predictions is observed. Assuming a coupling strength of unity between the mediator, a Standard Model quark and a dark quark, mediator masses up to 2.7 TeV are excluded at the 95% confidence level. Upper limits on the coupling strength are also derived.

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arXiv2023

The ATLAS Inner Detector Trigger performance in pp collisions at 13 TeV during LHC Run 2

The design and performance of the inner detector trigger for the high level trigger of the ATLAS experiment at the Large Hadron Collider during the 2016-18 data taking period is discussed. In 2016, 2017, and 2018 the ATLAS detector recorded 35.6 fb$^{-1}$, 46.9 fb$^{-1}$, and 60.6 fb$^{-1}$ respectively of proton-proton collision data at a centre-of-mass energy of 13 TeV. In order to deal with the very high interaction multiplicities per bunch crossing expected with the 13 TeV collisions the inner detector trigger was redesigned during the long shutdown of the Large Hadron Collider from 2013 until 2015. An overview of these developments is provided and the performance of the tracking in the trigger for the muon, electron, tau and $b$-jet signatures is discussed. The high performance of the inner detector trigger with these extreme interaction multiplicities demonstrates how the inner detector tracking continues to lie at the heart of the trigger performance and is essential in enabling the ATLAS physics programme.

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arXiv2023

Search for the $Zγ$ decay mode of new high-mass resonances in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This letter presents a search for narrow, high-mass resonances in the $Zγ$ final state with the $Z$ boson decaying into a pair of electrons or muons. The $\sqrt{s}=13$ TeV $pp$ collision data were recorded by the ATLAS detector at the CERN Large Hadron Collider and have an integrated luminosity of 140 fb$^{-1}$. The data are found to be in agreement with the Standard Model background expectation. Upper limits are set on the resonance production cross section times the decay branching ratio into $Zγ$. For spin-0 resonances produced via gluon-gluon fusion, the observed limits at 95% confidence level vary between 65.5 fb and 0.6 fb, while for spin-2 resonances produced via gluon-gluon fusion (or quark-antiquark initial states) limits vary between 77.4 (76.1) fb and 0.6 (0.5) fb, for the mass range from 220 GeV to 3400 GeV.

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arXiv2023

Search for a new pseudoscalar decaying into a pair of muons in events with a top-quark pair at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for a new pseudoscalar $a$-boson produced in events with a top-quark pair, where the $a$-boson decays into a pair of muons, is performed using $\sqrt{s} = 13$ TeV $pp$ collision data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of $139\, \mathrm{fb}^{-1}$. The search targets the final state where only one top quark decays to an electron or muon, resulting in a signature with three leptons $eμμ$ and $μμμ$. No significant excess of events above the Standard Model expectation is observed and upper limits are set on two signal models: $pp \rightarrow t\bar{t}a$ and $pp \rightarrow t\bar{t}$ with $t \rightarrow H^\pm b$, $H^\pm \rightarrow W^\pm a$, where $a\rightarrowμμ$, in the mass ranges $15$ GeV $ < m_a < 72$ GeV and $120$ GeV $ \leq m_{H^{\pm}} \leq 160$ GeV.

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arXiv2023

Search for Dark Photons in Rare $Z$ Boson Decays with the ATLAS Detector

A search for events with a dark photon produced in association with a dark Higgs boson via rare decays of the Standard Model $Z$ boson is presented, using 139 fb$^{-1}$ of $\sqrt{s} = 13$ TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. The dark Higgs boson decays into a pair of dark photons, and at least two of the three dark photons must each decay into a pair of electrons or muons, resulting in at least two same-flavor opposite-charge lepton pairs in the final state. The data are found to be consistent with the background prediction, and upper limits are set on the dark photon's coupling to the dark Higgs boson times the kinetic mixing between the Standard Model photon and the dark photon, $α_{D}\varepsilon^2$, in the dark photon mass range of $[5, 40]$ GeV except for the $Υ$ mass window $[8.8, 11.1]$ GeV. This search explores new parameter space not previously excluded by other experiments.

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arXiv2023

Search for high-mass $Wγ$ and $Zγ$ resonances using hadronic W/Z boson decays from 139 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=$ 13 TeV with the ATLAS detector

A search for high-mass charged and neutral bosons decaying to $Wγ$ and $Zγ$ final states is presented in this paper. The analysis uses a data sample of $\sqrt{s} = 13$ TeV proton-proton collisions with an integrated luminosity of 139 fb$^{-1}$ collected by the ATLAS detector during LHC Run 2 operation. The sensitivity of the search is determined using models of the production and decay of spin-1 charged bosons and spin-0/2 neutral bosons. The range of resonance masses explored extends from 1.0 TeV to 6.8 TeV. At these high resonance masses, it is beneficial to target the hadronic decays of the $W$ and $Z$ bosons because of their large branching fractions. The decay products of the high-momentum $W/Z$ bosons are strongly collimated and boosted-boson tagging techniques are employed to improve the sensitivity. No evidence of a signal above the Standard Model backgrounds is observed, and upper limits on the production cross-sections of these bosons times their branching fractions to $Wγ$ and $Zγ$ are derived for various boson production models.

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arXiv2023

Measurement of the Higgs boson mass with $H \rightarrow γγ$ decays in 140 fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collisions with the ATLAS detector

The mass of the Higgs boson is measured in the $H\toγγ$ decay channel, exploiting the high resolution of the invariant mass of photon pairs reconstructed from the decays of Higgs bosons produced in proton-proton collisions at a centre-of-mass energy $\sqrt{s}=13$ TeV. The dataset was collected between 2015 and 2018 by the ATLAS detector at the Large Hadron Collider, and corresponds to an integrated luminosity of 140 fb$^{-1}$. The measured value of the Higgs boson mass is $125.17 \pm 0.11 \mathrm{(stat.)} \pm 0.09 \mathrm{(syst.)}$ GeV and is based on an improved energy scale calibration for photons, whose impact on the measurement is about four times smaller than in the previous publication. A combination with the corresponding measurement using 7 and 8 TeV $pp$ collision ATLAS data results in a Higgs boson mass measurement of $125.22 \pm 0.11 \mathrm{(stat.)} \pm 0.09 \mathrm{(syst.)}$ GeV. With an uncertainty of 1.1 per mille, this is currently the most precise measurement of the mass of the Higgs boson from a single decay channel.

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arXiv2023

Search for displaced photons produced in exotic decays of the Higgs boson using 13 TeV $pp$ collisions with the ATLAS detector

A search is performed for delayed and nonpointing photons originating from the displaced decay of a neutral long-lived particle (LLP). The analysis uses the full Run 2 data set of proton-proton collisions delivered by the LHC at a center-of-mass energy of $\sqrt{s}=13$ TeV between 2015 and 2018 and recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb$^{-1}$. The capabilities of the ATLAS electromagnetic calorimeter are exploited to precisely measure the arrival times and trajectories of photons. The results are interpreted in a scenario where the LLPs are pair-produced in exotic decays of the 125 GeV Higgs boson, and each LLP subsequently decays into a photon and a particle that escapes direct detection, giving rise to missing transverse momentum. No significant excess is observed above the expectation due to Standard Model background processes. The results are used to set upper limits on the branching ratio of the exotic decay of the Higgs boson. A model-independent limit is also set on the production of photons with large values of displacement and time delay.

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arXiv2023

Measurement of the $t\bar{t}$ cross section and its ratio to the $Z$ production cross section using $pp$ collisions at $\sqrt{s} = 13.6$ TeV with the ATLAS detector

The inclusive top-quark-pair production cross section $σ_{t\bar{t}}$ and its ratio to the $Z$-boson production cross section have been measured in proton--proton collisions at $\sqrt{s} = 13.6$ TeV, using 29 fb${}^{-1}$ of data collected in 2022 with the ATLAS experiment at the Large Hadron Collider. Using events with an opposite-charge electron-muon pair and $b$-tagged jets, and assuming Standard Model decays, the top-quark-pair production cross section is measured to be $σ_{t\bar{t}} = 850 \pm 3\mathrm{(stat.)}\pm 18\mathrm{(syst.)}\pm 20\mathrm{(lumi.)}$ pb. The ratio of the $t\bar{t}$ and the $Z$-boson production cross sections is also measured, where the $Z$-boson contribution is determined for inclusive $e^+e^-$ and $μ^+μ^-$ events in a fiducial phase space. The relative uncertainty on the ratio is reduced compared to the $t\bar{t}$ cross section, thanks to the cancellation of several systematic uncertainties. The result for the ratio, $R_{t\bar{t}/Z} = 1.145 \pm 0.003\mathrm{(stat.)}\pm 0.021\mathrm{(syst.)}\pm 0.002\mathrm{(lumi.)}$ is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.

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arXiv2023

Search for dark matter produced in association with a Higgs boson decaying to tau leptons at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for dark matter produced in association with a Higgs boson in final states with two hadronically decaying $τ$-leptons and missing transverse momentum is presented. The analysis uses $139$ fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. No evidence for physics beyond the Standard Model is found. The results are interpreted in terms of a 2HDM+$a$ model. Exclusion limits at 95% confidence level are derived. Model-independent limits are also set on the visible cross section for processes beyond the Standard Model producing missing transverse momentum in association with a Higgs boson decaying to $τ$-leptons.

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arXiv2023

Performance of the reconstruction of large impact parameter tracks in the inner detector of ATLAS

Searches for long-lived particles (LLPs) are among the most promising avenues for discovering physics beyond the Standard Model at the Large Hadron Collider (LHC). However, displaced signatures are notoriously difficult to identify due to their ability to evade standard object reconstruction strategies. In particular, the default ATLAS track reconstruction applies strict pointing requirements which limit sensitivity to charged particles originating far from the primary interaction point. To recover efficiency for LLPs decaying within the tracking detector volume, the ATLAS Collaboration employs a dedicated large-radius tracking (LRT) pass with loosened pointing requirements. During Run 2 of the LHC, the LRT implementation produced many incorrectly reconstructed tracks and was therefore only deployed in small subsets of events. In preparation for LHC Run 3, ATLAS has significantly improved both standard and large-radius track reconstruction performance, allowing for LRT to run in all events. This development greatly expands the potential phase-space of LLP searches and streamlines LLP analysis workflows. This paper will highlight the above achievement and report on the readiness of the ATLAS detector for track-based LLP searches in Run 3.

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arXiv2023

Search for direct production of winos and higgsinos in events with two same-charge leptons or three leptons in $pp$ collision data at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for supersymmetry targeting the direct production of winos and higgsinos is conducted in final states with either two leptons ($e$ or $μ$) with the same electric charge, or three leptons. The analysis uses 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ TeV collected with the ATLAS detector during Run 2 of the Large Hadron Collider. No significant excess over the Standard Model expectation is observed. Simplified and complete models with and without $R$-parity conservation are considered. In topologies with intermediate states including either $Wh$ or $WZ$ pairs, wino masses up to 525 GeV and 250 GeV are excluded, respectively, for a bino of vanishing mass. Higgsino masses smaller than 440 GeV are excluded in a natural $R$-parity-violating model with bilinear terms. Upper limits on the production cross section of generic events beyond the Standard Model as low as 40 ab are obtained in signal regions optimised for these models and also for an $R$-parity-violating scenario with baryon-number-violating higgsino decays into top quarks and jets. The analysis significantly improves sensitivity to supersymmetric models and other processes beyond the Standard Model that may contribute to the considered final states.

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arXiv2023

Measurements of Higgs boson production cross-sections in the $H\toτ^{+}τ^{-}$ decay channel in $pp$ collisions at $\sqrt{s}=13\,\text{TeV}$ with the ATLAS detector

Measurements of the production cross-sections of the Standard Model (SM) Higgs boson ($H$) decaying into a pair of $τ$-leptons are presented. The measurements use data collected with the ATLAS detector from $pp$ collisions produced at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13\,\text{TeV}$, corresponding to an integrated luminosity of $139\,\text{fb}^{-1}$. Leptonic ($τ\to\ellν_{\ell}ν_τ$) and hadronic ($τ\to\text{hadrons}~ν_τ$) decays of the $τ$-lepton are considered. All measurements account for the branching ratio of $H\toττ$ and are performed with a requirement $|y_H|<2.5$, where $y_H$ is the true Higgs boson rapidity. The cross-section of the $pp\to H\toττ$ process is measured to be $2.94 \pm 0.21 \text{(stat)} ^{+\,0.37}_{-\,0.32} \text{(syst)}$ pb, in agreement with the SM prediction of $3.17\pm0.09~ \mbox{pb}$. Inclusive cross-sections are determined separately for the four dominant production modes: $2.65 \pm 0.41 \text{(stat)} ^{+\,0.91}_{-\,0.67} \text{(syst)}$ pb for gluon$-$gluon fusion, $0.197 \pm 0.028 \text{(stat)} ^{+\,0.032}_{-\,0.026} \text{(syst)}$ pb for vector-boson fusion, $0.115 \pm 0.058 \text{(stat)} ^{+\,0.042}_{-\,0.040} \text{(syst)}$ pb for vector-boson associated production, and $0.033 \pm 0.031 \text{(stat)} ^{+\,0.022}_{-\,0.017} \text{(syst)}$ pb for top-quark pair associated production. Measurements in exclusive regions of the phase space, using the simplified template cross-section framework, are also performed. All results are in agreement with the SM predictions.

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arXiv2023

Measurement of the $B_s^0 \to μμ$ Effective Lifetime with the ATLAS Detector

This paper reports the first ATLAS measurement of the $B_s^0 \to μμ$ effective lifetime. The measurement is based on the data collected in 2015-2016, amounting to 26.3 fb$^{-1}$ of 13 TeV LHC proton-proton collisions. The proper decay-time distribution of $58\pm13$ background-subtracted signal candidates is fit with simulated signal templates parameterised as a function of the $B_s^0$ effective lifetime, with statistical uncertainties extracted through a Neyman construction. The resulting effective measurement of the $B_s^0 \to μμ$ lifetime is $0.99^{+0.42}_{-0.07} \, (\text{stat.})\pm 0.17 \text{ (syst.)}\,\mathrm{ps}$ and it is found to be consistent with the Standard Model.

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