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Open Access Publications from the University of California

Scholarly Works (145 results)

  • Article
  • Peer Reviewed
UC Irvine Previously Published Works (2023)
A search is presented for a heavy resonance Formula Presented decaying into a Standard Model Higgs boson Formula Presented and a new particle Formula Presented in a fully hadronic final state. The full Large Hadron Collider run 2 dataset of proton-proton collisions at Formula Presented collected by the ATLAS detector from 2015 to 2018 is used and corresponds to an integrated luminosity of Formula Presented. The search targets the high Formula Presented-mass region, where the Formula Presented and Formula Presented have a significant Lorentz boost in the laboratory frame. A novel application of anomaly detection is used to define a general signal region, where events are selected solely because of their incompatibility with a learned background-only model. It is constructed using a jet-level tagger for signal-model-independent selection of the boosted Formula Presented particle, representing the first application of fully unsupervised machine learning to an ATLAS analysis. Two additional signal regions are implemented to target a benchmark Formula Presented decay into two quarks, covering topologies where the Formula Presented is reconstructed as either a single large-radius jet or two small-radius jets. The analysis selects Higgs boson decays into Formula Presented, and a dedicated neural-network-based tagger provides sensitivity to the boosted heavy-flavor topology. No significant excess of data over the expected background is observed, and the results are presented as upper limits on the production cross section Formula Presented) for signals with Formula Presented between 1.5 and 6 TeV and Formula Presented between 65 and 3000 GeV.
    Cover page: Anomaly detection search for new resonances decaying into a Higgs boson and a generic new particle X in hadronic final states using s=13 TeV pp collisions with the ATLAS detectorCreative Commons 'BY' version 4.0 license
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2023)
    A measurement of single top-quark production in the s-channel is performed in proton–proton collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to an integrated luminosity of 139 fb −1. The analysis is performed on events with an electron or muon, missing transverse momentum and exactly two b-tagged jets in the final state. A discriminant based on matrix element calculations is used to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W-boson production in association with jets. The observed (expected) signal significance over the background-only hypothesis is 3.3 (3.9) standard deviations, and the measured cross-section is σ=8.2−2.9+3.5 pb, consistent with the Standard Model prediction of σSM=10.32−0.36+0.40 pb. [Figure not available: see fulltext.]
      Cover page: Measurement of single top-quark production in the s-channel in proton–proton collisions at s = 13 TeV with the ATLAS detectorCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2023)
      Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the μ+μ-γ final state with a proton–proton collision data sample corresponding to an integrated luminosity of 139 fb - 1 collected at s=13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/ψγ , ψ(2S)γ , and Υ(1S,2S,3S)γ are found to be 2.0 × 10 - 4 , 10.5 × 10 - 4 , and (2.5 , 4.2 , 3.4) × 10 - 4 , respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2 × 10 - 6 , 2.4 × 10 - 6 , and (1.1 , 1.3 , 2.4) × 10 - 6 . An observed 95% CL interval of (- 133 , 175) is obtained for the κc/ κγ ratio of Higgs boson coupling modifiers, and a 95% CL interval of (- 37 , 40) is obtained for κb/ κγ .
        Cover page: Searches for exclusive Higgs and Z boson decays into a vector quarkonium state and a photon using 139 fb-1 of ATLAS s=13 TeV proton–proton collision dataCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2023)
        This paper presents a search for a new Z′ vector gauge boson with the ATLAS experiment at the Large Hadron Collider using pp collision data collected at s = 13 TeV, corresponding to an integrated luminosity of 139 fb −1. The new gauge boson Z′ is predicted by Lμ − Lτ models to address observed phenomena that can not be explained by the Standard Model. The search examines the four-muon (4μ) final state, using a deep learning neural network classifier to separate the Z′ signal from the Standard Model background events. The di-muon invariant masses in the 4μ events are used to extract the Z′ resonance signature. No significant excess of events is observed over the predicted background. Upper limits at a 95% confidence level on the Z′ production cross-section times the decay branching fraction of pp → Z′μμ → 4μ are set from 0.31 to 4.3 fb for the Z′ mass ranging from 5 to 81 GeV. The corresponding common coupling strengths, gZ′, of the Z′ boson to the second and third generation leptons above 0.003 – 0.2 have been excluded. [Figure not available: see fulltext.].
          Cover page: Search for a new Z′ gauge boson in 4μ events with the ATLAS experimentCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2023)
          A search for vector-like leptons in multilepton (two, three, or four-or-more electrons plus muons) final states with zero or more hadronic τ-lepton decays is presented. The search is performed using a dataset corresponding to an integrated luminosity of 139 fb−1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the LHC. To maximize the separation of signal and background, a machine-learning classifier is used. No excess of events is observed beyond the Standard Model expectation. Using a doublet vector-like lepton model, vector-like leptons coupling to third-generation Standard Model leptons are excluded in the mass range from 130 GeV to 900 GeV at the 95% confidence level, while the highest excluded mass is expected to be 970 GeV. [Figure not available: see fulltext.].
            Cover page: Search for third-generation vector-like leptons in pp collisions at s = 13 TeV with the ATLAS detectorCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2023)
            A search for long-lived particles decaying into hadrons is presented. The analysis uses 139 fb−1 of pp collision data collected at s = 13 TeV by the ATLAS detector at the LHC using events that contain multiple energetic jets and a displaced vertex. The search employs dedicated reconstruction techniques that significantly increase the sensitivity to long-lived particles decaying in the ATLAS inner detector. Background estimates for Standard Model processes and instrumental effects are extracted from data. The observed event yields are compatible with those expected from background processes. The results are used to set limits at 95% confidence level on model-independent cross sections for processes beyond the Standard Model, and on scenarios with pair-production of supersymmetric particles with long-lived electroweakinos that decay via a small R-parity-violating coupling. The pair-production of electroweakinos with masses below 1.5 TeV is excluded for mean proper lifetimes in the range from 0.03 ns to 1 ns. When produced in the decay of m(g~) = 2.4 TeV gluinos, electroweakinos with m(χ~10) = 1.5 TeV are excluded with lifetimes in the range of 0.02 ns to 4 ns. [Figure not available: see fulltext.].
              Cover page: Search for long-lived, massive particles in events with displaced vertices and multiple jets in pp collisions at s = 13 TeV with the ATLAS detectorCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Irvine Previously Published Works (2023)
              This paper presents the observation of four-top-quark (tt¯ tt¯) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 fb - 1 at a centre-of-mass energy of 13 TeV collected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of the measured tt¯ tt¯ signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The tt¯ tt¯ production cross section is measured to be 22.5-5.5+6.6 fb, consistent with the SM prediction of 12.0 ± 2.4 fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect tt¯ tt¯ production.
                Cover page: Observation of four-top-quark production in the multilepton final state with the ATLAS detectorCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2023)
                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 s = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. No evidence of physics beyond the Standard Model is found. The results are interpreted in terms of a 2HDM+a model featuring two scalar Higgs doublets and a pseudoscalar singlet field. Exclusion limits on the parameters of the model in selected benchmark scenarios are derived at 95% confidence level. 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 into τ-leptons.
                  Cover page: Search for dark matter produced in association with a Higgs boson decaying to tau leptons at s = 13 TeV with the ATLAS detectorCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2023)
                  Constraints on the Higgs boson self-coupling are set by combining double-Higgs boson analyses in the bb¯bb¯, bb¯τ+τ− and bb¯γγ decay channels with single-Higgs boson analyses targeting the γγ, ZZ⁎, WW⁎, τ+τ− and bb¯ decay channels. The data used in these analyses were recorded by the ATLAS detector at the LHC in proton–proton collisions at s=13 TeV and correspond to an integrated luminosity of 126–139 fb−1. The combination of the double-Higgs analyses sets an upper limit of μHH<2.4 at 95% confidence level on the double-Higgs production cross-section normalised to its Standard Model prediction. Combining the single-Higgs and double-Higgs analyses, with the assumption that new physics affects only the Higgs boson self-coupling (λHHH), values outside the interval −0.4<κλ=(λHHH/λHHHSM)<6.3 are excluded at 95% confidence level. The combined single-Higgs and double-Higgs analyses provide results with fewer assumptions, by adding in the fit more coupling modifiers introduced to account for the Higgs boson interactions with the other Standard Model particles. In this relaxed scenario, the constraint becomes −1.4<κλ<6.1 at 95% CL.
                    Cover page: Constraints on the Higgs boson self-coupling from single- and double-Higgs production with the ATLAS detector using pp collisions at s = 13 TeVCreative Commons 'BY' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UC Irvine Previously Published Works (2023)
                    Higgs boson production via gluon–gluon fusion is measured in the WW*? e?µ? decay channel. The dataset utilized corresponds to an integrated luminosity of 139 fb - 1 collected by the ATLAS detector from s=13 TeV proton–proton collisions delivered by the Large Hadron Collider between 2015 and 2018. Differential cross sections are measured in a fiducial phase space restricted to the production of at most one additional jet. The results are consistent with Standard Model expectations, derived using different Monte Carlo generators.
                      Cover page: Measurements of differential cross sections of Higgs boson production through gluon fusion in the H→WW∗→eνμν final state at s=13 TeV with the ATLAS detectorCreative Commons 'BY' version 4.0 license
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