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Open Access Publications from the University of California

Scholarly Works (176 results)

  • Article
  • Peer Reviewed
UC Irvine Previously Published Works (2016)
The results of a search for the top squark, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, jets, and missing transverse momentum are reported. The search uses the 2015 LHC pp collision data at a center-of-mass energy of s=13 TeV recorded by the ATLAS detector and corresponding to an integrated luminosity of 3.2 fb-1. The analysis targets two types of signal models: gluino-mediated pair production of top squarks with a nearly mass-degenerate top squark and neutralino and direct pair production of top squarks, decaying to the top quark and the lightest neutralino. The experimental signature in both signal scenarios is similar to that of a top quark pair produced in association with large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits on gluino and top squark masses are set at 95% confidence level. The results extend the LHC run-1 exclusion limit on the gluino mass up to 1460 GeV in the gluino-mediated scenario in the high gluino and low top squark mass region and add an excluded top squark mass region from 745 to 780 GeV for the direct top squark model with a massless lightest neutralino. The results are also reinterpreted to set exclusion limits in a model of vectorlike top quarks.
    Cover page: Search for top squarks in final states with one isolated lepton, jets, and missing transverse momentum in s=13 TeV pp collisions with the ATLAS detectorCreative Commons 'BY' version 4.0 license
    • Article
    • Peer Reviewed
    UC Santa Cruz Previously Published Works (2015)
    High transverse momentum jets produced in pp collisions at a centre of mass energy of 7 TeV are used to measure the transverse energy-energy correlation function and its associated azimuthal asymmetry. The data were recorded with the ATLAS detector at the LHC in the year 2011 and correspond to an integrated luminosity of 158 pb-1. The selection criteria demand the average transverse momentum of the two leading jets in an event to be larger than 250 GeV. The data at detector level are well described by Monte Carlo event generators. They are unfolded to the particle level and compared with theoretical calculations at next-to-leading-order accuracy. The agreement between data and theory is good and provides a precision test of perturbative Quantum Chromodynamics at large momentum transfers. From this comparison, the strong coupling constant given at the Z boson mass is determined to be as(mZ)=0.1173±0.0010 (exp.) -0.0026+0.0065 (theo.).
      Cover page: Measurement of transverse energy–energy correlations in multi-jet events in pp collisions at s=7 TeV using the ATLAS detector and determination of the strong coupling constant αs(mZ)Creative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2015)
        Cover page: Erratum to: Search for production of WW / WZ resonances decaying to a lepton, neutrino and jets in pp collisions at s=8 TeV with the ATLAS detectorCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2016)
        A measurement of the Bs0decay parameters in the Bs0→ J/ψϕ channel using an integrated luminosity of 14.3 fb−1collected by the ATLAS detector from 8 TeV pp collisions at the LHC is presented. The measured parameters include the CP -violating phase ϕs, the decay width Γs and the width difference between the mass eigenstates ΔΓs. The values measured for the physical parameters are statistically combined with those from 4.9 fb−1of 7 TeV data, leading to the following: Φs = -0.090 ± 0.078 (stat.) ± (syst.) rad Δ Γs = 0.085 ± 0.011 (stat.) ± 0.007 (syst.) Ps-1 Δ Γs = 0.675 ± 0.003 (stat.) ± 0.03 (syst.) Ps-1 In the analysis the parameter ΔΓs is constrained to be positive. Results for ϕs and ΔΓs are also presented as 68% and 95% likelihood contours in the ϕs-ΔΓs plane. Also measured in this decay channel are the transversity amplitudes and corresponding strong phases. All measurements are in agreement with the Standard Model predictions.[Figure not available: see fulltext.]
          Cover page: Measurement of the CP-violating phase ϕs and the Bs0 meson decay width difference with Bs0 → J/ψϕ decays in ATLASCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Santa Cruz Previously Published Works (2015)
          A search is presented for photonic signatures motivated by generalized models of gauge-mediated supersymmetry breaking. This search makes use of 20.3 fb-1 of proton-proton collision data at √s = 8 TeV recorded by the ATLAS detector at the LHC, and explores models dominated by both strong and electroweak production of supersymmetric partner states. Four experimental signatures incorporating an isolated photon and significant missing transverse momentum are explored. These signatures include events with an additional photon, lepton, b-quark jet, or jet activity not associated with any specific underlying quark flavor. No significant excess of events is observed above the Standard Model prediction and model-dependent 95% confidence-level exclusion limits are set.
            Cover page: Search for photonic signatures of gauge-mediated supersymmetry in 8 TeV pp collisions with the ATLAS detectorCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2016)
            Charged Higgs bosons produced in association with a single top quark and decaying via H±→τν are searched for with the ATLAS experiment at the LHC, using proton–proton collision data at s=13 TeV corresponding to an integrated luminosity of 3.2 fb−1. The final state is characterised by the presence of a hadronic τ decay and missing transverse momentum, as well as a hadronically decaying top quark, resulting in the absence of high-transverse-momentum electrons and muons. The data are found to be consistent with the expected background from Standard Model processes. A statistical analysis leads to 95% confidence-level upper limits on the production cross section times branching fraction, σ(pp→[b]tH±)×BR(H±→τν), between 1.9 pb and 15 fb, for charged Higgs boson masses ranging from 200 to 2000 GeV. The exclusion limits for this search surpass those obtained with the proton–proton collision data recorded at s=8 TeV.
              Cover page: Search for charged Higgs bosons produced in association with a top quark and decaying via H±→τν using pp collision data recorded at s=13 TeV by the ATLAS detectorCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Santa Cruz Previously Published Works (2016)
              This article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at [Formula: see text] TeV in 2015. Using a large sample of [Formula: see text] and [Formula: see text] decays from 3.2 fb[Formula: see text] of pp collision data, measurements of the reconstruction efficiency, as well as of the momentum scale and resolution, are presented and compared to Monte Carlo simulations. The reconstruction efficiency is measured to be close to [Formula: see text] over most of the covered phase space ([Formula: see text] and [Formula: see text] GeV). The isolation efficiency varies between 93 and [Formula: see text] depending on the selection applied and on the momentum of the muon. Both efficiencies are well reproduced in simulation. In the central region of the detector, the momentum resolution is measured to be [Formula: see text] ([Formula: see text]) for muons from [Formula: see text] ([Formula: see text]) decays, and the momentum scale is known with an uncertainty of [Formula: see text]. In the region [Formula: see text], the [Formula: see text] resolution for muons from [Formula: see text] decays is [Formula: see text] while the precision of the momentum scale for low-[Formula: see text] muons from [Formula: see text] decays is about [Formula: see text].
                Cover page: Muon reconstruction performance of the ATLAS detector in proton–proton collision data at s=13 TeVCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2016)
                The production of W boson pairs in proton-proton collisions at s=8 TeV is studied using data corresponding to 20.3 fb−1 of integrated luminosity collected by the ATLAS detector during 2012 at the CERN Large Hadron Collider. The W bosons are reconstructed using their leptonic decays into electrons or muons and neutrinos. Events with reconstructed jets are not included in the candidate event sample. A total of 6636 WW candidate events are observed. Measurements are performed in fiducial regions closely approximating the detector acceptance. The integrated measurement is corrected for all acceptance effects and for the W branching fractions to leptons in order to obtain the total WW production cross section, which is found to be 71.1 ± 1.1(stat) − 5.0+ 5.7(syst) ± 1.4(lumi) pb. This agrees with the next-to-next-to-leading-order Standard Model prediction of 63. 2− 1.4+ 1.6(scale) ± 1.2(PDF) pb. Fiducial differential cross sections are measured as a function of each of six kinematic variables. The distribution of the transverse momentum of the leading lepton is used to set limits on anomalous triple-gauge-boson couplings.[Figure not available: see fulltext.]
                  Cover page: Measurement of total and differential W+W− production cross sections in proton-proton collisions at s=8 TeV with the ATLAS detector and limits on anomalous triple-gauge-boson couplingsCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Santa Cruz Previously Published Works (2016)
                  This paper reports a detailed study of techniques for identifying boosted, hadronically decaying W bosons using 20.3 fb[Formula: see text] of proton-proton collision data collected by the ATLAS detector at the LHC at a centre-of-mass energy [Formula: see text]. A range of techniques for optimising the signal jet mass resolution are combined with various jet substructure variables. The results of these studies in Monte Carlo simulations show that a simple pairwise combination of groomed jet mass and one substructure variable can provide a 50 % efficiency for identifying W bosons with transverse momenta larger than 200 GeV while maintaining multijet background efficiencies of 2-4 % for jets with the same transverse momentum. These signal and background efficiencies are confirmed in data for a selection of tagging techniques.
                    Cover page: Identification of boosted, hadronically decaying W bosons and comparisons with ATLAS data taken at s=8 TeVCreative Commons 'BY' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UC Santa Cruz Previously Published Works (2016)
                    Combined analyses of the Higgs boson production and decay rates as well as its coupling strengths to vector bosons and fermions are presented. The combinations include the results of the analyses of the [Formula: see text] and [Formula: see text] decay modes, and the constraints on the associated production with a pair of top quarks and on the off-shell coupling strengths of the Higgs boson. The results are based on the LHC proton-proton collision datasets, with integrated luminosities of up to 4.7 [Formula: see text] at [Formula: see text] TeV and 20.3 [Formula: see text] at [Formula: see text] TeV, recorded by the ATLAS detector in 2011 and 2012. Combining all production modes and decay channels, the measured signal yield, normalised to the Standard Model expectation, is [Formula: see text]. The observed Higgs boson production and decay rates are interpreted in a leading-order coupling framework, exploring a wide range of benchmark coupling models both with and without assumptions on the Higgs boson width and on the Standard Model particle content in loop processes. The data are found to be compatible with the Standard Model expectations for a Higgs boson at a mass of 125.36 GeV for all models considered.
                      Cover page: Measurements of the Higgs boson production and decay rates and coupling strengths using pp collision data at s=7 and 8 TeV in the ATLAS experimentCreative Commons 'BY' version 4.0 license
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