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Open Access Publications from the University of California

Scholarly Works (116 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 Irvine Previously Published Works (2016)
    A search is performed for a heavy particle decaying into different flavour dilepton pairs ([Formula: see text], [Formula: see text] or [Formula: see text]), using 3.2 fb[Formula: see text] of proton-proton collision data at [Formula: see text] TeV collected in 2015 by the ATLAS detector at the Large Hadron Collider. No excess over the Standard Model prediction is observed. Limits at the 95 % credibility level are set on the mass of a [Formula: see text] boson with lepton-flavour-violating couplings at 3.0, 2.7 and 2.6 TeV, and on the mass of a supersymmetric [Formula: see text] sneutrino with R-parity-violating couplings at 2.3, 2.2 and 1.9 TeV, for [Formula: see text], [Formula: see text] and [Formula: see text] final states, respectively. The results are also interpreted as limits on the threshold mass for quantum black hole production.
      Cover page: Search for new phenomena in different-flavour high-mass dilepton final states 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 (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 Irvine Previously Published Works (2017)
        Unfortunately, in the original version of this article the copyright holder was wrong in the HTML. The original article has been corrected.
          Cover page: Correction to: Identification and rejection of pile-up jets at high pseudorapidity with the ATLAS detectorCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2017)
          Measurements of dijet pT correlations in Pb+Pb and pp collisions at a nucleon–nucleon centre-of-mass energy of sNN=2.76 TeV are presented. The measurements are performed with the ATLAS detector at the Large Hadron Collider using Pb+Pb and pp data samples corresponding to integrated luminosities of 0.14 nb−1 and 4.0 pb−1, respectively. Jets are reconstructed using the anti-kt algorithm with radius parameter values R=0.3 and R=0.4. A background subtraction procedure is applied to correct the jets for the large underlying event present in Pb+Pb collisions. The leading and sub-leading jet transverse momenta are denoted pT1 and pT2. An unfolding procedure is applied to the two-dimensional (pT1,pT2) distributions to account for experimental effects in the measurement of both jets. Distributions of (1/N)dN/dxJ, where xJ=pT2/pT1, are presented as a function of pT1 and collision centrality. The distributions are found to be similar in peripheral Pb+Pb collisions and pp collisions, but highly modified in central Pb+Pb collisions. Similar features are present in both the R=0.3 and R=0.4 results, indicating that the effects of the underlying event are properly accounted for in the measurement. The results are qualitatively consistent with expectations from partonic energy loss models.
            Cover page: Measurement of jet p T correlations in Pb+Pb and pp collisions at s NN = 2.76   TeV with the ATLAS detector
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2018)
            This Letter presents the measurement of differential cross sections of isolated prompt photons produced in association with a b-jet or a c-jet. These final states provide sensitivity to the heavy-flavour content of the proton and aspects related to the modelling of heavy-flavour quarks in perturbative QCD. The measurement uses proton–proton collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of up to 20.2 fb−1. The differential cross sections are measured for each jet flavour with respect to the transverse energy of the leading photon in two photon pseudorapidity regions: |ηγ|<1.37 and 1.56<|ηγ|<2.37. The measurement covers photon transverse energies 25
              Cover page: Measurement of differential cross sections of isolated-photon plus heavy-flavour jet production in pp collisions at s = 8 &nbsp;TeV using the ATLAS detector
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2017)
              The results of a search for squarks and gluinos in final states with an isolated electron or muon, multiple jets and large missing transverse momentum using proton-proton collision data at a center-of-mass energy of √ = 13 TeV are presented. The data set used was recorded during 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 36.1 fb-1. No significant excess beyond the expected background is found. Exclusion limits at 95% confidence level are set in a number of supersymmetric scenarios, reaching masses up to 2.1 TeV for gluino pair production and up to 1.25 TeV for squark pair production.
                Cover page: Search for squarks and gluinos in events with an isolated lepton, jets, and missing transverse momentum at s=13 TeV with the ATLAS detector
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2017)
                Two-particle pseudorapidity correlations are measured in sNN=2.76TeVPb+Pb, sNN=5.02TeVp+Pb, and s=13TeVpp collisions at the Large Hadron Collider (LHC), with total integrated luminosities of approximately 7μb-1, 28 nb-1, and 65 nb-1, respectively. The correlation function CN(η1,η2) is measured as a function of event multiplicity using charged particles in the pseudorapidity range |η|<2.4. The correlation function contains a significant short-range component, which is estimated and subtracted. After removal of the short-range component, the shape of the correlation function is described approximately by 1+(a12)1/2η1η2 in all collision systems over the full multiplicity range. The values of (a12)1/2 are consistent for the opposite-charge pairs and same-charge pairs, and for the three collision systems at similar multiplicity. The values of (a12)1/2 and the magnitude of the short-range component both follow a power-law dependence on the event multiplicity. The short-range component in p + Pb collisions, after symmetrizing the proton and lead directions, is found to be smaller at a given η than in pp collisions with comparable multiplicity.
                  Cover page: Measurement of forward-backward multiplicity correlations in lead-lead, proton-lead, and proton-proton collisions with the ATLAS detectorCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2018)
                  A search for an invisibly decaying Higgs boson or dark matter candidates produced in association with a leptonically decaying Z boson in proton–proton collisions at s=13 TeV is presented. This search uses 36.1 fb−1 of data collected by the ATLAS experiment at the Large Hadron Collider. No significant deviation from the expectation of the Standard Model backgrounds is observed. Assuming the Standard Model ZH production cross-section, an observed (expected) upper limit of 67% (39%) at the 95% confidence level is set on the branching ratio of invisible decays of the Higgs boson with mass mH=125 GeV. The corresponding limits on the production cross-section of the ZH process with the invisible Higgs boson decays are also presented. Furthermore, exclusion limits on the dark matter candidate and mediator masses are reported in the framework of simplified dark matter models.
                    Cover page: Search for an invisibly decaying Higgs boson or dark matter candidates produced in association with a Z boson in pp collisions at s = 13 &nbsp; TeV with the ATLAS detector
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2018)
                    A search for heavy resonances decaying into a Higgs boson (H) and a new particle (X) is reported, utilizing 36.1 fb−1 of proton–proton collision data at s=13TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. The particle X is assumed to decay to a pair of light quarks, and the fully hadronic final state XH→qq¯′bb¯ is analysed. The search considers the regime of high XH resonance masses, where the X and H bosons are both highly Lorentz-boosted and are each reconstructed using a single jet with large radius parameter. A two-dimensional phase space of XH mass versus X mass is scanned for evidence of a signal, over a range of XH resonance mass values between 1 TeV and 4 TeV, and for X particles with masses from 50 GeV to 1000 GeV. All search results are consistent with the expectations for the background due to Standard Model processes, and 95% CL upper limits are set, as a function of XH and X masses, on the production cross-section of the XH→qq¯′bb¯ resonance.
                      Cover page: A search for resonances decaying into a Higgs boson and a new particle X in the XH → qqbb final state with the ATLAS detectorCreative Commons 'BY' version 4.0 license
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