Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (28 results)

  • Article
  • Peer Reviewed
UC Davis Previously Published Works (2010)
First measurements of the W → ℓν and Z/γ * → ℓℓ (ℓ = e, μ) production cross sections in proton-proton collisions at √ s = 7TeV are presented using data recorded by the ATLAS experiment at the LHC. The results are based on 2250 W → ℓν and 179 Z/γ* → ℓℓ candidate events selected from a data set corresponding to an integrated luminosity of approximately 320 nb-1. The measured total W and Z/ γ*-boson production cross sections times the respective leptonic branching ratios for the combined electron and muon channels are σtotW · BR(W → ℓν) = 9.96 ± 0.23(stat) ± 0.50(syst) ± 1.10(lumi) nb and σtotZ/γ* · BR(Z/ γ* → ℓℓ) = 0.82 ± 0.06 (stat) ± 0.05 (syst) ± 0.09 (lumi) nb (within the invariant mass window 66 < mℓℓ < 116 GeV). The W/Z cross-section ratio is measured to be 11.7 ± 0.9(stat) ± 0.4(syst). In addition, measurements of the W+ and W- production cross sections and of the lepton charge asymmetry are reported. Theoretical predictions based on NNLO QCD calculations are found to agree with the measurements.
    Cover page: Measurement of the W → ℓν and Z/γ* → ℓℓ production cross sections in proton-proton collisions at with the ATLAS detectorCreative Commons 'BY' version 4.0 license
    • Article
    • Peer Reviewed
    UC Santa Cruz Previously Published Works (2011)
    A search for squarks and gluinos in final states containing jets, missing transverse momentum and no electrons or muons is presented. The data were recorded by the ATLAS experiment in √s=7 TeV proton-proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 35 pb-1 of analysed data. Gluino masses below 500 GeV are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino. The exclusion increases to 870 GeV for equal mass squarks and gluinos. In MSUGRA/CMSSM models with tanβ=3, A0=0 and μ>0, squarks and gluinos of equal mass are excluded below 775 GeV. These are the most stringent limits to date. © 2011 CERN.
      Cover page: Search for squarks and gluinos using final states with jets and missing transverse momentum with the ATLAS detector in s=7&nbsp;TeV proton–proton collisionsCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2011)
      Dijet angular distributions from the first LHC pp collisions at center-of-mass energy s=7 TeV have been measured with the ATLAS detector. The dataset used for this analysis represents an integrated luminosity of 3.1 pb-1. Dijet Χ distributions and centrality ratios have been measured up to dijet masses of 2.8 TeV, and found to be in good agreement with Standard Model predictions. Analysis of the λ distributions excludes quark contact interactions with a compositeness scale Χ below 3.4 TeV, at 95% confidence level, significantly exceeding previous limits. © 2010 CERN.
        Cover page: Search for quark contact interactions in dijet angular distributions in pp collisions at s=7 TeV measured with the ATLAS detectorCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2011)
        Measurements of charged particle distributions, sensitive to the underlying event, have been performed with the ATLAS detector at the LHC. The measurements are based on data collected using a minimum-bias trigger to select proton-proton collisions at center-of-mass energies of 900 GeV and 7 TeV. The "underlying event" is defined as those aspects of a hadronic interaction attributed not to the hard scattering process, but rather to the accompanying interactions of the rest of the proton. Three regions are defined in azimuthal angle with respect to the highest transverse momentum charged particle in the event, such that the region transverse to the dominant momentum-flow is most sensitive to the underlying event. In each of these regions, distributions of the charged particle multiplicity, transverse momentum density, and average pT are measured. The data show generally higher underlying event activity than that predicted by Monte Carlo models tuned to pre-LHC data. © 2011 American Physical Society.
          Cover page: Measurement of underlying event characteristics using charged particles in pp collisions at s=900 GeV and 7 TeV with the ATLAS detectorCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2010)
          More than half a million minimum-bias events of LHC collision data were collected by the ATLAS experiment in December 2009 at centre-of-mass energies of 0.9 TeV and 2.36 TeV. This paper reports on studies of the initial performance of the ATLAS detector from these data. Comparisons between data and Monte Carlo predictions are shown for distributions of several track-and calorimeter-based quantities. The good performance of the ATLAS detector in these first data gives confidence for successful running at higher energies.
            Cover page: Performance of the ATLAS detector using first collision dataCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Santa Cruz Previously Published Works (2011)
            A search is made for massive highly ionising particles with lifetimes in excess of 100 ns, with the ATLAS experiment at the Large Hadron Collider, using 3.1 pb-1 of pp collision data taken at √s=7 TeV. The signature of energy loss in the ATLAS inner detector and electromagnetic calorimeter is used. No such particles are found and limits on the production cross section for electric charges 6e≤|q|≤17e and masses 200 GeV≤m≤1000 GeV are set in the range 1-12 pb for different hypotheses on the production mechanism. © 2011 CERN.
              Cover page: Search for massive long-lived highly ionising particles with the ATLAS detector at the LHCCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Santa Cruz Previously Published Works (2011)
              The ATLAS detector is used to search for high-mass states, such as heavy charged gauge bosons (W′,W⁎), decaying to a charged lepton (electron or muon) and a neutrino. Results are presented based on the analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 36 pb−1. No excess beyond standard model expectations is observed. A W′ with sequential standard model couplings is excluded at 95% confidence level for masses below 1.49 TeV, and a W⁎ (charged chiral boson) for masses below 1.35 TeV.
                Cover page: Search for high-mass states with one lepton plus missing transverse momentum in proton–proton collisions at s=7 TeV with the ATLAS detectorCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Davis Previously Published Works (2010)
                A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the LHC and correspond to an integrated luminosity of 315  nb⁻¹ collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (q*) production as a function of q* mass. These exclude at the 95% C.L. the q* mass interval 0.30
                  Cover page: Search for new particles in two-jet final states in 7 TeV proton-proton collisions with the ATLAS detector at the LHC.Creative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Davis Previously Published Works (2010)
                  The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and chargedparticle multiplicity are measured for events with at least one charged particle in the kinematic range |η| < 2.5 and pT > 500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity at η = 0 is measured to be 1.333 ± 0.003(stat.) ± 0.040(syst.), which is 5-15% higher than the Monte Carlo models predict.
                    Cover page: Charged-particle multiplicities in pp interactions at s=900 GeV measured with the ATLAS detector at the LHCCreative Commons 'BY' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UC Santa Cruz Previously Published Works (2011)
                    Using the ATLAS detector, a centrality-dependent suppression has been observed in the yield of J/Ψ mesons produced in the collisions of lead ions at the Large Hadron Collider. In a sample of minimum-bias lead-lead collisions at a nucleon-nucleon centre of mass energy sNN=2.76 TeV, corresponding to an integrated luminosity of about 6.7 μb-1, J/Ψ mesons are reconstructed via their decays to μ+μ- pairs. The measured J/Ψ yield, normalized to the number of binary nucleon-nucleon collisions, is found to significantly decrease from peripheral to central collisions. The centrality dependence is found to be qualitatively similar to the trends observed at previous, lower energy experiments. The same sample is used to reconstruct Z bosons in the μ+μ- final state, and a total of 38 candidates are selected in the mass window of 66 to 116 GeV. The relative Z yields as a function of centrality are also presented, although no conclusion can be inferred about their scaling with the number of binary collisions, because of limited statistics. This analysis provides the first results on J/Ψ and Z production in lead-lead collisions at the LHC. © 2011 CERN.
                      Cover page: Measurement of the centrality dependence of J/ψ yields and observation of Z production in lead–lead collisions with the ATLAS detector at the LHCCreative Commons 'BY' version 4.0 license
                      Top