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1.
  • Aad, G., et al. (författare)
  • Search for heavy vector-like quarks coupling to light quarks in proton-proton collisions at root s=7 TeV with the ATLAS detector
  • 2012
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier. - 0370-2693 .- 1873-2445. ; 712:1-2, s. 22-39
  • Tidskriftsartikel (refereegranskat)abstract
    • This Letter presents a search for singly produced vector-like quarks, Q, coupling to light quarks, q. The search is sensitive to both charged current (CC) and neutral current (NC) processes, pp -> Qq -> Wqq' and pp -> Qq -> Zqq' with a leptonic decay of the vector gauge boson. In 1.04 fb(-1) of data taken in 2011 by the ATLAS experiment at a center-of-mass energy root s = 7 TeV, no evidence of such heavy vector-like quarks is observed above the expected Standard Model background. Limits on the heavy vector-like quark production cross section times branching ratio as a function of mass m(Q) are obtained. For a coupling K-qQ = v/m(Q), where v is the Higgs vacuum expectation value, 95% C.L. lower limits on the mass of a vector-like quark are set at 900 GeV and 760 GeV from CC and NC processes, respectively. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.
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2.
  • Aad, G., et al. (författare)
  • Jet energy measurement with the ATLAS detector in proton-proton collisions at root s=7 TeV
  • 2013
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer. - 1434-6044 .- 1434-6052. ; 73:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The jet energy scale and its systematic uncertainty are determined for jets measured with the ATLAS detector at the LHC in proton-proton collision data at a centre-of-mass energy of root s = 7 TeV corresponding to an integrated luminosity of 38 pb(-1). Jets are reconstructed with the anti-k(t) algorithm with distance parameters R = 0.4 or R = 0.6. Jet energy and angle corrections are determined from Monte Carlo simulations to calibrate jets with transverse momenta pT >= 20 GeV and pseudorapidities vertical bar eta vertical bar < 4.5. The jet energy systematic uncertainty is estimated using the single isolated hadron response measured in situ and in test-beams, exploiting the transverse momentum balance between central and forward jets in events with dijet topologies and studying systematic variations in Monte Carlo simulations. The jet energy uncertainty is less than 2.5 % in the central calorimeter region (vertical bar eta vertical bar < 0.8) for jets with 60 <= p(T) < 800 GeV, and is maximally 14 % for p(T) < 30 GeV in the most forward region 3.2 <= vertical bar eta vertical bar < 4.5. The jet energy is validated for jet transverse momenta up to 1 TeV to the level of a few percent using several in situ techniques by comparing a well-known reference such as the recoiling photon p(T), the sum of the transverse momenta of tracks associated to the jet, or a system of low-p(T) jets recoiling against a high-p(T) jet. More sophisticated jet calibration schemes are presented based on calorimeter cell energy density weighting or hadronic properties of jets, aiming for an improved jet energy resolution and a reduced flavour dependence of the jet response. The systematic uncertainty of the jet energy determined from a combination of in situ techniques is consistent with the one derived from single hadron response measurements over a wide kinematic range. The nominal corrections and uncertainties are derived for isolated jets in an inclusive sample of high-p(T) jets. Special cases such as event topologies with close-by jets, or selections of samples with an enhanced content of jets originating from light quarks, heavy quarks or gluons are also discussed and the corresponding uncertainties are determined.
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3.
  • Aad, G., et al. (författare)
  • Search for decays of stopped, long-lived particles from 7 TeV pp collisions with the ATLAS detector
  • 2012
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer. - 1434-6044 .- 1434-6052. ; 72:4
  • Tidskriftsartikel (refereegranskat)abstract
    • New metastable massive particles with electric and colour charge are features of many theories beyond the Standard Model. A search is performed for long-lived gluino-based R-hadrons with the ATLAS detector at the LHC using a data sample corresponding to an integrated luminosity of 31 pb(-1). We search for evidence of particles that have come to rest in the ATLAS detector and decay at some later time during the periods in the LHC bunch structure without proton-proton collisions. No significant deviations from the expected backgrounds are observed, and a cross-section limit is set. It can be interpreted as excluding gluino-based R-hadrons with masses less than 341 GeV at the 95 % C.L., for lifetimes from 10(-5) to 10(3) seconds and a neutralino mass of 100 GeV.
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4.
  • Aad, G., et al. (författare)
  • Measurement of the jet fragmentation function and transverse profile in proton-proton collisions at a center-of-mass energy of 7 TeV with the ATLAS detector
  • 2011
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer. - 1434-6044 .- 1434-6052. ; 71:11
  • Tidskriftsartikel (refereegranskat)abstract
    • The jet fragmentation function and transverse profile for jets with 25 GeV < p(Tjet) < 500 GeV and |eta(jet)| < 1.2 produced in proton-proton collisions with a center-of-mass energy of 7 TeV are presented. The measurement is performed using data with an integrated luminosity of 36 pb(-1). Jets are reconstructed and their momentum measured using calorimetric information. The momenta of the charged particle constituents are measured using the tracking system. The distributions corrected for detector effects are compared with various Monte Carlo event generators and generator tunes. Several of these choices show good agreement with the measured fragmentation function. None of these choices reproduce both the transverse profile and fragmentation function over the full kinematic range of the measurement.
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5.
  • Aad, G., et al. (författare)
  • A study of the material in the ATLAS inner detector using secondary hadronic interactions
  • 2012
  • Ingår i: Journal of Instrumentation. - : IOP Publishing. - 1748-0221. ; 7
  • Tidskriftsartikel (refereegranskat)abstract
    • The ATLAS inner detector is used to reconstruct secondary vertices due to hadronic interactions of primary collision products, so probing the location and amount of material in the inner region of ATLAS. Data collected in 7 TeV pp collisions at the LHC, with a minimum bias trigger, are used for comparisons with simulated events. The reconstructed secondary vertices have spatial resolutions ranging from similar to 200 mu m to 1 mm. The overall material description in the simulation is validated to within an experimental uncertainty of about 7%. This will lead to a better understanding of the reconstruction of various objects such as tracks, leptons, jets, and missing transverse momentum.
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6.
  • Aad, G., et al. (författare)
  • Measurement of inclusive jet and dijet production in pp collisions at root s=7 TeV using the ATLAS detector
  • 2012
  • Ingår i: Physical Review D (Particles, Fields, Gravitation and Cosmology). - : American Physical Society. - 1550-2368 .- 1550-7998. ; 86:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Inclusive jet and dijet cross sections have been measured in proton-proton collisions at a center-of-mass energy of 7 TeV using the ATLAS detector at the Large Hadron Collider. The cross sections were measured using jets clustered with the anti-kt algorithm with parameters R = 0.4 and R = 0.6. These measurements are based on the 2010 data sample, consisting of a total integrated luminosity of 37 pb(-1). Inclusive jet double-differential cross sections are presented as a function of jet transverse momentum, in bins of jet rapidity. Dijet double-differential cross sections are studied as a function of the dijet invariant mass, in bins of half the rapidity separation of the two leading jets. The measurements are performed in the jet rapidity range vertical bar y vertical bar < 4.4, covering jet transverse momenta from 20 GeV to 1.5 TeV and dijet invariant masses from 70 GeV to 5 TeV. The data are compared to expectations based on next-to-leading-order QCD calculations corrected for nonperturbative effects, as well as to next-to-leading-order Monte Carlo predictions. In addition to a test of the theory in a new kinematic regime, the data also provide sensitivity to parton distribution functions in a region where they are currently not well-constrained.
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7.
  • Aad, G., et al. (författare)
  • Measurement of the cross section for top-quark pair production in pp collisions at root s=7 TeV with the ATLAS detector using final states with two high-p(T) leptons
  • 2012
  • Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; :5
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement is reported of the production cross section of top-quark pairs (t (t) over bar) in proton-proton collisions at a center-of-mass energy of 7 TeV recorded with the ATLAS detector at the LHC. Candidate events have a signature consistent with containing two isolated leptons, large missing transverse momentum, and at least two jets. Using a data sample corresponding to an integrated luminosity of 0.70 fb(-1), a t (t) over bar production cross section sigma(t (t) over bar) = 176 +/- 5(stat.)(-11)(+14)(syst.) +/- 8(lum.) pb is measured for an assumed top-quark mass of m(t) = 172.5 GeV. This measurement is in good agreement with Standard Model predictions.
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8.
  • Aad, G., et al. (författare)
  • Search for anomaly-mediated supersymmetry breaking with the ATLAS detector based on a disappearing-track signature in pp collisions at root s=7 TeV
  • 2012
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer. - 1434-6044 .- 1434-6052. ; 72:4
  • Tidskriftsartikel (refereegranskat)abstract
    • In models of anomaly-mediated supersymmetry breaking (AMSB), the lightest chargino is predicted to have a lifetime long enough to be detected in collider experiments. This letter explores AMSB scenarios in pp collisions at root s = 7 TeV by attempting to identify decaying charginos which result in tracks that appear to have few associated hits in the outer region of the tracking system. The search was based on data corresponding to an integrated luminosity of 1.02 fb(-1) collected with the ATLAS detector in 2011. The p(T) spectrum of candidate tracks is found to be consistent with the expectation from Standard Model background processes and constraints on the lifetime and the production cross section were obtained. In the minimal AMSB framework with m(3/2) < 32 TeV, m(0) < 1.5 TeV, tan beta = 5 and mu > 0, a chargino having mass below 92 GeV and a lifetime between 0.5 ns and 2 ns is excluded at 95 % confidence level.
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9.
  • Aad, G., et al. (författare)
  • Search for diphoton events with large missing transverse momentum in 1 fb(-1) of 7 TeV proton-proton collision data with the ATLAS detector ATLAS Collaboration
  • 2012
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier. - 0370-2693 .- 1873-2445. ; 710:4-5, s. 519-537
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for diphoton events with large missing transverse momentum has been performed using 1.07 fb(-1) of proton-proton collision data at root s = 7 TeV recorded with the ATLAS detector. No excess of events was observed above the Standard Model prediction and 95% Confidence Level (CL) upper limits are set on the production cross section for new physics. The limits depend on each model parameter space and vary as follows: sigma < (22-129) fb in the context of a generalised model of gauge-mediated supersymmetry breaking (GGM) with a bino-like lightest neutralino, sigma < (27-91) fb in the context of a minimal model of gauge-mediated supersymmetry breaking (SPS8), and sigma < (15-27) fb in the context of a specific model with one universal extra dimension (UED). A 95% CL lower limit of 805 GeV, for bino masses above 50 GeV, is set on the GGM gluino mass. Lower limits of 145 TeV and 1.23 TeV are set on the SPS8 breaking scale Lambda and on the UED compactification scale 1/R, respectively. These limits provide the most stringent tests of these models to date. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.
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10.
  • Aad, G., et al. (författare)
  • Search for new phenomena in final states with large jet multiplicities and missing transverse momentum using root s=7 TeV pp collisions with the ATLAS detector
  • 2011
  • Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; :11
  • Tidskriftsartikel (refereegranskat)abstract
    • Results are presented of a search for any particle(s) decaying to six or more jets in association with missing transverse momentum. The search is performed using 1.34 fb(-1) of root s = 7TeV proton-proton collisions recorded by the ATLAS detector during 2011. Data-driven techniques are used to determine the backgrounds in kinematic regions that require at least six, seven or eight jets, well beyond the multiplicities required in previous analyses. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of a supersymmetry model (MSUGRA/CMSSM) where they extend previous constraints.
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