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41.
  • Aad, G., et al. (författare)
  • Differential top-antitop cross-section measurements as a function of observables constructed from final-state particles using pp collisions at root s=7 TeV in the ATLAS detector
  • 2015
  • Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; :6
  • Tidskriftsartikel (refereegranskat)abstract
    • Various differential cross-sections are measured in top-quark pair (t (t) over bar) events produced in proton-proton collisions at a centre-of-mass energy of root s = 7 TeV at the LHC with the ATLAS detector. These differential cross-sections are presented in a data set corresponding to an integrated luminosity of 4.6 fb(-1). The differential cross-sections are presented in terms of kinematic variables, such as momentum, rapidity and invariant mass, of a top-quark proxy referred to as the pseudo-top-quark as well as the pseudo-top-quark pair system. The dependence of the measurement on theoretical models is minimal. The measurements are performed on tt events in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of them tagged as originating from a b-quark. The hadronic and leptonic pseudo-top-quarks are defined via the leptonic or hadronic decay mode of the W boson produced by the top-quark decay in events with a single charged lepton. Differential cross-section measurements of the pseudo-top-quark variables are compared with several Monte Carlo models that implement next-to-leading order or leading-order multi-leg matrix-element calculations.
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42.
  • Aad, G., et al. (författare)
  • Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector
  • 2015
  • Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; :4
  • Tidskriftsartikel (refereegranskat)abstract
    • Results of a search for H -> tau tau decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb(-1) and 20.3 fb(-1) at centre-of-mass energies of root s = 7TeV and root s = 8 TeV respectively. All combinations of leptonic (tau -> l nu(nu) over bar with l = e, mu) and hadronic (tau -> hadrons nu) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of mu = 1.43(-0.37)(+0.43) is consistent with the predicted Yukawa coupling strength in the Standard Model.
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43.
  • Aad, G., et al. (författare)
  • Measurement of the branching ratio Gamma(Lambda(0)(b) -> Psi(2S)Lambda(0))/Gamma(Lambda(0.)(b) -> J/Psi Lambda(0)) with the ATLAS detector
  • 2015
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier. - 0370-2693 .- 1873-2445. ; 751, s. 63-80
  • Tidskriftsartikel (refereegranskat)abstract
    • An observation of the Lambda(0)(b) -> Psi (2S) Lambda(0) decay and a comparison of its branching fraction with that of the Lambda(0)(b) -> Psi (2S) Lambda(0) decay has been made with the ATLAS detector in proton-proton collisions at root s = 8 TeVat the LHC using an integrated luminosity of 20.6fb(-1). The J/Psi and Psi(2S) mesons are reconstructed in their decays to a muon pair, while the Lambda(0) -> p pi(-) decay is exploited for the Lambda(0) 0baryon reconstruction. The Lambda(0)(b) baryons are reconstructed with transverse momentum p(T)> 10 GeVand pseudorapidity vertical bar eta vertical bar < 2.1. The measured branching ratio of the Lambda(0)(b) -> Psi (2S) Lambda(0) and Lambda(0)(b) -> Psi (2S) Lambda(0) decays is Gamma(Lambda(0)(b) -> Psi (2S) Lambda(0)) / Gamma(Lambda(0)(b) -> Psi (2S) Lambda(0)) = 0.501 +/- 0.033(stat) +/- 0.019(syst), lower than the expectation from the covariant quark model. (C) 2015 CERN for the benefit of the ATLAS Collaboration. Published by Elsevier B.V.
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44.
  • Aad, G., et al. (författare)
  • Measurement of the charge asymmetry in dileptonic decays of top quark pairs in pp collisions at root s=7 TeV using the ATLAS detector
  • 2015
  • Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; :5
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of the top-antitop (t (t) over bar) charge asymmetry is presented using data corresponding to an integrated luminosity of 4.6 fb(-1) of LHC pp collisions at a centre-of-mass energy of 7 TeV collected by the ATLAS detector. Events with two charged leptons, at least two jets and large missing transverse momentum are selected. Two observables are studied: A(C)(ll) based on the identified charged leptons, and A(C)(t (t) over bar), based on the reconstructed t (t) over bar final state. The asymmetries are measured to be A(C)(ll) =0.024 +/- 0.015 (stat.) +/- 0.009 (syst.), A(C)(t (t) over bar) = 0.021 +/- 0.025 (stat.) +/- 0.017 (syst.). The measured values are in agreement with the Standard Model predictions.
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45.
  • Aad, G., et al. (författare)
  • Measurement of the correlation between flow harmonics of different order in lead-lead collisions at root S-NN=2.76 TeV with the ATLAS detector
  • 2015
  • Ingår i: Physical Review C (Nuclear Physics). - : American Physical Society. - 0556-2813 .- 1089-490X. ; 92:3
  • Tidskriftsartikel (refereegranskat)abstract
    • Correlations between the elliptic or triangular flow coefficients v(m) (m = 2 or 3) and other flow harmonics v(n) (n = 2 to 5) are measured using root S-NN = 2.76 TeV Pb + Pb collision data collected in 2010 by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 7 mu b(-1). The v(m)-v(n) correlations aremeasured in midrapidity as a function of centrality, and, for events within the same centrality interval, as a function of event ellipticity or triangularity defined in a forward rapidity region. For events within the same centrality interval, v(3) is found to be anticorrelated with v(2) and this anticorrelation is consistent with similar anticorrelations between the corresponding eccentricities, epsilon(2) and epsilon(3). However, it is observed that v(4) increases strongly with v(2), and v(5) increases strongly with both v(2) and v(3). The trend and strength of the v(m) -v(n) correlations for n = 4 and 5 are found to disagree with epsilon(m)-epsilon(n) correlations predicted by initial-geometry models. Instead, these correlations are found to be consistent with the combined effects of a linear contribution to v(n) and a nonlinear term that is a function of v(2)(2) or of v(2)v(3), as predicted by hydrodynamic models. A simple two-component fit is used to separate these two contributions. The extracted linear and nonlinear contributions to v(4) and v(5) are found to be consistent with previously measured event-plane correlations.
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46.
  • Aad, G., et al. (författare)
  • Measurement of the transverse polarization of Lambda and (Lambda)over-bar hyperons produced in proton-proton collisions at root s=7 TeV using the ATLAS detector
  • 2015
  • Ingår i: Physical Review D (Particles, Fields, Gravitation and Cosmology). - : American Physical Society. - 1550-2368 .- 1550-7998. ; 91:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The transverse polarization of Lambda and (Lambda) over bar hyperons produced in proton-proton collisions at a center-of-mass energy of 7 TeV is measured. The analysis uses 760 mu b(-1) of minimum bias data collected by the ATLAS detector at the LHC in the year 2010. The measured transverse polarization averaged over Feynman x(F) from 5 x 10(-5) to 0.01 and transverse momentum p(T) from 0.8 to 15 GeV is -0.010 +/- 0.005(stat) +/- 0.004(syst) for Lambda and 0.002 +/- 0.006(stat) +/- 0.004(syst) for (Lambda) over bar. It is also measured as a function of x(F) and p(T), but no significant dependence on these variables is observed. Prior to this measurement, the polarization was measured at fixed-target experiments with center-of-mass energies up to about 40 GeV. The ATLAS results are compatible with the extrapolation of a fit from previous measurements to the x(F) range covered by this measurement.
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47.
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48.
  • Aad, G, et al. (författare)
  • Measurement of three-jet production cross-sections in [Formula: see text] collisions at 7 [Formula: see text] centre-of-mass energy using the ATLAS detector.
  • 2015
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer. - 1434-6044. ; 75:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Double-differential three-jet production cross-sections are measured in proton-proton collisions at a centre-of-mass energy of [Formula: see text] using the ATLAS detector at the large hadron collider. The measurements are presented as a function of the three-jet mass [Formula: see text], in bins of the sum of the absolute rapidity separations between the three leading jets [Formula: see text]. Invariant masses extending up to 5 TeV are reached for [Formula: see text]. These measurements use a sample of data recorded using the ATLAS detector in 2011, which corresponds to an integrated luminosity of [Formula: see text]. Jets are identified using the anti-[Formula: see text] algorithm with two different jet radius parameters, [Formula: see text] and [Formula: see text]. The dominant uncertainty in these measurements comes from the jet energy scale. Next-to-leading-order QCD calculations corrected to account for non-perturbative effects are compared to the measurements. Good agreement is found between the data and the theoretical predictions based on most of the available sets of parton distribution functions, over the full kinematic range, covering almost seven orders of magnitude in the measured cross-section values.
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49.
  • Aad, G, et al. (författare)
  • Measurements of the [Formula: see text] production cross sections in association with jets with the ATLAS detector.
  • 2015
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer. - 1434-6044. ; 75:2
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents cross sections for the production of a [Formula: see text] boson in association with jets, measured in proton-proton collisions at [Formula: see text] with the ATLAS experiment at the large hadron collider. With an integrated luminosity of [Formula: see text], this data set allows for an exploration of a large kinematic range, including jet production up to a transverse momentum of [Formula: see text] and multiplicities up to seven associated jets. The production cross sections for [Formula: see text] bosons are measured in both the electron and muon decay channels. Differential cross sections for many observables are also presented including measurements of the jet observables such as the rapidities and the transverse momenta as well as measurements of event observables such as the scalar sums of the transverse momenta of the jets. The measurements are compared to numerous QCD predictions including next-to-leading-order perturbative calculations, resummation calculations and Monte Carlo generators.
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50.
  • Aad, G, et al. (författare)
  • Performance of the ATLAS muon trigger in pp collisions at [Formula: see text] TeV.
  • 2015
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer. - 1434-6044 .- 1434-6052. ; 75:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The performance of the ATLAS muon trigger system is evaluated with proton-proton collision data collected in 2012 at the Large Hadron Collider at a centre-of-mass energy of 8 TeV. It is primarily evaluated using events containing a pair of muons from the decay of [Formula: see text] bosons. The efficiency of the single-muon trigger is measured for muons with transverse momentum [Formula: see text] GeV, with a statistical uncertainty of less than 0.01 % and a systematic uncertainty of 0.6 %. The [Formula: see text] range for efficiency determination is extended by using muons from decays of [Formula: see text] mesons, [Formula: see text] bosons, and top quarks. The muon trigger shows highly uniform and stable performance. The performance is compared to the prediction of a detailed simulation.
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