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1.
  • Aad, G., et al. (författare)
  • Constraints on new phenomena via Higgs boson couplings and invisible decays with the ATLAS detector
  • 2015
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :11
  • Tidskriftsartikel (refereegranskat)abstract
    • The ATLAS experiment at the LHC has measured the Higgs boson couplings and mass, and searched for invisible Higgs boson decays, using multiple production and decay channels with up to 4.7 fb(-1) of pp collision data at root S = 7 TeV and 20.3 fb(-1) at root s = 8 TeV. In the current study, the measured production and decay rates of the observed Higgs boson in the gamma gamma, ZZ, WW, Z gamma, bb, tau tau, and mu mu decay channels, along with results from the associated production of a Higgs boson with a top-quark pair, are used to probe the scaling of the couplings with mass. Limits are set on parameters in extensions of the Standard Model including a composite Higgs boson, an additional electroweak singlet, and two-Higgs-doublet models. Together with the measured mass of the scalar Higgs boson in the gamma gamma and ZZ decay modes, a lower limit is set on the pseudoscalar Higgs boson mass of m(A) > 370 GeV in the "hMSSM" simplified Minimal Supersymmetric Standard Model. Results from direct searches for heavy Higgs bosons are also interpreted in the hMSSM. Direct searches for invisible Higgs boson decays in the vector-boson fusion and associated production of a Higgs boson with W/Z (Z -> ll, W/Z -> jj) modes are statistically combined to set an upper limit on the Higgs boson invisible branching ratio of 0.25. The use of the measured visible decay rates in a more general coupling fit improves the upper limit to 0.23, constraining a Higgs portal model of dark matter.
2.
  • Aad, G., et al. (författare)
  • Measurement of event-plane correlations in root s(NN)=2.76 TeV lead-lead collisions with the ATLAS detector
  • 2014
  • Ingår i: Physical Review C (Nuclear Physics). - American Physical Society. - 0556-2813. ; 90:2
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of event-plane correlations involving two or three event planes of different order is presented as a function of centrality for 7 mu b(-1) Pb + Pb collision data at v root s(NN) = 2.76 TeV, recorded by the ATLAS experiment at the Large Hadron Collider. Fourteen correlators are measured using a standard event-plane method and a scalar-product method, and the latter method is found to give a systematically larger correlation signal. Several different trends in the centrality dependence of these correlators are observed. These trends are not reproduced by predictions based on the Glauber model, which includes only the correlations from the collision geometry in the initial state. Calculations that include the final-state collective dynamics are able to describe qualitatively, and in some cases also quantitatively, the centrality dependence of the measured correlators. These observations suggest that both the fluctuations in the initial geometry and the nonlinear mixing between different harmonics in the final state are important for creating these correlations in momentum space.
3.
  • Aad, G., et al. (författare)
  • Measurement of the t(t)over-barW and t(t)over-barZ production cross sections in pp collisions at root s=8 TeV with the ATLAS detector
  • 2015
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :11
  • Tidskriftsartikel (refereegranskat)abstract
    • The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from pp collisions at root s = 8 TeV. The dataset corresponds to an integrated luminosity of 20.3 fb(-1) collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the t (t) over barW and t (t) over barZ processes simultaneously yields a significance of 5.0 sigma (4.2 sigma) over the background-only hypothesis for t (t) over barW (t (t) over barZ) production. The measured cross sections are sigma(t (t) over barW) = 369(-91)(+100) fb and sigma(t (t) over barZ) = 176(-52)(+58) fb. The background-only hypothesis with neither t (t) over barW nor t (t) over barZ production is excluded at 7.1 sigma. All measurements are consistent with next-to-leading-order calculations for the t (t) over barW and t (t) over barZ processes.
4.
  • Aad, G, et al. (författare)
  • Measurements of fiducial cross-sections for [Formula: see text] production with one or two additional b-jets in pp collisions at [Formula: see text]=8 TeV using the ATLAS detector.
  • 2016
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 76
  • Tidskriftsartikel (refereegranskat)abstract
    • Fiducial cross-sections for [Formula: see text] production with one or two additional b-jets are reported, using an integrated luminosity of 20.3 fb[Formula: see text] of proton-proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for [Formula: see text] events with at least one additional b-jet is measured to be 950 [Formula: see text] 70 (stat.) [Formula: see text] (syst.) fb in the lepton-plus-jets channel and 50 [Formula: see text] 10 (stat.) [Formula: see text] (syst.) fb in the [Formula: see text] channel. The cross-section times branching ratio for events with at least two additional b-jets is measured to be 19.3 [Formula: see text] 3.5 (stat.) [Formula: see text] 5.7 (syst.) fb in the dilepton channel ([Formula: see text], [Formula: see text], and ee) using a method based on tight selection criteria, and 13.5 [Formula: see text] 3.3 (stat.) [Formula: see text] 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 [Formula: see text] 0.33 (stat.) [Formula: see text] 0.28 (syst.)% for the ratio of [Formula: see text] production with two additional b-jets to [Formula: see text] production with any two additional jets. All measurements are in good agreement with recent theory predictions.
5.
  • Aad, G., et al. (författare)
  • Search for lepton-flavour-violating H -> mu tau decays of the Higgs boson with the ATLAS detector
  • 2015
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :11
  • Tidskriftsartikel (refereegranskat)abstract
    • A direct search for lepton-flavour-violating H -> mu tau decays of the recently discovered Higgs boson with the ATLAS detector at the LHC is presented. The analysis is performed in the H -> mu tau(had) channel, where tau(had) is a hadronically decaying tau-lepton. The search is based on the data sample of proton-proton collisions collected by the ATLAS experiment corresponding to an integrated luminosity of 20.3 fb(-1) at a centre-of-mass energy of root s = 8TeV. No statistically significant excess of data over the predicted background is observed. The observed (expected) 95% confidence-level upper limit on the branching fraction, Br(H -> mu tau), is 1.85% (1.24%).
6.
  • Aad, G., et al. (författare)
  • Search for pair production of a new heavy quark that decays into a W boson and a light quark in pp collisions at root s=8 TeV with the ATLAS detector
  • 2015
  • Ingår i: Physical Review D (Particles, Fields, Gravitation and Cosmology). - American Physical Society. - 1550-2368. ; 92:11
  • Tidskriftsartikel (refereegranskat)abstract
    • A search is presented for pair production of a new heavy quark (Q) that decays into aW boson and a light quark (q) in the final state where one W boson decays leptonically (to an electron or muon plus a neutrino) and the other W boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb(-1) of pp collisions at root s = 8 TeV collected by the ATLAS detector at the LHC. No evidence of Q (Q) over bar production is observed. New chiral quarks with masses below 690 GeVare excluded at 95% confidence level, assuming BR(Q -> Wq) = 1. Results are also interpreted in the context of vectorlike quark models, resulting in the limits on the mass of a vectorlike quark in the two-dimensional plane of BR(Q -> Wq) versus BR(Q -> Hq).
7.
  • Aad, G., et al. (författare)
  • Search for top quark decays t -> q H with H -> gamma gamma using the ATLAS detector
  • 2014
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :6
  • Tidskriftsartikel (refereegranskat)abstract
    • A search is performed for flavour-changing neutral currents in the decay of a top quark to an up-type (c, u) quark and a Higgs boson, where the Higgs boson decays to two photons. The proton-proton collision data set used corresponds to 4.7 fb(-1) at root s = 7TeV and 20.3 fb(-1) at root s = 8TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for in which one top quark decays to qH and the other decays to bW. Both the hadronic and the leptonic decay modes of the W boson are used. No significant signal is observed and an upper limit is set on the t -> qH branching ratio of 0.79% at the 95% confidence level. The corresponding limit on the tqH coupling combination root lambda(2)(tcH) + root lambda(2)(tuH) is 0.17.
8.
  • Aad, G., et al. (författare)
  • Summary of the ATLAS experiment's sensitivity to supersymmetry after LHC Run 1-interpreted in the phenomenological MSSM
  • 2015
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :10
  • Tidskriftsartikel (refereegranskat)abstract
    • A summary of the constraints from the ATLAS experiment on R-parity-conserving supersymmetry is presented. Results from 22 separate ATLAS searches are considered, each based on analysis of up to 20.3 fb(-1) of proton-proton collision data at centre-of-mass energies of root s = 7 and 8TeV at the Large Hadron Collider. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, in which the lightest supersymmetric particle is a neutralino, taking into account constraints from previous precision electroweak and flavour measurements as well as from dark matter related measurements. The results are presented in terms of constraints on supersymmetric particle masses and are compared to limits from simplified models. The impact of ATLAS searches on parameters such as the dark matter relic density, the couplings of the observed Higgs boson, and the degree of electroweak fine-tuning is also shown. Spectra for surviving supersymmetry model points with low fine-tunings are presented.
9.
  • Aad, G, et al. (författare)
  • Evidence for Electroweak Production of W^{±}W^{±}jj in pp Collisions at sqrt[s]=8 TeV with the ATLAS Detector.
  • 2014
  • Ingår i: Physical Review Letters. - American Physical Society. - 1079-7114. ; 113:14
  • Tidskriftsartikel (refereegranskat)abstract
    • This Letter presents the first study of W^{±}W^{±}jj, same-electric-charge diboson production in association with two jets, using 20.3 fb^{-1} of proton-proton collision data at sqrt[s]=8 TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with two reconstructed same-charge leptons (e^{±}e^{±}, e^{±}μ^{±}, and μ^{±}μ^{±}) and two or more jets are analyzed. Production cross sections are measured in two fiducial regions, with different sensitivities to the electroweak and strong production mechanisms. First evidence for W^{±}W^{±}jj production and electroweak-only W^{±}W^{±}jj production is observed with a significance of 4.5 and 3.6 standard deviations, respectively. The measured production cross sections are in agreement with standard model predictions. Limits at 95% confidence level are set on anomalous quartic gauge couplings.
10.
  • Aad, G., et al. (författare)
  • Light-quark and gluon jet discrimination in collisions at root s=7 TeV with the ATLAS detector
  • 2014
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 74:8
  • Tidskriftsartikel (refereegranskat)abstract
    • A likelihood-based discriminant for the identification of quark- and gluon-initiated jets is built and validated using 4.7 fb of proton-proton collision data at collected with the ATLAS detector at the LHC. Data samples with enriched quark or gluon content are used in the construction and validation of templates of jet properties that are the input to the likelihood-based discriminant. The discriminating power of the jet tagger is established in both data and Monte Carlo samples within a systematic uncertainty of 10-20 %. In data, light-quark jets can be tagged with an efficiency of while achieving a gluon-jet mis-tag rate of in a range between and for jets in the acceptance of the tracker. The rejection of gluon-jets found in the data is significantly below what is attainable using a Pythia 6 Monte Carlo simulation, where gluon-jet mis-tag rates of 10 % can be reached for a 50 % selection efficiency of light-quark jets using the same jet properties.
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