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Sökning: WFRF:(Armbruster A. J.) > Uppsala universitet

  • Resultat 1-10 av 453
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1.
  • 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.
2.
  • Aad, G., et al. (författare)
  • ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider.
  • 2015
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 75:10
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper reviews and extends searches for the direct pair production of the scalar supersymmetric partners of the top and bottom quarks in proton-proton collisions collected by the ATLAS collaboration during the LHC Run 1. Most of the analyses use 20 [Formula: see text] of collisions at a centre-of-mass energy of [Formula: see text] TeV, although in some case an additional [Formula: see text] of collision data at [Formula: see text] TeV are used. New analyses are introduced to improve the sensitivity to specific regions of the model parameter space. Since no evidence of third-generation squarks is found, exclusion limits are derived by combining several analyses and are presented in both a simplified model framework, assuming simple decay chains, as well as within the context of more elaborate phenomenological supersymmetric models.
3.
  • Aad, G., et al. (författare)
  • Determination of the Ratio of b-Quark Fragmentation Fractions f_{s}/f_{d} in pp Collisions at sqrt[s]=7 TeV with the ATLAS Detector.
  • 2015
  • Ingår i: Physical Review Letters. - American Physical Society. - 1079-7114. ; 115:26
  • Tidskriftsartikel (refereegranskat)abstract
    • With an integrated luminosity of 2.47 fb^{-1} recorded by the ATLAS experiment at the LHC, the exclusive decays B_{s}^{0}→J/ψϕ and B_{d}^{0}→J/ψK^{*0} of B mesons produced in pp collisions at sqrt[s]=7 TeV are used to determine the ratio of fragmentation fractions f_{s}/f_{d}. From the observed B_{s}^{0}→J/ψϕ and B_{d}^{0}→J/ψK^{*0} yields, the quantity (f_{s}/f_{d})[B(B_{s}^{0}→J/ψϕ)/B(B_{d}^{0}→J/ψK^{*0})] is measured to be 0.199±0.004(stat)±0.008(syst). Using a recent theory prediction for [B(B_{s}^{0}→J/ψϕ)/B(B_{d}^{0}→J/ψK^{*0})] yields (f_{s}/f_{d})=0.240±0.004(stat)±0.010(syst)±0.017(th). This result is based on a new approach that provides a significant improvement of the world average.
4.
  • 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. ; 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.
5.
  • Aad, G., et al. (författare)
  • Measurements of the Higgs boson production and decay rates and coupling strengths using pp collision data at [Formula: see text] and 8 TeV in the ATLAS experiment.
  • 2016
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 76
  • Tidskriftsartikel (refereegranskat)abstract
    • Combined analyses of the Higgs boson production and decay rates as well as its coupling strengths to vector bosons and fermions are presented. The combinations include the results of the analyses of the [Formula: see text] and [Formula: see text] decay modes, and the constraints on the associated production with a pair of top quarks and on the off-shell coupling strengths of the Higgs boson. The results are based on the LHC proton-proton collision datasets, with integrated luminosities of up to 4.7 [Formula: see text] at [Formula: see text] TeV and 20.3 [Formula: see text] at [Formula: see text] TeV, recorded by the ATLAS detector in 2011 and 2012. Combining all production modes and decay channels, the measured signal yield, normalised to the Standard Model expectation, is [Formula: see text]. The observed Higgs boson production and decay rates are interpreted in a leading-order coupling framework, exploring a wide range of benchmark coupling models both with and without assumptions on the Higgs boson width and on the Standard Model particle content in loop processes. The data are found to be compatible with the Standard Model expectations for a Higgs boson at a mass of 125.36 GeV for all models considered.
6.
  • Aad, G, et al. (författare)
  • Search for an additional, heavy Higgs boson in the decay channel at in collision data with the ATLAS detector
  • 2016
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 76:1
  • Tidskriftsartikel (refereegranskat)abstract
    • A search is presented for a high-mass Higgs boson in the , , , and decay modes using the ATLAS detector at the CERN Large Hadron Collider. The search uses proton-proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb. The results of the search are interpreted in the scenario of a heavy Higgs boson with a width that is small compared with the experimental mass resolution. The Higgs boson mass range considered extends up to for all four decay modes and down to as low as 140 , depending on the decay mode. No significant excess of events over the Standard Model prediction is found. A simultaneous fit to the four decay modes yields upper limits on the production cross-section of a heavy Higgs boson times the branching ratio to boson pairs. 95 % confidence level upper limits range from 0.53 pb at GeV to 0.008 pb at GeV for the gluon-fusion production mode and from 0.31 pb at GeV to 0.009 pb at GeV for the vector-boson-fusion production mode. The results are also interpreted in the context of Type-I and Type-II two-Higgs-doublet models.
7.
  • Aad, G., et al. (författare)
  • Search for flavour-changing neutral current top quark decays t -> Hq in pp collisions at root s=8 TeV with the ATLAS detector
  • 2015
  • Ingår i: Journal of High Energy Physics. - Springer. - 1126-6708. ; :12
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for flavour-changing neutral current decays of a top quark to an up-type quark (q = u; c) and the Standard Model Higgs boson, where the Higgs boson decays to b (b) over bar, is presented. The analysis searches for top quark pair events in which one top quark decays to W b, with the W boson decaying leptonically, and the other top quark decays to Hq. The search is based on pp collisions at root s = 8TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and uses an integrated luminosity of 20.3 fb(-1). Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of b quark jets characteristic of signal events, and employs a likelihood discriminant that uses the kinematic differences between the signal and the background, which is dominated by t (t) over bar -> WbWb decays. No significant excess of events above the background expectation is found, and observed (expected) 95% CL upper limits of 0.56% (0.42%) and 0.61% (0.64%) are derived for the t -> Hc and t -> Hu branching ratios respectively. The combination of this search with other ATLAS searches in the H -> gamma gamma and H -> WW*, tau tau decay modes significantly improves the sensitivity, yielding observed (expected) 95% CL upper limits on the t -> Hc and t -> Hu branching ratios of 0.46% (0.25%) and 0.45% (0.29%) respectively. The corresponding combined observed (expected) upper limits on the vertical bar lambda(tcH)vertical bar and vertical bar lambda(tuH)vertical bar couplings are 0.13 (0.10) and 0.13 (0.10) respectively. These are the most restrictive direct bounds on t q H interactions measured so far.
8.
  • 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. - 1126-6708. ; :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%).
9.
  • Aad, G., et al. (författare)
  • Search for metastable heavy charged particles with large ionisation energy loss in pp collisions at root s=8 TeV using the ATLAS experiment
  • 2015
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 75:9
  • Tidskriftsartikel (refereegranskat)abstract
    • Many extensions of the Standard Model predict the existence of charged heavy long-lived particles, such as R-hadrons or charginos. These particles, if produced at the Large Hadron Collider, should bemoving non-relativistically and are therefore identifiable through the measurement of an anomalously large specific energy loss in the ATLAS pixel detector. Measuring heavy long-lived particles through their track parameters in the vicinity of the interaction vertex provides sensitivity to metastable particles with lifetimes from 0.6 ns to 30 ns. A search for such particles with the ATLAS detector at the Large Hadron Collider is presented, based on a data sample corresponding to an integrated luminosity of 18.4 fb(-1) of pp collisions at root s = 8 TeV. No significant deviation from the Standard Model background expectation is observed, and lifetime-dependent upper limits on R-hadrons and chargino production are set. Gluino R-hadrons with 10 ns lifetime and masses up to 1185 GeV are excluded at 95 % confidence level, and so are charginos with 15 ns lifetime and masses up to 482 GeV.
10.
  • 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).
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