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

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1.
  • Aad, G, et al. (författare)
  • Measurements of Four-Lepton Production at the Z Resonance in pp Collisions at sqrts=7 and 8 TeV with ATLAS.
  • 2014
  • Ingår i: Physical review letters. - American Physical Society. - 0031-9007 .- 1079-7114. ; 112:23, s. 231806
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of four-lepton (4ℓ, ℓ=e,μ) production cross sections at the Z resonance in pp collisions at the LHC with the ATLAS detector are presented. For dilepton and four-lepton invariant mass regions m_{ℓ^{+}ℓ^{-}}>5 GeV and 80<m_{4ℓ}<100 GeV, the measured cross sections are 76±18(stat)±4(syst)±1.4(lumi) fb and 107±9(stat)±4(syst)±3.0(lumi) fb at sqrts=7 and 8 TeV, respectively. By subtracting the nonresonant 4ℓ production contributions and normalizing with Z→μ^{+}μ^{-} events, the branching fraction for the Z boson decay to 4ℓ is determined to be (3.20±0.25(stat)±0.13(syst))×10^{-6}, consistent with the standard model prediction.
2.
  • Aad, G., et al. (författare)
  • Search for direct production of charginos, neutralinos and sleptons in final states with two leptons and missing transverse momentum in pp collisions at root s=8TeV with the ATLAS detector
  • 2014
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :5, s. 1-52
  • Tidskriftsartikel (refereegranskat)abstract
    • Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb(-1) of proton-proton collision data at root s = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.
3.
  • Aad, G., et al. (författare)
  • Search for direct top squark pair production in events with a boson, -jets and missing transverse momentum in TeV collisions with the ATLAS detector
  • 2014
  • Ingår i: European Physical Journal C. - Springer. - 1434-6044. ; 74:6, s. 2883
  • Tidskriftsartikel (refereegranskat)abstract
    • A search is presented for direct top squark pair production using events with at least two leptons including a same-flavour opposite-sign pair with invariant mass consistent with the boson mass, jets tagged as originating from -quarks and missing transverse momentum. The analysis is performed with proton-proton collision data at collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3 fb. No excess beyond the Standard Model expectation is observed. Interpretations of the results are provided in models based on the direct pair production of the heavier top squark state () followed by the decay to the lighter top squark state () via , and for pair production in natural gauge-mediated supersymmetry breaking scenarios where the neutralino () is the next-to-lightest supersymmetric particle and decays producing a boson and a gravitino () via the process.
4.
  • Aad, G., et al. (författare)
  • Search for Higgs boson decays to a photon and a Z boson in pp collisions at root s=7 and 8 TeV with the ATLAS detector
  • 2014
  • Ingår i: Physics Letters B. - Elsevier. - 0370-2693. ; 732, s. 8-27
  • Tidskriftsartikel (refereegranskat)abstract
    • A search is reported for a neutral Higgs boson in the decay channel H -> Z gamma, Z -> l(+)l(-) (l = e, mu), using 4.5 fb(-1) of pp collisions at root s = 7 TeV and 20.3 fb(-1) of pp collisions at root s = 8 TeV, recorded by the ATLAS detector at the CERN Large Hadron Collider. The observed distribution of the invariant mass of the three final-state particles, m(ll gamma), is consistent with the Standard Model hypothesis in the investigated mass range of 120-150 GeV. For a Higgs boson with a mass of 125.5 GeV, the observed upper limit at the 95% confidence level is 11 times the Standard Model expectation. Upper limits are set on the cross section times branching ratio of a neutral Higgs boson with mass in the range 120-150 GeV between 0.13 and 0.5 pb for root s = 8 TeV at 95% confidence level. (C) 2014 The Authors. Published by Elsevier B.V.
5.
  • Aad, G., et al. (författare)
  • Measurement of distributions sensitive to the underlying event in inclusive Z-boson production in pp collisions at root s=7 TeV with the ATLAS detector
  • 2014
  • Ingår i: European Physical Journal C. - Springer. - 1434-6044. ; 74:12, s. 3195
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of charged-particle distributions sensitive to the properties of the underlying event is presented for an inclusive sample of events containing a Z-boson, decaying to an electron or muon pair. The measurement is based on data collected using the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 7 TeV with an integrated luminosity of 4.6fb(-1). Distributions of the charged particle multiplicity and of the charged particle transverse momentum are measured in regions of azimuthal angle defined with respect to the Z-boson direction. The measured distributions are compared to similar distributions measured in jet events, and to the predictions of various Monte Carlo generators implementing different underlying event models.
6.
  • Aad, G., et al. (författare)
  • Evidence for the spin-0 nature of the Higgs boson using ATLAS data
  • 2013
  • Ingår i: Physics Letters B. - Elsevier Science Bv. - 0370-2693. ; 726:1-3, s. 120-144
  • Tidskriftsartikel (refereegranskat)abstract
    • Studies of the spin and parity quantum numbers of the Higgs boson are presented, based on protonproton collision data collected by the ATLAS experiment at the LHC. The Standard Model spin-parity J(P) = 0(+) hypothesis is compared with alternative hypotheses using the Higgs boson decays H -> gamma gamma, H -> ZZ* -> 4l and H -> WW* -> l nu l nu, as well as the combination of these channels. The analysed dataset corresponds to an integrated luminosity of 20.7 fb(-1) collected at a centre-of-mass energy of root s = 8 TeV. For the H -> ZZ* -> 4l decay mode the dataset corresponding to an integrated luminosity of 4.6 fb(-1) collected at root s = 7 TeV is included. The data are compatible with the Standard Model J(P) = 0+ quantum numbers for the Higgs boson, whereas all alternative hypotheses studied in this Letter, namely some specific J(P) = 0(-), 1(+), 1(-), 2(+) models, are excluded at confidence levels above 97.8%. This exclusion holds independently of the assumptions on the coupling strengths to the Standard Model particles and in the case of the J(P) = 2(+) model, of the relative fractions of gluon-fusion and quark-antiquark production of the spin-2 particle. The data thus provide evidence for the spin-0 nature of the Higgs boson, with positive parity being strongly preferred. (C) 2013 CERN. Published by Elsevier B.V. All rights reserved.
7.
  • Aad, G., et al. (författare)
  • Measurement of the inclusive jet cross-section in proton-proton collisions at root s=7 TeV using 4.5 fb(-1) of data with the ATLAS detector
  • 2015
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :2, s. 153
  • Tidskriftsartikel (refereegranskat)abstract
    • The inclusive jet cross-section is measured in proton-proton collisions at a centre-of-mass energy of 7 TeV using a data set corresponding to an integrated luminosity of 4.5 fb(-1) collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-k(t) algorithm with radius parameter values of 0.4 and 0.6. The double-differential cross-sections are presented as a function of the jet transverse momentum and the jet rapidity, covering jet transverse momenta from 100 GeV to 2 TeV. Next-to-leading-order QCD calculations corrected for non-perturbative effects and electroweak effects, as well as Monte Carlo simulations with next-to-leading-order matrix elements interfaced to parton showering, are compared to the measured cross-sections. A quantitative comparison of the measured cross-sections to the QCD calculations using several sets of parton distribution functions is performed.
8.
  • Aad, G., et al. (författare)
  • Measurement of the total cross section from elastic scattering in pp collisions at root s=7 TeV with the ATLAS detector
  • 2014
  • Ingår i: Nuclear Physics B. - Elsevier Science BV. - 0550-3213. ; 889, s. 486-548
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of the total pp cross section at the LHC at root s = 7 TeV is presented. In a special run with high-beta* beam optics, an integrated luminosity of 80 mu b(-1) was accumulated in order to measure the differential elastic cross section as a function of the Mandelstam momentum transfer variable t. The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in the vertical bar t vertical bar range from 0.01 GeV2 to 0.1 GeV2 to extrapolate to vertical bar t vertical bar --> 0, the total cross section, sigma(tot)(pp --> X), is measured via the optical theorem to be: sigma(tot)(pp --> X) = 95.35 +/- 0.38 (stat.) +/- 1.25 (exp.) +/- 0.37 (extr.) mb, where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation to vertical bar t vertical bar --> 0. In addition, the slope of the elastic cross section at small vertical bar t vertical bar is determined to be B = 19.73 +/- 0.14 (stat.) +/- 0.26 (syst.) GeV-2. (C) 2014 The Authors. Published by Elsevier B.V.
9.
  • Aad, G., et al. (författare)
  • Measurements of spin correlation in top-antitop quark events from proton-proton collisions at root s=7 TeV using the ATLAS detector
  • 2014
  • Ingår i: Physical Review D. - American Physical Society. - 1550-7998. ; 90:11, s. 112016
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of spin correlation in top quark pair production are presented using data collected with the ATLAS detector at the LHC with proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 4.6 fb(-1). Events are selected in final states with two charged leptons and at least two jets and in final states with one charged lepton and at least four jets. Four different observables sensitive to different properties of the top quark pair production mechanism are used to extract the correlation between the top and antitop quark spins. Some of these observables are measured for the first time. The measurements are in good agreement with the Standard Model prediction at next-to-leading-order accuracy.
10.
  • Aad, G., et al. (författare)
  • Search for anomalous production of prompt same-sign lepton pairs and pair-produced doubly charged Higgs bosons with root s=8 TeV pp collisions using the ATLAS detector
  • 2015
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :3, s. 041
  • Tidskriftsartikel (refereegranskat)abstract
    • A low-background inclusive search for new physics in events with same-sign dileptons is presented. The search uses proton-proton collisions corresponding to 20.3 fb(-1) of integrated luminosity taken in 2012 at a centre-of-mass energy of 8TeV with the ATLAS detector at the LHC. Pairs of isolated leptons with the same electric charge and large transverse momenta of the type e(+/-) e(+/-), e(+/-) mu(+/-), and mu(+/-) mu(+/-) are selected and their invariant mass distribution is examined. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the cross-sections for processes beyond the Standard Model. Limits are placed as a function of the dilepton invariant mass within a fiducial region corresponding to the signal event selection criteria. Exclusion limits are also derived for a specific model of doubly charged Higgs boson production.
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