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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 dark matter candidates and large extra dimensions in events with a jet and missing transverse momentum with the ATLAS detector
  • 2013
  • Ingår i: JOURNAL OF HIGH ENERGY PHYSICS. - SPRINGER. - 1029-8479. ; :4
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for new phenomena in events with a high-energy jet and large missing transverse momentum is performed using data from proton-proton collisions at root s = 7 TeV with the ATLAS experiment at the Large flatiron Collider. Four kinematic regions are explored using a dataset corresponding to an integrated luminosity of 4.7 fb(-1). No excess of events beyond expectations from Standard Model processes is observed, and limits are set on large extra dimensions and the pair production of dark matter particles.
5.
  • Aad, G., et al. (författare)
  • Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in root s=8 TeV pp collisions with the ATLAS detector
  • 2014
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :4, s. 169
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb(-1) of root s = 8TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R-parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino ((chi) over tilde (+/-)(1) ) and next-to-lightest neutralino ((chi) over tilde (0)(2)) production with decays to lightest neutralino ((chi) over tilde (0)(1)) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons, (chi) over tilde (+/-)(1) and (chi) over tilde (0)(2) masses are excluded up to 700GeV, 380GeV, 345GeV, or 148GeV respectively, for a massless (chi) over tilde (0)(1).
6.
  • Aad, G., et al. (författare)
  • Search for new phenomena in photon plus jet events collected in proton-proton collisions at root s=8 TeV with the ATLAS detector
  • 2014
  • Ingår i: Physics Letters B. - Elsevier Science Bv. - 0370-2693. ; 728, s. 562-578
  • Tidskriftsartikel (refereegranskat)abstract
    • This Letter describes a model-independent search for the production of new resonances in photon + jet (gamma + jet) events using 20 fb(-1) of proton-proton LHC data recorded with the ATLAS detector at a centre-of-mass energy of root s = 8 TeV. The gamma + jet mass distribution is compared to a background model fit from data; no significant deviation from the background-only hypothesis is found. Limits are set at 95% credibility level on generic Gaussian-shaped signals and two benchmark phenomena beyond the Standard Model: non-thermal quantum black holes and excited quarks. Non-thermal quantum black holes are excluded below masses of 4.6 TeV and excited quarks are excluded below masses of 3.5 TeV. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.
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, s. 008
  • 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)
  • Study of heavy-flavor quarks produced in association with top-quark pairs at root s=7 TeV using the ATLAS detector
  • 2014
  • Ingår i: Physical Review D. - American Physical Society. - 1550-7998. ; 89:7, s. 072012
  • Tidskriftsartikel (refereegranskat)abstract
    • Using a sample of dilepton top-quark pair ((tt) over bar) candidate events, a study is performed of the production of top-quark pairs together with heavy-flavor (HF) quarks, the sum of (tt) over bar + b + X and (tt) over bar + c + X, collectively referred to as (tt) over bar + HF. The data set used corresponds to an integrated luminosity of 4.7 fb(-1) of proton-proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The presence of additional HF (b or c) quarks in the (tt) over bar sample is inferred by looking for events with at least three b-tagged jets, where two are attributed to the b quarks from the (tt) over bar decays and the third to additional HF production. The dominant background to (tt) over bar + HF in this sample is (tt) over bar + jet events in which a light-flavor jet is misidentified as a heavy-flavor jet. To determine the heavy-and light-flavor content of the additional b-tagged jets, a fit to the vertex mass distribution of b-tagged jets in the sample is performed. The result of the fit shows that 79 +/- 14 (stat) +/- 22 (syst) of the 105 selected extra b-tagged jets originate from HF quarks, 3 standard deviations away from the hypothesis of zero (tt) over bar + HF production. The result for extra HF production is quoted as a ratio (R-HF) of the cross section for (tt) over bar + HF production to the cross section for (tt) over bar production with at least one additional jet. Both cross sections are measured in a fiducial kinematic region within the ATLAS acceptance. R-HF is measured to be 6.2 +/- 1.1(stat) +/- 1.8 (syst)% for jets with p(T) > 25 GeV and vertical bar eta vertical bar < 2.5, in agreement with the expectations from Monte Carlo generators.
9.
  • Aad, G., et al. (författare)
  • Measurement of the electroweak production of dijets in association with a Z-boson and distributions sensitive to vector boson fusion in proton-proton collisions at=8 TeV using the ATLAS detector
  • 2014
  • Ingår i: Journal of High Energy Physics. - Springer. - 1029-8479. ; :4, s. 031
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of fiducial cross sections for the electroweak production of two jets in association with a Z-boson are presented. The measurements are performed using 20.3 fb(-1) of proton-proton collision data collected at a centre-of-mass energy of = 8 TeV by the ATLAS experiment at the Large Hadron Collider. The electroweak component is extracted by a fit to the dijet invariant mass distribution in a fiducial region chosen to enhance the electroweak contribution over the dominant background in which the jets are produced via the strong interaction. The electroweak cross sections measured in two fiducial regions are in good agreement with the Standard Model expectations and the background-only hypothesis is rejected with significance above the 5 sigma level. The electroweak process includes the vector boson fusion production of a Z-boson and the data are used to place limits on anomalous triple gauge boson couplings. In addition, measurements of cross sections and differential distributions for inclusive Z-boson-plus-dijet production are performed in five fiducial regions, each with different sensitivity to the electroweak contribution. The results are corrected for detector effects and compared to predictions from the Sherpa and Powheg event generators.
10.
  • Aad, G., et al. (författare)
  • Dynamics of isolated-photon plus jet production in pp collisions at root s=7 TeV with the ATLAS detector
  • 2013
  • Ingår i: Nuclear Physics B. - Elsevier Science Bv. - 0550-3213. ; 875:3, s. 483-535
  • Tidskriftsartikel (refereegranskat)abstract
    • The dynamics of isolated-photon plus jet production in pp collisions at a centre-of-mass energy of 7 TeV has been studied with the ATLAS detector at the LHC using an integrated luminosity of 37 pb(-1). Measurements of isolated-photon plus jet bin-averaged cross sections are presented as functions of photon transverse energy, jet transverse momentum and jet rapidity. In addition, the bin-averaged cross sections as functions of the difference between the azimuthal angles of the photon and the jet, the photon jet invariant mass and the scattering angle in the photon jet centre-of-mass frame have been measured. Next-to-leading-order QCD calculations are compared to the measurements and provide a good description of the data, except for the case of the azimuthal opening angle. (C) 2013 CERN. Published by Elsevier B.V. All rights reserved.
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