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

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1.
  • 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. - Springer. - 1434-6044. ; 74:8, s. 3023
  • 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.
2.
  • 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.
3.
  • 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.
4.
  • 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.
5.
  • 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.
6.
  • 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.
7.
  • 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).
8.
  • Aad, G., et al. (författare)
  • Search for Invisible Decays of a Higgs Boson Produced in Association with a Z Boson in ATLAS
  • 2014
  • Ingår i: Physical Review Letters. - American Physical Society. - 0031-9007. ; 112:20, s. 201802
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for evidence of invisible-particle decay modes of a Higgs boson produced in association with a Z boson at the Large Hadron Collider is presented. No deviation from the standard model expectation is observed in 4.5 fb(-1) (20.3 fb(-1)) of 7 (8) TeV pp collision data collected by the ATLAS experiment. Assuming the standard model rate for ZH production, an upper limit of 75%, at the 95% confidence level is set on the branching ratio to invisible-particle decay modes of the Higgs boson at a mass of 125.5 GeV. The limit on the branching ratio is also interpreted in terms of an upper limit on the allowed dark matter-nucleon scattering cross section within a Higgs-portal dark matter scenario. Within the constraints of such a scenario, the results presented in this Letter provide the strongest available limits for low-mass dark matter candidates. Limits are also set on an additional neutral Higgs boson, in the mass range 110 < m(H) < 400 GeV, produced in association with a Z boson and decaying to invisible particles.
9.
  • 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.
10.
  • Aad, G., et al. (författare)
  • Search for pair-produced third-generation squarks decaying via charm quarks or in compressed supersymmetric scenarios in pp collisions at root s = 8 TeV with the ATLAS detector
  • 2014
  • Ingår i: Physical Review D. - American Physical Society. - 1550-7998. ; 90:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Results of a search for supersymmetry via direct production of third-generation squarks are reported, using 20.3 fb(-1) of proton-proton collision data at root s = 8 TeV recorded by the ATLAS experiment at the LHC in 2012. Two different analysis strategies based on monojetlike and c-tagged event selections are carried out to optimize the sensitivity for direct top squark-pair production in the decay channel to a charm quark and the lightest neutralino ((t) over tilde (1) -> c + (chi) over tilde (0)(1)) across the top squark-neutralino mass parameter space. No excess above the Standard Model background expectation is observed. The results are interpreted in the context of direct pair production of top squarks and presented in terms of exclusion limits in the (m((t) over tilde1), m((chi) over tilde 10)) parameter space. A top squark of mass up to about 240 GeV is excluded at 95% confidence level for arbitrary neutralino masses, within the kinematic boundaries. Top squark masses up to 270 GeV are excluded for a neutralino mass of 200 GeV. In a scenario where the top squark and the lightest neutralino are nearly degenerate in mass, top squark masses up to 260 GeV are excluded. The results from the monojetlike analysis are also interpreted in terms of compressed scenarios for top squark-pair production in the decay channel (t) over tilde (1) -> b + ff' + (chi) over tilde (0)(1) and sbottom pair production with (b) over tilde -> b + (chi) over tilde (0)(1), leading to a similar exclusion for nearly mass-degenerate third-generation squarks and the lightest neutralino. The results in this paper significantly extend previous results at colliders.
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