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Sökning: WFRF:(Zhang Y. H.) > (2010-2014)

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1.
  • Aad, G., et al. (författare)
  • Search for nonpointing and delayed photons in the diphoton and missing transverse momentum final state in 8 TeV pp collisions at the LHC using the ATLAS detector
  • 2014
  • Ingår i: Physical Review D (Particles, Fields, Gravitation and Cosmology). - : American Physical Society. - 1550-2368 .- 1550-7998. ; 90:11
  • Tidskriftsartikel (refereegranskat)abstract
    • A search has been performed, using the full 20.3 fb(-1) data sample of 8 TeV proton-proton collisions collected in 2012 with the ATLAS detector at the LHC, for photons originating from a displaced vertex due to the decay of a neutral long-lived particle into a photon and an invisible particle. The analysis investigates the diphoton plus missing transverse momentum final state, and is therefore most sensitive to pair production of long-lived particles. The analysis technique exploits the capabilities of the ATLAS electromagnetic calorimeter to make precise measurements of the flight direction, as well as the time of flight, of photons. No excess is observed over the Standard Model predictions for background. Exclusion limits are set within the context of gauge mediated supersymmetry breaking models, with the lightest neutralino being the next-to-lightest supersymmetric particle and decaying into a photon and gravitino with a lifetime in the range from 250 ps to about 100 ns.
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2.
  • Aad, G., et al. (författare)
  • Search for neutral Higgs bosons of the minimal supersymmetric standard model in pp collisions at root s=8 TeV with the ATLAS detector
  • 2014
  • Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; :11
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for the neutral Higgs bosons predicted by the Minimal Supersymmetric Standard Model (MSSM) is reported. The analysis is performed on data from proton-proton collisions at a centre-of-mass energy of 8 TeV collected with the ATLAS detector at the Large Hadron Collider. The samples used for this search were collected in 2012 and correspond to integrated luminosities in the range 19.5-20.3 fb(-1). The MSSM Higgs bosons are searched for in the tau tau final state. No significant excess over the expected background is observed, and exclusion limits are derived for the production cross section times branching fraction of a scalar particle as a function of its mass. The results are also interpreted in the MSSM parameter space for various benchmark scenarios.
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3.
  • Aad, G., et al. (författare)
  • Measurement of Higgs boson production in the diphoton decay channel in pp collisions at center-of-mass energies of 7 and 8 TeV with the ATLAS detector
  • 2014
  • Ingår i: Physical Review D (Particles, Fields, Gravitation and Cosmology). - : American Physical Society. - 1550-2368 .- 1550-7998. ; 90:11
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of the production processes of the recently discovered Higgs boson is performed in the two-photon final state using 4.5 fb(-1) of proton-proton collisions data at root s = 7 TeV and 20.3 fb(-1) at root s = 8 TeV collected by the ATLAS detector at the Large Hadron Collider. The number of observed Higgs boson decays to diphotons divided by the corresponding Standard Model prediction, called the signal strength, is found to be mu = 1.17 +/- 0.27 at the value of the Higgs boson mass measured by ATLAS, m(H) = 125.4 GeV. The analysis is optimized to measure the signal strengths for individual Higgs boson production processes at this value of m(H). They are found to be mu(ggF) = 1.32 +/- 0.38, mu(VBF) = 0.8 +/- 0.7, mu(WH) = 1.0 +/- 1.6, mu(ZH) = 0.1(-0.1)(+3.7), and mu t (t) over barH = 1.6(-1.8)(+2.7), for Higgs boson production through gluon fusion, vector-boson fusion, and in association with a W or Z boson or a top-quark pair, respectively. Compared with the previously published ATLAS analysis, the results reported here also benefit from a new energy calibration procedure for photons and the subsequent reduction of the systematic uncertainty on the diphoton mass resolution. No significant deviations from the predictions of the Standard Model are found.
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4.
  • 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, Section B. - : North-Holland. - 0550-3213 .- 1873-1562. ; 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.
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5.
  • Aad, G., et al. (författare)
  • Search for long-lived neutral particles decaying into lepton jets in proton-proton collisions at root s=8 Tev with the ATLAS detector
  • 2014
  • Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; :11
  • Tidskriftsartikel (refereegranskat)abstract
    • Several models of physics beyond the Standard Model predict neutral particles that decay into final states consisting of collimated jets of light leptons and hadrons (so-called "lepton jets"). These particles can also be long-lived with decay length comparable to, or even larger than, the LHC detectors' linear dimensions. This paper presents the results of a search for lepton jets in proton-proton collisions at the centre-of-mass energy of root s = 8 TeV in a sample of 20.3 fb(-1) collected during 2012 with the ATLAS detector at the LHC. Limits on models predicting Higgs boson decays to neutral long-lived lepton jets are derived as a function of the particle's proper decay length.
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6.
  • Aad, G., et al. (författare)
  • Search for the lepton flavor violating decay Z -> e mu in pp collisions at root s=8 TeV with the ATLAS detector
  • 2014
  • Ingår i: Physical Review D (Particles, Fields, Gravitation and Cosmology). - : American Physical Society. - 1550-2368 .- 1550-7998. ; 90:7
  • Tidskriftsartikel (refereegranskat)abstract
    • The ATLAS detector at the Large Hadron Collider is used to search for the lepton flavor violating process Z -> e mu in pp collisions using 20.3 fb(-1) of data collected at root s = 8 TeV. An enhancement in the e mu invariant mass spectrum is searched for at the Z-boson mass. The number of Z bosons produced in the data sample is estimated using events of similar topology, Z -> ee and mu mu, significantly reducing the systematic uncertainty in the measurement. There is no evidence of an enhancement at the Z-boson mass, resulting in an upper limit on the branching fraction, B(Z -> e mu) < 7.5 x 10(-7) at the 95% confidence level.
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7.
  • Aad, G., et al. (författare)
  • Fiducial and differential cross sections of Higgs boson production measured in the four-lepton decay channel in pp collisions at root s=8 TeV with the ATLAS detector
  • 2014
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier. - 0370-2693 .- 1873-2445. ; 738, s. 234-253
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of fiducial and differential cross sections of Higgs boson production in the H -> ZZ* -> 4l decay channel are presented. The cross sections are determined within a fiducial phase space and corrected for detection efficiency and resolution effects. They are based on 20.3 fb(-1) of pp collision data, produced at root s= 8 TeV centre-of-mass energy at the LHC and recorded by the ATLAS detector. The differential measurements are performed in bins of transverse momentum and rapidity of the four-lepton system, the invariant mass of the subleading lepton pair and the decay angle of the leading lepton pair with respect to the beam line in the four-lepton rest frame, as well as the number of jets and the transverse momentum of the leading jet. The measured cross sections are compared to selected theoretical calculations of the Standard Model expectations. No significant deviation from any of the tested predictions is found. Published by Elsevier B.V.
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8.
  • 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. Particles and Fields. - : Springer. - 1434-6044 .- 1434-6052. ; 74:12
  • 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.
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9.
  • Aad, G., et al. (författare)
  • Search for pair and single production of new heavy quarks that decay to a Z boson and a third-generation quark in pp collisions at root s=8 TeV with the ATLAS detector
  • 2014
  • Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; :11
  • Tidskriftsartikel (refereegranskat)abstract
    • A search is presented for the production of new heavy quarks that decay to a Z boson and a third-generation Standard Model quark. In the case of a new charge +2/3 quark (T), the decay targeted is T -> Zt, while the decay targeted for a new charge -1/3 quark (B) is B -> Zb. The search is performed with a dataset corresponding to 20.3 fb(-1) of p p collisions at root s = 8TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum Z boson candidate reconstructed from a pair of oppositely charged same-flavor leptons (electrons or muons), and are analyzed in two channels defined by the absence or presence of a third lepton. Hadronic jets, in particular those with properties consistent with the decay of a b-hadron, are also required to be present in selected events. Different requirements are made on the jet activity in the event in order to enhance the sensitivity to either heavy quark pair production mediated by the strong interaction, or single production mediated by the electroweak interaction. No significant excess of events above the Standard Model expectation is observed, and lower limits are derived on the mass of vector-like T and B quarks under various branching ratio hypotheses, as well as upper limits on the magnitude of electroweak coupling parameters.
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10.
  • Aad, G., et al. (författare)
  • Measurement of inclusive jet charged-particle fragmentation functions in Pb plus Pb collisions at root S-NN=2.76 TeV with the ATLAS detector
  • 2014
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier. - 0370-2693 .- 1873-2445. ; 739, s. 320-342
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of charged-particle fragmentation functions of jets produced in ultra-relativistic nuclear collisions can provide insight into the modification of parton showers in the hot, dense medium created in the collisions. ATLAS has measured jets in root S-NN = 2.76 TeV Pb+Pb collisions at the LHC using a data set recorded in 2011 with an integrated luminosity of 0.14 nb(-1). Jets were reconstructed using the anti-k(t) algorithm with distance parameter values R = 0.2, 0.3, and 0.4. Distributions of charged-particle transverse momentum and longitudinal momentum fraction are reported for seven bins in collision centrality for R = 0.4 jets with p(T)(jet) > 100 GeV. Commensurate minimum p(T) values are used for the other radii. Ratios of fragment distributions in each centrality bin to those measured in the most peripheral bin are presented. These ratios show a reduction of fragment yield in central collisions relative to peripheral collisions at intermediate z values, 0.04 less than or similar to z less than or similar to 0.2, and an enhancement in fragment yield for z less than or similar to 0.04. A smaller, less significant enhancement is observed at large z and large p(T) in central collisions. (C) 2014 The Authors. Published by Elsevier B.V.
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