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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Observation of Associated Near-Side and Away-Side Long-Range Correlations in root S-NN=5.02 TeV Proton-Lead Collisions with the ATLAS Detector. Phys. Rev. Lett., 110(18), 182302–18pp.
Abstract: Two-particle correlations in relative azimuthal angle (Delta phi) and pseudorapidity (Delta eta) are measured in root S-NN = 5.02 TeV p + Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1 μb(-1) of data as a function of transverse momentum (p(T)) and the transverse energy (Sigma E-T(Pb)) summed over 3.1 < eta < 4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2 < vertical bar Delta eta vertical bar < 5) “near-side” (Delta phi similar to 0) correlation that grows rapidly with increasing Sigma E-T(Pb). A long-range “away-side” (Delta phi similar to pi) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small Sigma E-T(Pb), is found to match the near-side correlation in magnitude, shape (in Delta eta and Delta phi) and Sigma E-T(Pb) dependence. The resultant Delta phi correlation is approximately symmetric about pi/2, and is consistent with a dominant cos2 Delta phi modulation for all Sigma E-T(Pb) ranges and particle p(T).
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Determination of the Strange-Quark Density of the Proton from ATLAS Measurements of the W -> lv and Z -> ll Cross Sections. Phys. Rev. Lett., 109(1), 012001–17pp.
Abstract: A QCD analysis is reported of ATLAS data on inclusive W-+/- and Z boson production in pp collisions at the LHC, jointly with ep deep-inelastic scattering data from HERA. The ATLAS data exhibit sensitivity to the light quark sea composition and magnitude at Bjorken x similar to 0:01. Specifically, the data support the hypothesis of a symmetric composition of the light quark sea at low x. The ratio of the strange-to-down sea quark distributions is determined to be 1:00(-0:28)(+0.25) at absolute four-momentum transfer squared Q(2) = 1: 9 GeV2 and x = 0: 023.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for Down-Type Fourth Generation Quarks with the ATLAS Detector in Events with One Lepton and Hadronically Decaying W Bosons. Phys. Rev. Lett., 109(3), 032001–19pp.
Abstract: This Letter presents a search for pair production of heavy down-type quarks decaying via b' -> Wt in the lepton + jets channel, as b'(b) over bar' -> W(-)tW(+)(t) over bar -> b (b) over barW(+)W(-)W(+)W- -> l(+/-) nu b (b) over barq (q) over barq (q) over barq ($) over barq. In addition to requiring exactly one lepton, large missing transverse momentum, and at least six jets, the invariant mass of nearby jet pairs is used to identify high transverse momentum W bosons. In data corresponding to an integrated luminosity of 1.04 fb(-1) from pp collisions at root s = 7 TeV recorded with the ATLAS detector, a heavy down-type quark with mass less than 480 GeV can be excluded at the 95% confidence level.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for tb Resonances in Proton-Proton Collisions at root s=7 TeV with the ATLAS Detector. Phys. Rev. Lett., 109(8), 081801–19pp.
Abstract: This Letter presents a search for tb resonances in 1.04 fb(-1) of LHC proton-proton collision data collected by the ATLAS detector at a center-of-mass energy of 7 TeV. Events with a lepton, missing transverse momentum, and two jets are selected and the invariant mass of the corresponding final state is reconstructed. The search exploits the shape of the tb invariant mass distribution compared to the expected standard model backgrounds. The model of a right-handed W'(R) with standard model-like couplings is chosen as the benchmark model for this search. No statistically significant excess of events is observed in the data, and upper limits on the cross section times the branching ratio of W'(R) resonances at 95% C.L. lie in the range of 6.1-1.0 pb for W'(R) masses ranging from 0.5 to 2.0 TeV. These limits are translated into a lower bound on the allowed right-handed W'(R) mass, giving mW'(R) > 1.13 TeV at 95% C.L.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for Pair Production of a New b ' Quark that Decays into a Z Boson and a Bottom Quark with the ATLAS Detector. Phys. Rev. Lett., 109(7), 071801–19pp.
Abstract: A search is reported for the pair production of a new quark b' with at least one b' decaying to a Z boson and a bottom quark. The data, corresponding to 2.0 fb(-1) of integrated luminosity, were collected from pp collisions at root s = 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. Using events with a b-tagged jet and a Z boson reconstructed from opposite-charge electrons, the mass distribution of large transverse momentum b' candidates is tested for an enhancement. No evidence for a b' signal is detected in the observed mass distribution, resulting in the exclusion at a 95% confidence level of b' quarks with masses m(b)' < 400 GeV that decay entirely via b' -> Z + b. In the case of a vectorlike singlet b' mixing solely with the third standard model generation, masses m(b)' < 358 GeV are excluded.
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