ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Measurement of the correlation between flow harmonics of different order in lead-lead collisions at root S-NN=2.76 TeV with the ATLAS detector. Phys. Rev. C, 92(3), 034903–30pp.
Abstract: Correlations between the elliptic or triangular flow coefficients v(m) (m = 2 or 3) and other flow harmonics v(n) (n = 2 to 5) are measured using root S-NN = 2.76 TeV Pb + Pb collision data collected in 2010 by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 7 μb(-1). The v(m)-v(n) correlations aremeasured in midrapidity as a function of centrality, and, for events within the same centrality interval, as a function of event ellipticity or triangularity defined in a forward rapidity region. For events within the same centrality interval, v(3) is found to be anticorrelated with v(2) and this anticorrelation is consistent with similar anticorrelations between the corresponding eccentricities, epsilon(2) and epsilon(3). However, it is observed that v(4) increases strongly with v(2), and v(5) increases strongly with both v(2) and v(3). The trend and strength of the v(m) -v(n) correlations for n = 4 and 5 are found to disagree with epsilon(m)-epsilon(n) correlations predicted by initial-geometry models. Instead, these correlations are found to be consistent with the combined effects of a linear contribution to v(n) and a nonlinear term that is a function of v(2)(2) or of v(2)v(3), as predicted by hydrodynamic models. A simple two-component fit is used to separate these two contributions. The extracted linear and nonlinear contributions to v(4) and v(5) are found to be consistent with previously measured event-plane correlations.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., Costa, M. J., et al. (2016). Measurement of the correlation between the polar angles of leptons from top quark decays in the helicity basis at root s=7 TeV using the ATLAS detector. Phys. Rev. D, 93(1), 012002–23pp.
Abstract: A measurement of the correlations between the polar angles of leptons from the decay of pair-produced t and (t) over bar quarks in the helicity basis is reported, using proton-proton collision data collected by the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 4.6 fb(-1) at a center-of-mass energy of root s = 7 TeV collected during 2011. Candidate events are selected in the dilepton topology with large missing transverse momentum and at least two jets. The angles theta(1) and theta(2) between the charged leptons and the direction of motion of the parent quarks in the t (t) over bar rest frame are sensitive to the spin information, and the distribution of cos theta(1).cos theta(2) is sensitive to the spin correlation between the t and (t) over bar quarks. The distribution is unfolded to parton level and compared to the next-to-leading order prediction. A good agreement is observed.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2016). Measurement of the CP-violating phase phi(s) and the B-s(0) meson decay width difference with B-s(0) -> J/psi phi decays in ATLAS. J. High Energy Phys., 08(8), 147–45pp.
Abstract: A measurement of the B-s(0) decay parameters in the B-s(0) -> J/psi/phi channel using an integrated luminosity of 14.3 fb(-1) collected by the ATLAS detector from 8TeV pp collisions at the LHC is presented. The measured parameters include the CP-violating phase phi(s), the decay width Gamma(s) and the width di ff erence between the mass eigenstates Delta Gamma(s). The values measured for the physical parameters are statistically combined with those from 4.9 fb-1 of 7TeV data, leading to the following: phi(s) = -0.090 +/- 0.078 (stat.) +/- 0.041 (syst.) rad Delta Gamma s = 0.085 +/- 0.011 (stat.) +/- 0.007 (syst.) ps(-1) Gamma(s) = 0.675 +/- 0.003 (stat.) +/- 0.003 (syst:) ps(-1). In the analysis the parameter Delta Gamma(s) is constrained to be positive. Results for phi(s) and Delta Gamma(s) are also presented as 68% and 95% likelihood contours in the phi(s)-Delta Gamma(s) plane. Also measured in this decay channel are the transversity amplitudes and corresponding strong phases. All measurements are in agreement with the Standard Model predictions.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Escobar, C., et al. (2012). Measurement of the cross section for the production of a W boson in association with b-jets in pp collisions at root s=7 TeV with the ATLAS detector. Phys. Lett. B, 707(5), 418–437.
Abstract: A measurement is presented of the cross section for the production of a W boson with one or two jets, of which at least one must be a b-jet, in pp collisions at root s = 7 TeV. Production via top decay is not included in the signal definition. The measurement is based on 35 pb(-1) of data collected with the ATLAS detector at the LHC. The W + b-jet cross section is defined for jets reconstructed with the anti-k(t) clustering algorithm with transverse momentum above 25 GeV and rapidity within +/-2.1. The b-jets are identified by reconstructing secondary vertices. The fiducial cross section is measured both for the electron and muon decay channel of the W boson and is found to be 10.2 +/- 1.9(stat) +/- 2.6(syst) pb for one lepton flavour. The results are compared with next-to-leading order QCD calculations, which predict a cross section smaller than, though consistent with, the measured value.
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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). Measurement of the cross section for top-quark pair production in pp collisions at root s=7 TeV with the ATLAS detector using final states with two high-p(T) leptons. J. High Energy Phys., 05(5), 059–35pp.
Abstract: A measurement is reported of the production cross section of top-quark pairs (t (t) over bar) in proton-proton collisions at a center-of-mass energy of 7 TeV recorded with the ATLAS detector at the LHC. Candidate events have a signature consistent with containing two isolated leptons, large missing transverse momentum, and at least two jets. Using a data sample corresponding to an integrated luminosity of 0.70 fb(-1), a t (t) over bar production cross section sigma(t (t) over bar) = 176 +/- 5(stat.)(-11)(+14)(syst.) +/- 8(lum.) pb is measured for an assumed top-quark mass of m(t) = 172.5 GeV. This measurement is in good agreement with Standard Model predictions.
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