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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 b-jets produced in association with a Z boson at root s=7 TeV with the ATLAS detector ATLAS Collaboration. Phys. Lett. B, 706(4-5), 295–313.
Abstract: A measurement is presented of the inclusive cross-section for b-jet production in association with a Z boson in pp collisions at a centre-of-mass energy of root s = 7 TeV. The analysis uses the data sample collected by the ATLAS experiment in 2010, corresponding to an integrated luminosity of approximately 36 pb(-1). The event selection requires a Z boson decaying into high P-T electrons or muons, and at least one b-jet, identified by its displaced vertex, with transverse momentum p(T) > 25 GeV and rapidity vertical bar y vertical bar < 2.1. After subtraction of background processes, the yield is extracted from the vertex mass distribution of the candidate b-jets. The ratio of this cross-section to the inclusive Z cross-section (the average number of b-jets per Z event) is also measured. Both results are found to be in good agreement with perturbative QCD predictions at next-to-leading order.
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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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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 charge asymmetry in top quark pair production in pp collisions at root s=7 TeV using the ATLAS detector. Eur. Phys. J. C, 72(6), 2039–27pp.
Abstract: A measurement of the top-antitop production charge asymmetry A(C) is presented using data corresponding to an integrated luminosity of 1.04 fb(-1) of pp collisions at root s = 7 TeV collected by the ATLAS detector at the LHC. Events are selected with a single lepton (electron or muon), missing transverse momentum and at least four jets of which at least one jet is identified as coming from a b-quark. A kinematic fit is used to reconstruct the t (t) over bar event topology. After background subtraction, a Bayesian unfolding procedure is performed to correct for acceptance and detector effects. The measured value of A(C) is A(C) =-0.019+/-0.028 (stat.)+/-0.024 (syst.), consistent with the prediction from the MC@NLO Monte Carlo generator of A(C) = 0.006+/-0.002. Measurements of AC in two ranges of invariant mass of the top-antitop pair are also shown.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2016). Measurement of the centrality dependence of the charged-particle pseudorapidity distribution in proton-lead collisions at root s(NN)=5.02 TeV with the ATLAS detector. Eur. Phys. J. C, 76(4), 199–30pp.
Abstract: The centrality dependence of the mean charged-particle multiplicity as a function of pseudorapidity is measured in approximately 1 μb(-1) of proton-lead collisions at a nucleon-nucleon centre-of-mass energy of root s(NN) = 5.02 TeV using the ATLAS detector at the Large Hadron Collider. Charged particles with absolute pseudorapidity less than 2.7 are reconstructed using the ATLAS pixel detector. The p + Pb collision centrality is characterised by the total transverse energy measured in the Pb-going direction of the forward calorimeter. The charged-particle pseudorapidity distributions are found to vary strongly with centrality, with an increasing asymmetry between the proton-going and Pb-going directions as the collisions become more central. Three different estimations of the number of nucleons participating in the p + Pb collision have been carried out using the Glauber model as well as two Glauber-Gribov inspired extensions to the Glauber model. Charged-particle multiplicities per participant pair are found to vary differently for these three models, highlighting the importance of including colour fluctuations in nucleon-nucleon collisions in the modelling of the initial state of p + Pb collisions.
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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 centrality dependence of the charged particle pseudorapidity distribution in lead-lead collisions at root s(NN)=2.76 TeV with the ATLAS detector. Phys. Lett. B, 710(3), 363–382.
Abstract: The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions over vertical bar eta vertical bar < 2 in lead-lead collisions at a nucleon-nucleon centre-of-mass energy of root s(NN) = 2.76 TeV. In order to include particles with transverse momentum as low as 30 MeV, the data were recorded with the central solenoid magnet off. Charged particles were reconstructed with two algorithms (2-point “tracklets” and full tracks) using information from the pixel detector only. The lead-lead collision centrality was characterized by the total transverse energy in the forward calorimeter in the range 3.2 < vertical bar eta vertical bar< 4.9. Measurements are presented of the per-event charged particle pseudorapidity distribution, dN(ch)/d eta, and the average charged particle multiplicity in the pseudorapidity interval vertical bar eta vertical bar < 0.5 in several intervals of collision centrality. The results are compared to previous mid-rapidity measurements at the LHC and RHIC. The variation of the mid-rapidity charged particle yield per colliding nucleon pair with the number of participants is consistent with lower root s(NN) results. The shape of the dN(ch)/d eta distribution is found to be independent of centrality within the systematic uncertainties of the measurement.
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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. (2011). Measurement of the centrality dependence of J/psi yields and observation of Z production in lead-lead collisions with the ATLAS detector at the LHC. Phys. Lett. B, 697(4), 294–312.
Abstract: Using the ATLAS detector, a centrality-dependent suppression has been observed in the yield of J/psi mesons produced in the collisions of lead ions at the Large Hadron Collider. In a sample of minimum-bias lead-lead collisions at a nucleon-nucleon centre of mass energy root s(NN) = 2.76 TeV, corresponding to an integrated luminosity of about 6.7 μb(-1), J/psi mesons are reconstructed via their decays to mu(+)mu(-) pairs. The measured J/psi yield, normalized to the number of binary nucleon-nucleon collisions, is found to significantly decrease from peripheral to central collisions. The centrality dependence is found to be qualitatively similar to the trends observed at previous, lower energy experiments. The same sample is used to reconstruct Z bosons in the mu(+)mu(-) final state, and a total of 38 candidates are selected in the mass window of 66 to 116 GeV. The relative Z yields as a function of centrality are also presented, although no conclusion can be inferred about their scaling with the number of binary collisions, because of limited statistics. This analysis provides the first results on J/psi and Z production in lead-lead collisions at the LHC.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2012). Measurement of the b-hadron production cross section using decays to D*(+)mu X- final states in pp collisions at root s=7 TeV with the ATLAS detector. Nucl. Phys. B, 864(3), 341–381.
Abstract: The b-hadron production cross section is measured with the ATLAS detector in pp collisions at root s = 7 TeV, using 3.3 pb(-1) of integrated luminosity, collected during the 2010 LHC run. The b-hadrons are selected by partially reconstructing D*(+)mu X- final states. Differential cross sections are measured as functions of the transverse momentum and pseudorapidity. The measured production cross section for a b-hadron with p(T) > 9 GeV and vertical bar eta vertical bar < 2.5 is 32.7 +/- 0.8(stat.)(-6.8)(+4.5)(syst.) μb, higher than the next-to-leading-order QCD predictions but consistent within the experimental and theoretical uncertainties.
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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 azimuthal ordering of charged hadrons with the ATLAS detector. Phys. Rev. D, 86(5), 052005–25pp.
Abstract: This paper presents a study of the possible ordering of charged hadrons in the azimuthal angle relative to the beam axis in high-energy proton-proton collisions at the Large Hadron Collider (LHC). A spectral analysis of correlations between longitudinal and transverse components of the momentum of the charged hadrons, driven by the search for phenomena related to the structure of the QCD field, is performed. Data were recorded with the ATLAS detector at center-of-mass energies of root s = 900 GeV and root s = 7 TeV. The correlations measured in a kinematic region dominated by low-p(T) particles are not well described by conventional models of hadron production. The measured spectra show features consistent with the fragmentation of a QCD string represented by a helixlike ordered gluon chain.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2018). Measurement of the azimuthal anisotropy of charged particles produced in root s NN=5.02 TeV Pb+ Pb collisions with the ATLAS detector. Eur. Phys. J. C, 78(12), 997–35pp.
Abstract: Measurements of the azimuthal anisotropy in lead-lead collisions at v s NN = 5.02 TeV are presented using a data sample corresponding to 0.49 nb -1 integrated luminosity collected by the ATLAS experiment at the LHC in 2015. The recorded minimum-bias sample is enhanced by triggers for “ ultra-central” collisions, providing an opportunity to perform detailed study of flow harmonics in the regime where the initial state is dominated by fluctuations. The anisotropy of the charged-particle azimuthal angle distributions is characterized by the Fourier coefficients, v2-v7, which are measured using the two-particle correlation, scalar-product and event-plane methods. The goal of the paper is to provide measurements of the differential as well as integrated flow harmonics vn over wide ranges of the transverse momentum, 0.5 < pT < 60 GeV, the pseudorapidity, |.| < 2.5, and the collision centrality 0-80%. Results from different methods are compared and discussed in the context of previous and recent measurements in Pb+ Pb collisions at v s NN = 2.76TeV and 5.02TeV. In particular, the shape of the pT dependence of elliptic or triangular flow harmonics is observed to be very similar at different centralities after scaling the vn and pT values by constant factors over the centrality interval 0-60% and the pT range 0.5 < pT < 5 GeV.
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