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 polarisation of W bosons produced with large transverse momentum in pp collisions at root s=7 TeV with the ATLAS experiment. Eur. Phys. J. C, 72(5), 2001–30pp.
Abstract: This paper describes an analysis of the angular distribution of W -> e nu and W -> μnu decays, using data from pp collisions at root s = 7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb(-1). Using the decay lepton transverse momentum and the missing transverse momentum, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f(0), f(L) and f(R) over two ranges of W transverse momentum (p(T)(W)) : 35 < p(T)(W) < 50 GeV and p(T)(W) > 50 GeV. Good agreement is found with theoretical predictions. For p(T)(W) > 50 GeV, the values of f(0) and f(L) – f(R), averaged over charge and lepton flavour, are measured to be: f(0) = 0.127 +/- 0.030 +/- 0.108 and f(L) – f(R) = 0.252 +/- 0.017 +/- 0.030, where the first uncertainties are statistical, and the second include all systematic effects.
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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 anisotropy for charged particle production in root s(NN)=2.76 TeV lead-lead collisions with the ATLAS detector. Phys. Rev. C, 86(1), 014907–41pp.
Abstract: Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at root sNN = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 μb(-1). This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle relative to the reaction plane, with the coefficients v(n) denoting the magnitude of the anisotropy. Significant v(2)-v(6) values are obtained as a function of transverse momentum (0.5 < p(T) < 20 GeV), pseudorapidity (|eta| < 2.5), and centrality using an event plane method. The v(n) values for n >= 3 are found to vary weakly with both eta and centrality, and their p(T) dependencies are found to follow an approximate scaling relation, v(n)(1/n)(p(T)) proportional to v(2)(1/2)(p(T)), except in the top 5% most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Delta phi = phi(a)-phi(b)) is performed to extract the coefficients v(n,n) = < cos n Delta phi >. For pairs of charged particles with a large pseudorapidity gap (|Delta eta = eta(a) – eta(b)| > 2) and one particle with p(T) < 3 GeV, the v(2,2)-v(6,6) values are found to factorize as v(n,n)(p(T)(a), p(T)(b)) approximate to v(n) (p(T)(a))v(n)(p(T)(b)) in central and midcentral events. Such factorization suggests that these values of v(2,2)-v(6,6) are primarily attributable to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v(1,1)(p(T)(a), p(T)(b)) data are consistent with the combined contributions from a rapidity-even v(1) and global momentum conservation. A two-component fit is used to extract the v(1) contribution. The extracted v(1) isobserved to cross zero at pT approximate to 1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for v(3), and decreases at higher 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). Forward-backward correlations and charged-particle azimuthal distributions in pp interactions using the ATLAS detector. J. High Energy Phys., 07(7), 019–46pp.
Abstract: Using inelastic proton-proton interactions at root s = 900 GeV and 7 TeV, recorded by the ATLAS detector at the LHC, measurements have been made of the correlations between forward and backward charged-particle multiplicities and, for the first time, between forward and backward charged-particle summed transverse momentum. In addition, jet-like structure in the events is studied by means of azimuthal distributions of charged particles relative to the charged particle with highest transverse momentum in a, selected kinematic region of the event. The results are compared with predictions from tunes of the PYTHIA and HERWIG++ Monte Carlo generators, which in most cases are found to provide a reasonable description of the data.
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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 production cross section of an isolated photon associated with jets in proton-proton collisions at root s=7 TeV with the ATLAS detector. Phys. Rev. D, 85(9), 092014–30pp.
Abstract: A measurement of the cross section for the production of an isolated photon in association with jets in proton-proton collisions at a center-of-mass energy root s = 7 TeV is presented. Photons are reconstructed in the pseudorapidity range vertical bar eta(gamma)vertical bar < 1.37 and with a transverse energy E-T(gamma) > 25 GeV. Jets are reconstructed in the rapidity range vertical bar y(jet)vertical bar < 4.4 and with a transverse momentum p(T)(jet) > 20 GeV. The differential cross section d sigma/dE(T)(gamma) is measured, as a function of the photon transverse energy, for three different rapidity ranges of the leading-p(T) jet: vertical bar y(jet)vertical bar < 1.2, 1.2 <= vertical bar y(jet)vertical bar < 2.8 and 2.8 <= vertical bar y(jet)vertical bar < 4.4. For each rapidity configuration the same-sign (eta(gamma)y(jet) >= 0) and opposite-sign (eta(gamma) y(jet) < 0) cases are studied separately. The results are based on an integrated luminosity of 37 pb(-1), collected with the ATLAS detector at the LHC. Next-to-leading order perturbative QCD calculations are found to be in fair agreement with the data, except for E-T(gamma) <= 45 GeV, where the theoretical predictions overestimate the measured cross sections.
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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 second generation scalar leptoquarks in pp collisions at root s=7 TeV with the ATLAS detector. Eur. Phys. J. C, 72(9), 2151–21pp.
Abstract: The results of a search for the production of second generation scalar leptoquarks are presented for final states consisting of either two muons and at least two jets or a muon plus missing transverse momentum and at least two jets. A total of 1.03 fb(-1) integrated luminosity of proton-proton collision data produced by the Large Hadron Collider at root s = 7 TeV and recorded by the ATLAS detector is used for the search. The event yields in the signal regions are found to be consistent with the Standard Model background expectations. The production of second generation leptoquarks is excluded for a leptoquark mass m(LQ) < 594 (685) GeV at 95 % confidence level, for a branching ratio of 0.5 (1.0) for leptoquark decay to a muon and a quark.
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