Abstract: The inclusive production of four isolated charged leptons in pp collisions is analysed for the presence of hard double-parton scattering, using 20.2 fb(-1) of data recorded in the ATLAS detector at the LHC at centre-of-mass energy root s = 8 TeV. In the four-lepton invariant-mass range of 80 < m(4l) < 1000 GeV, an artificial neural network is used to enhance the separation between single- and double-parton scattering based on the kinematics of the four leptons in the final state. An upper limit on the fraction of events originating from double-parton scattering is determined at 95% confidence level to be f(DPS) = 0.042, which results in an estimated lower limit on the effective cross section at 95% confidence level of 1.0 mb.

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.

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).

Abstract: In several extensions of the Standard Model, the top quark can decay into a bottom quark and a light charged Higgs boson H+, t -> bH(+), in addition to the Standard Model decay t -> bW. Since W bosons decay to the three lepton generations equally, while H+ may predominantly decay into tau nu, charged Higgs bosons can be searched for using the violation of lepton universality in top quark decays. The analysis in this paper is based on 4.6 fb(-1) of proton-proton collision data at root s = 7 TeV collected by the ATLAS experiment at the Large Hadron Collider. Signatures containing leptons (e or mu) and/or a hadronically decaying tau (tau(had)) are used. Event yield ratios between e+ tau(had) and e + mu, as well as between μ+ tau(had) and μ+ e, final states are measured in the data and compared to predictions from simulations. This ratio-based method reduces the impact of systematic uncertainties in the analysis. No significant deviation from the Standard Model predictions is observed. With the assumption that the branching fraction B(H+ -> tau nu) is 100%, upper limits in the range 3.2%-4.4% can be placed on the branching fraction B(t -> bH(+)) for charged Higgs boson masses m(H+) in the range 90-140 GeV. After combination with results from a search for charged Higgs bosons in t (t) over bar decays using the tau(had) + jets final state, upper limits on B(t -> bH(+)) can be set in the range 0.8%-3.4%, for m(H+) in the range 90-160 GeV.

Abstract: This paper reports a measurement of the W+b-jets (W+b+X and W+b (b) over bar +X) production cross-section in proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC. These results are based on data corresponding to an integrated luminosity of 4.6 fb(-1), collected with the ATLAS detector. Cross-sections are presented as a function of jet multiplicity and of the transverse momentum of the leading b-jet for both the muon and electron decay modes of the W boson. The W+b-jets cross-section, corrected for all known detector effects, is quoted in a limited kinematic range. Combining the muon and electron channels, the fiducial cross-section for W+b-jets is measured to be 7.1 +/- 0.5 (stat) +/- 1.4 (syst) pb, consistent with the next-to-leading order QCD prediction, corrected for non-perturbative and double-parton interactions (DPI) contributions, of 4.70 +/- 0.09 (stat) (+0.60)(-0.49) (scale) +/- 0.06 (PDF) +/- 0.16 (non-pert) (+0.52)(-0.38) (DPI) pb.