Abstract: This Letter presents evidence for single top-quark production in the s-channel using proton-proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The analysis is performed on events containing one isolated electron or muon, large missing transverse momentum and exactly two b-tagged jets in the final state. The analysed data set corresponds to an integrated luminosity of 20.3 fb(-1). The signal is extracted using a maximum-likelihood fit of a discriminant which is based on the matrix element method and optimized in order to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W boson production in association with heavy-flavour jets. The measurement leads to an observed signal significance of 3.2 standard deviations and a measured cross-section of sigma s = 4.8 +/- 0.8(stat.)(-1.3)(+1.6)(syst.) pb, which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.

Abstract: ATLAS has measured two-particle correlations as a function of the relative azimuthal angle, Delta phi, and pseudorapidity, Delta eta, in root s = 13 and 2.76 TeV pp collisions at the LHC using charged particles measured in the pseudorapidity interval vertical bar eta vertical bar < 2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at Delta phi similar to 0 that extends over a wide range of Delta eta, which has been referred to as the “ridge.” Per-trigger-particle yields, Y(Delta phi) are measured over 2 < vertical bar Delta eta vertical bar < 5. For both collision energies, the Y(Delta phi) distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 2 phi reconstructed tracks, and a constant combinatoric contribution modulated by cos (2 Delta phi). The fitted Fourier coefficient, nu(2,2), exhibits factorization, suggesting that the ridge results from per-event cos (2 phi) modulation of the single-particle distribution with Fourier coefficients nu(2). The nu(2) values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a p(T) dependence similar to that measured in p + Pb and Pb + Pb collisions. The nu(2) values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in pp collisions arises from the same or similar underlying physics as observed in p + Pb collisions, and that the dynamics responsible for the ridge has no strong root s dependence.

Abstract: This paper presents a new method of reconstructing the individual charged and neutral hadrons in tau decays with the ATLAS detector. The reconstructed hadrons are used to classify the decay mode and to calculate the visible four-momentum of reconstructed tau candidates, significantly improving the resolution with respect to the calibration in the existing tau reconstruction. The performance of the reconstruction algorithm is optimised and evaluated using simulation and validated using samples of Z -> tau tau and Z(-> μmu)+jets events selected from proton-proton collisions at a centre-of-mass energy root s = 8 TeV, corresponding to an integrated luminosity of 5 fb(-1).

Abstract: The production of W boson pairs in proton-proton collisions at root s = 8TeV is studied using data corresponding to 20.3 fb(-1) of integrated luminosity collected by the ATLAS detector during 2012 at the CERN Large Hadron Collider. The W bosons are reconstructed using their leptonic decays into electrons or muons and neutrinos. Events with reconstructed jets are not included in the candidate event sample. A total of 6636 W W candidate events are observed. Measurements are performed in fiducial regions closely approximating the detector acceptance. The integrated measurement is corrected for all acceptance effects and for the W branching fractions to leptons in order to obtain the total W W production cross section, which is found to be 71.1 +/- 1.1( stat) + (5.7) (5.0) (syst) +/- 1 : 4( lumi) pb. This agrees with the next-to-next-to-leading-order Standard Model prediction of 63.2(+1.6) (-1.4) (scale) +/- 1.2(PDF) pb. Fiducial differential cross sections are measured as a function of each of six kinematic variables. The distribution of the transverse momentum of the leading lepton is used to set limits on anomalous triple-gauge-boson couplings.

Abstract: Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, t (t) over bar system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of root s = 8 TeV. The observables have been chosen to emphasize the t (t) over bar production process and to be sensitive to effects of initial-and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb(-1), recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.