Abstract: A search for Higgs boson production in association with a pair of top quarks (t (t) over barH) is performed, where the Higgs boson decays to b (b) over bar, and both top quarks decay hadronically. The data used correspond to an integrated luminosity of 20.3 fb(-1) of pp collisions at root s = 8 TeV collected with the ATLAS detector at the Large Hadron Collider. The search selects events with at least six energetic jets and uses a boosted decision tree algorithm to discriminate between signal and Standard Model background. The dominant multijet background is estimated using a dedicated data-driven technique. For a Higgs boson mass of 125 GeV, an upper limit of 6.4 (5.4) times the Standard Model cross section is observed (expected) at 95% confidence level. The best-fit value for the signal strength is μ= 1.6 +/- 2.6 times the Standard Model expectation for m(H) = 125 GeV. Combining all t (t) over barH searches carried out by ATLAS at root s = 8 and 7 TeV, an observed (expected) upper limit of 3.1 (1.4) times the Standard Model expectation is obtained at 95% confidence level, with a signal strength μ= 1.7 +/- 0.8.

Abstract: This paper presents studies of the performance of several jet-substructure techniques, which are used to identify hadronically decaying top quarks with high transverse momentum contained in large-radius jets. The efficiency of identifying top quarks is measured using a sample of top-quark pairs and the rate of wrongly identifying jets from other quarks or gluons as top quarks is measured using multijet events collected with the ATLAS experiment in 20.3 fb(-1) of 8TeV proton-proton collisions at the Large Hadron Collider. Predictions from Monte Carlo simulations are found to provide an accurate description of the performance. The techniques are compared in terms of signal efficiency and background rejection using simulations, covering a larger range in jet transverse momenta than accessible in the dataset. Additionally, a novel technique is developed that is optimized to reconstruct top quarks in events with many jets.

Abstract: The angular distributions of Drell-Yan charged lepton pairs in the vicinity of the Z-boson mass peak probe the underlying QCD dynamics of Z-boson production. This paper presents a measurement of the complete set of angular coefficients Lambda(0-7) describing these distributions in the Z-boson Collins-Soper frame. The data analysed correspond to 20.3 fb(-1) of pp collisions at root s = 8TeV, collected by the ATLAS detector at the CERN LHC. The measurements are compared to the most precise fixed-order calculations currently available (O (alpha(2)(s))) and with theoretical predictions embedded in Monte Carlo generators. The measurements are precise enough to probe QCD corrections beyond the formal accuracy of these calculations and to provide discrimination between different parton-shower models. A signi fi cant deviation from the O (alpha(2)(s)) predictions is observed for A(0)-A(2). Evidence is found for non-zero A(5,6, 7), consistent with expectations.

Abstract: Measurements of the jet activity in t (t) over bar events produced in proton-proton collisions at root s = 8 TeV are presented, using 20.3 fb(-1) of data collected by the ATLAS experiment at the Large Hadron Collider. The events were selected in the dilepton e μdecay channel with two identified b-jets. The numbers of additional jets for various jet transverse momentum ( p(T)) thresholds, and the normalised differential cross-sections as a function of p(T) for the five highest-p(T) additional jets, were measured in the jet pseudorapidity range broken vertical bar eta broken vertical bar < 4.5. The gap fraction, the fraction of events which do not contain an additional jet in a central rapidity region, was measured for several rapidity intervals as a function of the minimum p(T) of a single jet or the scalar sum of p(T) of all additional jets. These fractions were also measured in different intervals of the invariant mass of the e μb<(b)over bar> b system. All measurements were corrected for detector effects, and found to be mostly well-described by predictions from next-to-leading-order and leading-order t (t) over bar t event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.

Abstract: A selection of searches by the ATLAS experiment at the LHC for the electroweak production of SUSY particles are used to study their impact on the constraints on dark matter candidates. The searches use 20 fb(-1) of proton-proton collision data at root s = 8 TeV. A likelihood-driven scan of a five-dimensional effective model focusing on the gaugino-higgsino and Higgs sector of the phenomenological minimal supersymmetric Standard Model is performed. This scan uses data from direct dark matter detection experiments, the relic dark matter density and precision flavour physics results. Further constraints from the ATLAS Higgs mass measurement and SUSY searches at LEP are also applied. A subset of models selected from this scan are used to assess the impact of the selected ATLAS searches in this five-dimensional parameter space. These ATLAS searches substantially impact those models for which the mass m((chi) over tilde (0)(1)) of the lightest neutralino is less than 65 GeV, excluding 86% of such models. The searches have limited impact on models with larger m((chi) over tilde (0)(1)) due to either heavy electroweakinos or compressed mass spectra where the mass splittings between the produced particles and the lightest supersymmetric particle is small.