Abstract: A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb(-1) of proton-proton collision data collected at a centre-of-mass energy of root s = 8TeV. The lepton-plus-jets final state is used, where the top pair decays to W (+) bW(-)(b) over bar, with one W boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow Z' boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour Z' boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with Gamma/m = 15% decaying to tt. These range from 4.8 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.

Abstract: An analysis is presented of events containing jets including at least one b tagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb(-1) of p p collisions at p root s = 8TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events; the ones considered here are production of vector-like quarks, enhanced fourtop- quark production, pair production of chiral b'-quarks, and production of two positively charged top quarks. Eleven signal regions are defined; subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral b'-quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models.

Abstract: The prompt and non-prompt production cross-sections for the chi(c1) and chi(c2) charmonium states are measured in pp collisions at root s = 7TeV with the ATLAS detector at the LHC using 4.5 fb(-1) of integrated luminosity. The chi(c) states are reconstructed through the radiative decay chi c -> J/psi gamma ( with J/psi -> mu(+)mu(-)) where photons are reconstructed from gamma -> e(+)e(-) conversions. The production rate of the chi(c2) state relative to the chi(c1) state is measured for prompt and non-prompt chi(c) as a function of J/psi transverse momentum. The prompt chi(c) cross-sections are combined with existing measurements of prompt J/psi production to derive the fraction of prompt J/psi produced in feed-down from chi(c) decays. The fractions of chi(c1) and chi(c2) produced in b-hadron decays are also measured.