#20.3fb

A search for flavour changing neutral currents in top quark decays , with  at  GeV, using 20 fb of = 8 TeV and 5 fb of  = 7 TeV data collected by the ATLAS experiment at the LHC, is reported. The analysis is based on  events in which one top decays to , the other decays to , and both the hadronic and the leptonic decay modes of the  are used and combined. No significant signal is observed and an upper limit on the branching ratio of 0.83% (0.53% expected) at the 95% confidence level is set. The corresponding limit on the  coupling is 0.17 (0.14 expected).

A search by the ATLAS experiment at the Large Hadron Collider for a Higgs boson in the  channel in the range 260 GeV  1 TeV is presented. The analysis uses proton-proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.7 fb. A Higgs boson with Standard Model-like couplings is excluded at 95\% confidence level in the mass range 260 GeV  642 GeV.

The ATLAS detector is used to search for heavy neutral gauge bosons () decaying to an electron-positron pair or a muon-antimuon pair. Results are presented based on the analysis of  collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of approximately 20 . A  with Standard Model  couplings to fermions is excluded at 95\% C.L. for masses below 2.79 TeV in the electron channel, 2.48 TeV in the muon channel, and 2.86 TeV in the two channels combined. Limits on other model interpretations are also presented, including a Grand Unification model based on the  gauge group, a Randall-Sundrum graviton and a technicolor model with a composite Higgs boson.

A search for a dijet resonance with an invariant mass in the range between 130 and 300~GeV is performed in the processes  with  and  with  (). The data used correspond to 20.3 fb of  collision data recorded at  8~TeV with the ATLAS detector at the CERN Large Hadron Collider. The results are interpreted in terms of constraints on the Low Scale Technicolor model. No significant deviation from the Standard Model background prediction is observed. Upper limits on the production cross section times branching fraction are set for a hypothetical technipion produced in association with a  or  boson from the decay of a technirho particle.

A search is presented for microscopic black holes in a like-sign dimuon final state in proton--proton collisions at sqrt(s)= 8 TeV. The data were collected with the ATLAS detector at the Large Hadron Collider in 2012 and correspond to an integrated luminosity of 20.3 fb-1. Using a high track multiplicity requirement, 0.6 +- 0.2 background events from Standard Model processes are predicted and none observed. This result is interpreted in the context of low-scale gravity models and 95% CL lower limits on microscopic black hole masses are set for different model assumptions.

This Letter describes a model-independent search for the production of new resonances in photon + jet events using 20 inverse fb of proton--proton LHC data recorded with the ATLAS detector at a centre-of-mass energy of sqrt(s) = 8 TeV. The photon + jet mass distribution is compared to a background model fit from data; no significant deviation from the background-only hypothesis is found. Limits are set at 95% credibility level on generic Gaussian-shaped signals and two benchmark phenomena beyond the Standard Model: non-thermal quantum black holes and excited quarks. Non-thermal quantum black holes are excluded below masses of 4.6 TeV and excited quarks are excluded below masses of 3.5 TeV.

This Letter presents a search for quantum black-hole production using 20.3 inverse fb of data collected with the ATLAS detector in pp collisions at the LHC at sqrt(s) = 8 TeV. The quantum black holes are assumed to decay into a lepton (electron or muon) and a jet. In either channel, no event with a lepton-jet invariant mass of 3.5 TeV or more is observed, consistent with the expected background. Limits are set on the product of cross sections and branching fractions for the lepton+jet final states of quantum black holes produced in a search region for invariant masses above 1 TeV. The combined 95% confidence level upper limit on this product for quantum black holes with threshold mass above 3.5 TeV is 0.18 fb. This limit constrains the threshold quantum black-hole mass to be above 5.3 TeV in the model considered.