Abstract:

A search for the production of single top-quarks via flavour-changing neutral-currents is presented. Data collected with the ATLAS detector at a centre-of-mass energy of √s=7 TeV, corresponding to an integrated luminosity of 2.05 fb-1, are used. Candidate events with a semileptonic top-quark decay signature are classified as signal- or background-like events by using several kinematic variables as input to a neural network. No signal is observed in the neural network output distribution and a Bayesian upper limit is placed on the production cross-section. The observed upper limit at 95% confidence level on the cross-section multiplied by the t→Wb branching fraction is measured to be σqg→t×B(t→Wb)<3.9 pb. This upper limit is converted using a model-independent approach into upper limits on the coupling strengths κugt/Λ<6.9{dot operator}10-3 TeV-1 and κcgt/Λ<1.6{dot operator}10-2 TeV-1, where Λ is the new physics scale, and on the branching fractions B(t→ug)<5.7{dot operator}10-5 and B(t→cg)<2.7{dot operator}10-4. © 2012 CERN.

Abstract:

A search is presented for a narrow resonance decaying to a pair of Z bosons using data corresponding to 1.02fb-1 of integrated luminosity collected by the ATLAS experiment from pp collisions at √s=7TeV. Events containing either four charged leptons (ℓℓℓℓ) or two charged leptons and two jets (ℓℓjj) are analyzed and found to be consistent with the Standard Model background expectation. Lower limits on a resonance mass are set using the Randall-Sundrum (RS1) graviton model as a benchmark. Using both ℓℓℓℓ and ℓℓjj events, an RS1 graviton with k/m-pl=0.1 and mass between 325 and 845 GeV is excluded at 95% confidence level. In addition, the ℓℓℓℓ events are used to set a model-independent fiducial cross section limit of σfid(pp→X→ZZ)<0.92pb at 95% confidence level for any new sources of ZZ production with mZZ greater than 300 GeV. © 2012 CERN.

Abstract:

A measurement is reported of the production cross section of top-quark pairs (tt̄) in proton-proton collisions at a center-of-mass energy of 7 TeV recorded with the ATLAS detector at the LHC. Candidate events have a signature consistent with containing two isolated leptons, large missing transverse momentum, and at least two jets. Using a data sample corresponding to an integrated luminosity of 0.70 fb-1, a tt̄ production cross section σtt̄ = 176 ± 5(stat.)-1111+(syst.) ± 8(lum.) pb is measured for an assumed top-quark mass of mt = 172.5 GeV. This measurement is in good agreement with Standard Model predictions. © CERN.

Abstract:

The top quark mass has been measured using the template method in the tt̄ → lepton+jets channel based on data recorded in 2011 with the ATLAS detector at the LHC. The data were taken at a proton-proton centre-of-mass energy of √s = 7 TeV and correspond to an integrated luminosity of 1. 04 fb-1. The analyses in the e+jets and μ+jets decay channels yield consistent results. The top quark mass is measured to be mtop=174. 5±0. 6stat±2. 3syst GeV. © 2012 CERN for the benefit of the ATLAS collaboration.

Abstract:

This Letter presents a search for contact interactions in the dielectron and dimuon channels using data from proton-proton collisions produced by the LHC at s=7 TeV and recorded by the ATLAS detector. The data sample, collected in 2011, corresponds to an integrated luminosity of 1.08 and 1.21 fb-1 in the e+e- and μ+μ- channels, respectively. No significant deviations from the standard model are observed. Using a Bayesian approach with a prior flat in 1/Λ2, the following 95% CL lower limits are placed on the energy scale of ℓℓqq contact interactions: Λ->10.1 TeV (Λ+>9.4 TeV) in the electron channel and Λ->8.0 TeV (Λ+>7.0 TeV) in the muon channel for constructive (destructive) interference in the left-left isoscalar contact interaction model. Limits are also provided for a prior flat in 1/Λ4. © 2012 CERN.