© 2011 CERN. A measurement is presented of the cross section for the production of a W boson with one or two jets, of which at least one must be a b-jet, in pp collisions at √s = 7 TeV. Production via top decay is not included in the signal definition. The measurement is based on 35 pb-1 of data collected with the ATLAS detector at the LHC. The W + b-jet cross section is defined for jets reconstructed with the antikt clustering algorithm with transverse momentum above 25 GeV and rapidity within ±2.1. The b-jets are identified by reconstructing secondary vertices. The fiducial cross section is measured both for the electron and muon decay channel of the W boson and is found to be 10.2 ± 1.9(stat) ± 2.6(syst) pb for one lepton flavour. The results are compared with next-to-leading order QCD calculations, which predict a cross section smaller than, though consistent with, the measured value.

The inclusive J/Ψ production cross-section and fraction of J/Ψ mesons produced in B-hadron decays are measured in proton-proton collisions at √s=7 TeV with the ATLAS detector at the LHC, as a function of the transverse momentum and rapidity of the J/Ψ, using 2.3 pb-1 of integrated luminosity. The cross-section is measured from a minimum pT of 1 GeV to a maximum of 70 GeV and for rapidities within |y|<2.4 giving the widest reach of any measurement of J/Ψ production to date. The differential production cross-sections of prompt and non-prompt J/Ψ are separately determined and are compared to Colour Singlet NNLO*, Colour Evaporation Model, and FONLL predictions. © 2011 CERN.

A measurement 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 Large Hadron Collider is reported. Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. Using a data sample corresponding to an integrated luminosity of 35pb-1, a tt̄ production cross section σtt̄=177±20(stat.)±14(syst.)±7(lum.)pb is measured for an assumed top quark mass of mt=172.5GeV. A second measurement requiring at least one jet identified as coming from a b quark yields a comparable result, demonstrating that the dilepton final states are consistent with being accompanied by b-quark jets. These measurements are in good agreement with Standard Model predictions. © 2011 CERN.

© 2012 CERN A measurement of the cross section of top quark pair production in proton–proton collisions recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb−1. Events with an isolated electron or muon and a τ lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, σtt¯=186±13(stat)±20(syst.)±7(lumi.) pb, is in good agreement with the Standard Model prediction.

Distributions sensitive to the underlying event are studied in events containing one or more charged-particle jets produced in pp collisions at √s=7TeV with the ATLAS detector at the Large Hadron Collider (LHC). These measurements reflect 800μb-1 of data taken during 2010. Jets are reconstructed using the anti-kt algorithm with radius parameter R varying between 0.2 and 1.0. Distributions of the charged-particle multiplicity, the scalar sum of the transverse momentum of charged particles, and the average charged-particle pT are measured as functions of pTjet in regions transverse to and opposite the leading jet for 4GeV

This paper describes searches for the pair production of first or second generation scalar leptoquarks using 35pb-1 of proton-proton collision data recorded by the ATLAS detector at √s=7TeV. Leptoquarks are searched in events with two oppositely-charged muons or electrons and at least two jets, and in events with one muon or electron, missing transverse momentum and at least two jets. After event selection, the observed yields are consistent with the predicted backgrounds. Leptoquark production is excluded at the 95 CL for masses MLQ<376 (319) GeV and MLQ<422 (362) GeV for first and second generation scalar leptoquarks, respectively, when assuming the branching fraction of a leptoquark to a charged lepton is equal to 1.0 (0.5). © 2011 American Physical Society.

This Letter presents a search for high mass e+e- or μ+μ- resonances in pp collisions at √s=7 TeV at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of ~40 pb-1. No statistically significant excess above the Standard Model expectation is observed in the search region of dilepton invariant mass above 110 GeV. Upper limits at the 95% confidence level are set on the cross section times branching ratio of Z' resonances decaying to dielectrons and dimuons as a function of the resonance mass. A lower mass limit of 1.048 TeV on the Sequential Standard Model Z' boson is derived, as well as mass limits on Z* and E6-motivated Z' models. © 2011 CERN.

The measurement of missing transverse momentum in the ATLAS detector, described in this paper, makes use of the full event reconstruction and a calibration based on reconstructed physics objects. The performance of the missing transverse momentum reconstruction is evaluated using data collected in pp collisions at a centre-of-mass energy of 7 TeV in 2010. Minimum bias events and events with jets of hadrons are used from data samples corresponding to an integrated luminosity of about 0.3 nb1 and 600 nb1 respectively, together with events containing a Z boson decaying to two leptons (electrons or muons) or aW boson decaying to a lepton (electron or muon) and a neutrino, from a data sample corresponding to an integrated luminosity of about 36 pb1. An estimate of the systematic uncertainty on the missing transverse momentum scale is presented. © CERN for the benefit of the ATLAS collaboration 2011.

The ATLAS Inner Detector is a composite tracking system consisting of silicon pixels, silicon strips and straw tubes in a 2 T magnetic field. Its installation was completed in August 2008 and the detector took part in data-taking with single LHC beams and cosmic rays. The initial detector operation, hardware commissioning and in-situ calibrations are described. Tracking performance has been measured with 7. 6 million cosmic-ray events, collected using a tracking trigger and reconstructed with modular pattern-recognition and fitting software. The intrinsic hit efficiency and tracking trigger efficiencies are close to 100%. Lorentz angle measurements for both electrons and holes, specific energy-loss calibration and transition radiation turn-on measurements have been performed. Different alignment techniques have been used to reconstruct the detector geometry. After the initial alignment, a transverse impact parameter resolution of 22. 1±0. 9 μm and a relative momentum resolution σp/p=(4. 83±0. 16)×10-4 GeV-1×pT have been measured for high momentum tracks. © 2010 CERN for the benefit of the ATLAS collaboration.

The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions over |η|<2 in lead-lead collisions at a nucleon-nucleon centre-of-mass energy of √s NN=2.76 TeV. In order to include particles with transverse momentum as low as 30 MeV, the data were recorded with the central solenoid magnet off. Charged particles were reconstructed with two algorithms (2-point "tracklets" and full tracks) using information from the pixel detector only. The lead-lead collision centrality was characterized by the total transverse energy in the forward calorimeter in the range 3.2<|η|<4.9. Measurements are presented of the per-event charged particle pseudorapidity distribution, dN ch/dη, and the average charged particle multiplicity in the pseudorapidity interval |η|<0.5 in several intervals of collision centrality. The results are compared to previous mid-rapidity measurements at the LHC and RHIC. The variation of the mid-rapidity charged particle yield per colliding nucleon pair with the number of participants is consistent with lower sNN results. The shape of the dN ch/dη distribution is found to be independent of centrality within the systematic uncertainties of the measurement. © 2012 CERN.