Search for the Standard Model Higgs Boson in the Diphoton Decay Channel with 4.9 fb^(-1) of pp Collision Data at √s = 7 TeV with ATLAS

A search for the standard model Higgs boson is performed in the diphoton decay channel. The data used correspond to an integrated luminosity of 4.9  fb^(-1) collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of √s = 7  TeV. In the diphoton mass range 110–150 GeV, the largest excess with respect to the background-only hypothesis is observed at 126.5 GeV, with a local significance of 2.8 standard deviations. Taking the look-elsewhere effect into account in the range 110–150 GeV, this significance becomes 1.5 standard deviations. The standard model Higgs boson is excluded at 95% confidence level in the mass ranges of 113–115 GeV and 134.5–136 GeV

Measurement of the W →τν_τ cross section in pp collisions at √s = 7 TeV with the ATLAS experiment

The cross section for the production of W bosons with subsequent decay W→τν_τ is measured with the ATLAS detector at the LHC. The analysis is based on a data sample that was recorded in 2010 at a proton–proton center-of-mass energy of √s = 7TeV and corresponds to an integrated luminosity of 34 pb^(−1). The cross section is measured in a region of high detector acceptance and then extrapolated to the full phase space. The product of the total W production cross section and the W→τν_τ branching ratio is measured to be σ^(tot) _(W→τντ) = 11.1±0.3 (stat)±1.7 (syst)±0.4 (lumi) nb

Measurement of the W^±Z production cross section and limits on anomalous triple gauge couplings in proton–proton collisions at √s = 7 TeV with the ATLAS detector

This Letter presents a measurement of W^± Z production in 1.02 fb^(−1) of pp collision data at √s = 7 TeV collected by the ATLAS experiment in 2011. Doubly leptonic decay events are selected with electrons, muons and missing transverse momentum in the final state. In total 71 candidates are observed, with a background expectation of 12.1 ± 1.4(stat.)^(+4.1)_(−2.0)(syst.) events. The total cross section for W^± Z production for Z/γ^∗ masses within the range 66 GeV to 116 GeV is determined to be σ^(tot)_(WZ) = 20.5^(+3.1)_(−2.8)(stat.)^(+1.4)_(−1.3)(syst.)^(+0.9)_(−0.8)(lumi.) pb, which is consistent with the Standard Model expectation of 17.3^(+1.3) _(0.8) pb. Limits on anomalous triple gauge boson couplings are extracted

Search for a Light Higgs Boson Decaying to Long-Lived Weakly Interacting Particles in Proton-Proton Collisions at √s = 7 TeV with the ATLAS Detector

A search for the decay of a light Higgs boson (120–140 GeV) to a pair of weakly interacting, long-lived particles in 1.94  fb^(-1) of proton-proton collisions at √s=7  TeV recorded in 2011 by the ATLAS detector is presented. The search strategy requires that both long-lived particles decay inside the muon spectrometer. No excess of events is observed above the expected background and limits on the Higgs boson production times branching ratio to weakly interacting, long-lived particles are derived as a function of the particle proper decay length

Search for contact interactions in dilepton events from pp collisions at √s = 7 TeV with the ATLAS detector

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/Λ^2, 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/Λ^4

Search for an excess of events with an identical flavour lepton pair and significant missing transverse momentum in √s =7 TeV proton–proton collisions with the ATLAS detector

Results are presented of a search for particles decaying into final states with significant missing transverse momentum and exactly two identical flavour leptons (e, μ) of opposite charge in √s = 7 TeV collisions at the Large Hadron Collider. This channel is particularly sensitive to supersymmetric particle cascade decays producing flavour correlated lepton pairs. Flavour uncorrelated backgrounds are subtracted using a sample of opposite flavour lepton pair events. Observation of an excess beyond Standard Model expectations following this subtraction procedure would offer one of the best routes to measuring the masses of supersymmetric particles. In a data sample corresponding to an integrated luminosity of 35 pb^(−1) no such excess is observed. Model-independent limits are set on the contribution to these final states from supersymmetry and are used to exclude regions of a phenomenological supersymmetric parameter space

Electron performance measurements with the ATLAS detector using the 2010 LHC proton-proton collision data

Detailed measurements of the electron performance of the ATLAS detector at the LHC are reported, using decays of the Z, W and J/ψ particles. Data collected in 2010 at s=7~TeV are used, corresponding to an integrated luminosity of almost 40 pb^(−1). The inter-alignment of the inner detector and the electromagnetic calorimeter, the determination of the electron energy scale and resolution, and the performance in terms of response uniformity and linearity are discussed. The electron identification, reconstruction and trigger efficiencies, as well as the charge misidentification probability, are also presented

Performance of the ATLAS detector using first collision data

More than half a million minimum-bias events of LHC collision data were collected by the ATLAS experiment in December 2009 at centre-of-mass energies of 0.9 TeV and 2.36 TeV. This paper reports on studies of the initial performance of the ATLAS detector from these data. Comparisons between data and Monte Carlo predictions are shown for distributions of several track- and calorimeter-based quantities. The good performance of the ATLAS detector in these first data gives confidence for successful running at higher energies