Measurement of the inclusive isolated prompt photon cross section in pp collisions at √s=7 TeV with the ATLAS detector

A measurement of the cross section for the inclusive production of isolated prompt photons in pp collisions at a center-of-mass energy √s=7  TeV is presented. The measurement covers the pseudorapidity ranges |ηγ|<1.37 and 1.52≤|ηγ|<1.81 in the transverse energy range 15≤ETγ<100  GeV. The results are based on an integrated luminosity of 880  nb-1, collected with the ATLAS detector at the Large Hadron Collider. Photon candidates are identified by combining information from the calorimeters and from the inner tracker. Residual background in the selected sample is estimated from data based on the observed distribution of the transverse isolation energy in a narrow cone around the photon candidate. The results are compared to predictions from next-to-leading-order perturbative QCD calculations

Search for the Higgs boson in the H->WW->lνjj decay channel in pp collisions at √s=7 TeV with the ATLAS detector

A search for a Higgs boson has been performed in the H→WW→ℓνjj channel in 1.04  fb-1 of pp collision data at √s=7  TeV recorded with the ATLAS detector at the Large Hadron Collider. No significant excess of events is observed over the expected background and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range 240  GeV<mH<600  GeV. The best sensitivity is reached for mH=400  GeV, where the 95% confidence level upper bound on the cross section for H→WW production is 3.1 pb, or 2.7 times the standard model prediction

Measurement of D*± meson production in jets from pp collisions at √s=7 TeV with the ATLAS detector

This paper reports a measurement of D*± meson production in jets from proton-proton collisions at a center-of-mass energy of √s=7  TeV at the CERN Large Hadron Collider. The measurement is based on a data sample recorded with the ATLAS detector with an integrated luminosity of 0.30  pb-1 for jets with transverse momentum between 25 and 70 GeV in the pseudorapidity range |η|<2.5. D*± mesons found in jets are fully reconstructed in the decay chain: D*+→D0π+, D0→K-π+, and its charge conjugate. The production rate is found to be N(D*±)/N(jet)=0.025±0.001(stat.)±0.004(syst.) for D*± mesons that carry a fraction z of the jet momentum in the range 0.3<z<1. Monte Carlo predictions fail to describe the data at small values of z, and this is most marked at low jet transverse momentum

Measurement of τ polarization in W→τν decays with the ATLAS detector in pp collisions at √s=7 TeV

In this paper, a measurement of τ polarization in W→τν decays is presented. It is measured from the energies of the decay products in hadronic τ decays with a single final state charged particle. The data, corresponding to an integrated luminosity of 24 pb−1, were collected by the ATLAS experiment at the Large Hadron Collider in 2010. The measured value of the τ polarization is Pτ=−1.06±0.04 (stat)+0.05−0.07 (syst), in agreement with the Standard Model prediction, and is consistent with a physically allowed 95 % CL interval [−1,−0.91]. Measurements of τ polarization have not previously been made at hadron colliders

Search for supersymmetry in final states with jets, missing transverse momentum and one isolated lepton in √s=7 TeV pp collisions using 1 fb-1 of ATLAS data

We present an update of a search for supersymmetry in final states containing jets, missing transverse momentum, and one isolated electron or muon, using 1.04  fb-1 of proton-proton collision data at √s=7  TeV recorded by the ATLAS experiment at the LHC in the first half of 2011. The analysis is carried out in four distinct signal regions with either three or four jets and variations on the (missing) transverse momentum cuts, resulting in optimized limits for various supersymmetry models. No excess above the standard model background expectation is observed. Limits are set on the visible cross section of new physics within the kinematic requirements of the search. The results are interpreted as limits on the parameters of the minimal supergravity framework, limits on cross sections of simplified models with specific squark and gluino decay modes, and limits on parameters of a model with bilinear R-parity violation

Measurement of the production cross section for Z/γ* in association with jets in pp collisions at √s=7 TeV with the ATLAS detector

Results are presented on the production of jets of particles in association with a Z/γ* boson, in proton-proton collisions at √s=7  TeV with the ATLAS detector. The analysis includes the full 2010 data set, collected with a low rate of multiple proton-proton collisions in the accelerator, corresponding to an integrated luminosity of 36  pb-1. Inclusive jet cross sections in Z/γ* events, with Z/γ* decaying into electron or muon pairs, are measured for jets with transverse momentum pT>30  GeV and jet rapidity |y|<4.4. The measurements are compared to next-to-leading-order perturbative QCD calculations, and to predictions from different Monte Carlo generators implementing leading-order matrix elements supplemented by parton showers

Evidence for the associated production of a W boson and a top quark in ATLAS at √s = 7 TeV

This Letter presents evidence for the associated production of a W boson and a top quark using 2.05 fb −1 of pp collision data at √ s = 7 TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. An approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions is performed to determine the crosssection. The result is incompatible with the background-only hypothesis at the 3.3σ level, the expected sensitivity assuming the Standard Model production rate being 3.4σ. The corresponding cross-section is determined and found to be σWt = 16.8 ±2.9 (stat)±4.9 (syst) pb, in good agreement with the Standard Model expectation. From this result the CKM matrix element |Vtb| = 1.03 +0.16 −0.19 is derived assuming that the Wt production through |Vts | and |Vtd| is smal

Search for new phenomena in events with three charged leptons at √s=7 TeV with the ATLAS detector

A generic search for anomalous production of events with at least three charged leptons is presented. The search uses a pp-collision data sample at a center-of-mass energy of √s=7  TeV corresponding to 4.6  fb-1 of integrated luminosity collected in 2011 by the ATLAS detector at the CERN Large Hadron Collider. Events are required to contain at least two electrons or muons, while the third lepton may either be an additional electron or muon, or a hadronically decaying tau lepton. Events are categorized by the presence or absence of a reconstructed tau-lepton or Z-boson candidate decaying to leptons. No significant excess above backgrounds expected from Standard Model processes is observed. Results are presented as upper limits on event yields from non-Standard-Model processes producing at least three prompt, isolated leptons, given as functions of lower bounds on several kinematic variables. Fiducial efficiencies for model testing are also provided. The use of the results is illustrated by setting upper limits on the production of doubly charged Higgs bosons decaying to same-sign lepton pairs

Search for R-parity-violating supersymmetry in events with four or more leptons in √s=7 TeV pp collisions with the ATLAS detector

A search for new phenomena in final states with four or more leptons (electrons or muons) is presented. The analysis is based on 4.7 fb(-1) of √s = 7TeV proton-proton collisions delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in two signal regions: one that requires moderate values of missing transverse momentum and another that requires large effective mass. The results are interpreted in a simplified model of R-parity-violating supersymmetry in which a 95% CL exclusion region is set for charged wino masses up to 540GeV. In an R-parity-violating MSUGRA/CMSSM model, values of m(1/2) up to 820GeV are excluded for 10 < tan β < 40

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

A search for nonresonant new phenomena, originating from either contact interactions or large extra spatial dimensions, has been carried out using events with two isolated electrons or muons. These events, produced at the LHC in proton-proton collisions at √s=7  TeV, were recorded by the ATLAS detector. The data sample, collected throughout 2011, corresponds to an integrated luminosity of 4.9 and 5.0  fb-1 in the e+e- and μ+μ- channels, respectively. No significant deviations from the Standard Model expectation are observed. Using a Bayesian approach, 95% confidence level lower limits ranging from 9.0 to 13.9 TeV are placed on the energy scale of ℓℓqq contact interactions in the left-left isoscalar model. Lower limits ranging from 2.4 to 3.9 TeV are also set on the string scale in large extra dimension models. After combining these limits with results from a similar search in the diphoton channel, slightly more stringent limits are obtained