XLIX International Winter Meeting on Nuclear Physics January 2011 Bormio, Italy G. Cattani, on behalf of the ATLAS Collaboration Measurement of W and Z production cross sections with the ATLAS experiment at √s = 7 TeV QCD Backgrounds Except for the Z →  final state, the QCD components of the backgrounds were estimated from the data. The electroweak and ttbar components were obtained for all channels from Monte-Carlo simulation. Z Background: Electrons – QCD background estimated relaxing lepton requirement from “medium” to “loose”. Invariant-mass distribution pairs was used as a template. Muons - QCD background was determined from Monte-Carlo simulation bbar production. W Background: Electrons - QCD background estimated using the distribution of the missing transverse energy ETmiss as measured in data. Events were selected by applying all cuts used in theW selection, except the ETmiss cut at 25 GeV. Signal and background components in this sample were obtained from a binned maximum likelihood template fit. Muons – QCD background was estimated from a comparison of the number of events seen in data after the full W selection, to the number of events observed if the muon isolation requirement is not applied. Estimate cross-checked: extrapolate (“ABCD” method) from control region in ETmiss and muon isolation. Compatible results. Selection of W → lν and Z → ll candidates Lepton Reconstruction and Identification Electrons - The electron identification selections are based on criteria using calorimeter and tracker information. The ATLAS standard electron reconstruction and identification algorithm is designed to provide various levels of background rejection for high identification efficiencies for calorimeter transverse energy E T > 20 GeV, over the full acceptance of the Inner Detector (ID) system: ● Loose: uses EM shower shape information from the second layer of the EM calorimeter and energy leakage into the hadronic calorimeters as discriminant variables ● Medium: evaluates the energy deposit patterns in the first layer of the EM calorimeter, requires cuts on track quality variables and applies a cluster-track matching. ● Tight: makes requirements on the ratio of cluster energy to track momentum, on the number of hits in the TRT and on the ratio of high-threshold hits to the total number of hits in the TRT. Electrons from conversions are rejected by requiring at least one hit in the first layer of the pixel detector. Muons - The ATLAS muon identification and reconstruction algorithms associate a stand-alone Muon Spectrometer (MS) track to an ID track. The combined track parameters are derived either from a statistical combination of the two tracks or from a refit of the full track (combined muon). The results presented here are based on the statistical combination of MS and ID measurement. Missing E T - Transverse missing energy reconstruction is based on calorimeter information. It relies on a cell-based algorithm which sums the electromagnetic-scale energy deposits of calorimeter cells inside three-dimensional topological clusters. In the muon channel It is calculated by adding the reconstructed momenta of isolated and non-isolated muons. mT of the lepton-ETMiss system after all requirements for the electron channel. Invariant mass of Z candidates Muons mT of the lepton-ETMiss system after all requirements for the electron channel. Electrons m T of the lepton- E T,Miss system Z event selection A pair of oppositely charged leptons of the same flavour Electrons: medium identification requirements, E T > 20 GeV |  | < 2.47 (transition region excluded) Muons: combined muon, p T > 20 GeV, p T,MS > 10 GeV, |  || < 2.4, |  p T(ID-MS) | < 15 GeV, relative tracking isolation (in  R cone of 0.4) < 0.2 Invariant mass: 66 < m ll < 116 GeV Measurement of the W → lν charge asymmetry ● The measurement of the charge asymmetry of the W-bosons produced at hadron colliders provides important information about parton distribution functions. ● The W-boson charge asymmetry is obtained from the charge of the decay leptons. Combined charge asymmetry in two eta bins (barrel |eta|<1.37 and endcap 1.52<|eta|<2.4) for an integrated luminosity of 315 nb-1 in the electron channel and 310 nb-1 in the muon channel and compared to different theoretical predictions. Statistical and systematic uncertainties are included. *No acceptance correction factors are needed. Cross-sections not affected by significant theoretical uncertainties * * * * Cross-section Measurement ● Acceptance (A W, A Z ): fraction of decays satisfying the geometrical and kinematical constraints at generator level (fiducial acceptance) ● Correction factors (C W,C Z ): ● central values computed using Monte-Carlo full simulation through ATLAS detector ● trigger efficiencies for muons from data Ratio of the W to Z cross-section ● Important test of Standard Model ● Measured with a higher relative precision ● Both experimental and theoretical uncertainties partially cancel ● Sensitive to new physics processes (stat) (syst) Introduction Abstract: First determinations of the W -> lnu and Z -> ll (l = e,mu) production cross sections for proton- proton collisions at sqrt(s) = 7 TeV were performed using about 320/nb of data recorded by the ATLAS experiment at the LHC. The results of these measurements for W and Z bosons for proton- proton collisions at sqrt(s) = 7 TeV are presented. In addition first measurements of the ratio between the W and Z/gamma*-cross sections and of the W -> lnu charge asymmetry are also discussed. Motiovation: LHC energy will allow for detailed studies of W and Z boson properties in a previously unexplored kinematic domain of low parton momentum fraction and high energy scale thus providing new constraints on the parton distribution functions and precise tests of perturbative QCD. W event selection Same as Z selection but requiring only one lepton Electrons: tight identification requirements Muons: same as Z selection Missing transverse energy: E T,Miss > 25 GeV Transverse Mass: m T > 40 GeV ATLAS Detector Collision candidate events: A primary vertex with at least three tracks and a position along beam axis to be within 15 cm of the nominal position Trigger: hardware-based L1 trigger, The L1 calorimeter trigger selects photon and electron candidates within |  | < 2.5 using calorimeter information in trigger towers of dimension D  xD  = 0.1x0.1. The L1 muon trigger searches for patterns of hits within |  | < 2.4 consistent with high-pT muons originating from the interaction region. The algorithm requiresa coincidence of hits in the different trigger stations along a road which follows the path of a muon from the interaction point through the detector. The width of the road is related to the pT threshold to be applied. p T of W → e p T of Z →  Invariant mass of Z candidates References The ATLAS Collaboration, Measurement of the W → lnu and Z/gamma* → ll production cross sections in proton-proton collisions at √s = 7 TeV with the ATLAS detector, arXiv: [hep-ex]. W → l Z → ll R W/Z ETmiss vs muon isolation Opposite charge loose-loose electron pair invariant mass