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First Results from ATLAS Experiment on Production of W and Z Bosons In 1 First results from ATLAS experiment on production of W 2 Z s = 7 and bosons in proton-proton collisions at √ TeV ∗ † 3 PawelMalecki 4 Henryk Niewodniczanski Inst. of Nucl. Physics, PAN, Cracow, Poland 5 First results for the W and Z boson production cross-sections at √s = 6 7 TeV obtained with the ATLAS detector are presented. The measure- 7 ments are performed in electron and muon channels, with a data sample of −1 8 around 320 nb collected between March and July 2010. Methods of signal 9 extraction and background estimation are briefly presented. The measured 10 values of W production cross-sections in electron and muon channels are 11 σW BR(W eν)=10.51 1.45 nb and σW BR(W µν)=9.58 × → ± × → ± 12 1.20 nb. The Z cross-section measurements yield σZ BR(Z ee) = × → 13 0.75 0.14 nb and σZ BR(Z µµ)=0.87 0.14 nb. The obtained 14 results± are in good agreement× with→ theoretical predictions.± A preliminary 15 measurement of the W boson charge asymmetry is also presented. 16 1. Introduction 17 A very important point in the ATLAS program of first physics measure- 18 ments is the observation of W and Z bosons and their properties. The- 19 oretical predictions for W/Z production cross-sections are nowadays very 20 precise [1], so a fine measurement is needed to test these predictions, which 21 are sensitive to higher-order (NNLO) QCD corrections as well as Parton Dis- 22 tribution Functions (P DF ) of the proton [2]. New constraints on the latter 23 are expected to be obtained with LHC measurements of W and Z bosons. 24 In addition, these bosons as first sources of high-transverse-momentum iso- 25 lated leptons are important for detector and algorithm commissioning. 26 The existence of W and Z bosons has been first proposed by Glashow, 27 Weinberg and Salam in the 1960’s [3]. After their discovery by the UA1 28 and UA2 experiments at the Super Proton Synchrotron (SPS) at CERN [4] 29 in proton-antiproton collisions, their properties have been thoroughly mea- 30 sured at LEP, SLC, RHIC and Tevatron, at different center-of-mass energies ∗ Presented at the Cracow Epiphany Conference on the first year of the LHC, Cracow, Poland, January 10-12, 2011 † On behalf of the ATLAS Collaboration (1) 2 proceedings printed on March 31, 2011 31 varying from √s = 500 GeV up to √s = 1.96 TeV. Further confirmation 32 of previous results, improved accuracy and extension of measurements into 33 previously unexplored kinematic domains are the current goals for LHC 34 experiments. 35 The studies presented here are based on electron and muon final states, 36 either in a lepton-neutrino pair or in two oppositely charged leptons. This 37 ensures a clean experimental signature based on high-pT isolated charged 38 leptons, which can be easily identified and separated from the background 39 dominated by QCD processes. The following analyses have been published 40 by the ATLAS Collaboration in [5]. 41 2. The ATLAS detector 42 The ATLAS detector [6] is the largest of the four detectors at the Large 43 Hadron Collider (LHC) and one of two so-called general-purpose detectors, 44 designed for most precision measurements, including Standard Model (SM) 45 tests, Higgs boson searches, SUSY searches, but also aspects of B-physics 46 and Heavy Ion collisions. It provides almost full solid-angle coverage around 47 the beam crossing point and allows for detailed and precise studies of colli- 48 sion events. 49 In the ATLAS coordinate system the z-axis is parallel to the beam-line 50 and points towards the anti-clockwise direction of the beam in LHC ring. 51 The x-axis points towards the ring center, setting the φ = 0 value of the 52 azimuthal angle and the y-axis points upwards. The transverse quantities 53 Miss (like transverse momentum pT or missing transverse energy ET ) are 54 defined in the x y plane. − 55 The detector is built of concentric layers around the beam-line. The 56 innermost subsystem is responsible for tracking of charged particles. It is 57 immersed in a 2T axial magnetic field parallel to the beam-line. Further 58 layers consist of calorimeters and muon chambers. The muon system has 59 its own toroidal magnetic field surrounding the detector. The Inner De- 60 tector (ID) provides coverage over full azimuthal angle and pseudorapidity 61 within η < 2.47. It consists of three subsystems, the two innermost be- | | 62 ing silicon detectors, namely the Pixel and Semiconductor Tracker (SCT), 63 and the outer being the gaseous Transition Radiation Tracker (TRT). The 64 calorimeters surround the ID and solenoid magnet and cover up to η < 4.9. | | 65 Liquid Argon (LAr) and scintillating tiles are used as active media, with 66 lead and steel being the respective absorber material. The calorimeters are 67 segmented, both longitudinally and transversely, to allow for shower-shape 68 studies, useful for particle identification. The electromagnetic (EM) part of 69 the ATLAS calorimeter system has a depth of at least 22 radiation-lengths, 70 whereas the hadronic part is approximately 10 interaction lengths deep. The proceedings printed on March 31, 2011 3 71 Muon Spectrometer (MS) provides the possibility to measure muon tracks 72 within η < 2.7. Tracking capabilities in the MS are assured by drift tube | | 73 detectors and cathode-strip chambers. 74 3. Data samples and Monte Carlo simulations 75 The presented analysis is based on the data sample collected by ATLAS 76 between March and July 2010. It corresponds to an integrated luminos- −1 77 ity of approximately 310 - 331 nb (depending on the channel). For the 78 electron channel, the sample was selected by a Level-1 calorimetric trigger, 79 requiring an electromagnetic cluster with transverse energy ET > 10 GeV. 80 Muon events were selected by comparing the signal pattern from the Muon 81 Spectrometer with reference tables based on muon tracks, starting at an 82 effective pT threshold of 6 GeV. 83 Monte Carlo samples were generated by PYTHIA [7] using MRSTLO* 84 [2] Parton Distribution Functions. Detector simulation [8] is performed by 85 the GEANT4 [9] package. 86 4. Object reconstruction 87 4.1. Electrons 88 The reconstruction of electrons in ATLAS is based on electromagnetic 89 clusters in the calorimeter. Such a cluster is matched to a track recon- 90 structed in the Inner Detector. The matched candidate object subsequently 91 undergoes the identification procedure, based on cuts applied on electromag- 92 netic shower shapes (both longitudinal and transverse), track quality and 93 the actual distance (in η φ) between track and cluster. Three sets of selec- − 94 tion criteria are used as a reference, namely the Loose, Medium and Tight 95 cuts. They provide strongly increasing rejection power against the back- 96 ground, with a small decrease of signal efficiency. Loose criteria are based 97 on the energy deposits in the second layer of electromagnetic calorimeter, 98 Medium ones add more restrictive criteria on the shower shape in the first 99 layer and on track-quality variables. The Tight requirements, in addition 100 to even tighter track selection and shower-shape restrictions reject electrons 101 from photon conversions. 102 4.2. Muons 103 In this analysis the combined muon objects are considered. They utilize 104 the information from both the Muon Spectrometer and Inner Detector. The 105 association between the tracks from these two subsystems is performed using 2 106 a χ test comparing the differences between track parameters weighted by 107 their covariance matrices. 4 proceedings printed on March 31, 2011 108 4.3. Missing transverse energy 109 To account for a neutrino that appears in W ℓν and is not re- 110 Miss → constructed, a missing transverse energy (ET ) is calculated, based on 111 calorimeter information. A vector sum of energy deposits in cells that 112 belong to reconstructed clusters is calculated in both x and y directions, 113 Miss and then combined into ET . This is an important tool to discriminate 114 against QCD background in W ℓν signal extraction procedures. In these → 115 studies, the missing transverse energy is reconstructed assuming all energy 116 is deposited by electromagnetic processes, and then corrected for different 117 calorimeter response for hadrons. In the analysis of the muon channel, the 118 missing transverse energy is corrected for the muon momentum. 119 5. Event selection 120 5.1. Preselection of high-pT leptons 121 Data samples containing high transverse momentum leptons are first 122 selected from the datasets available after trigger filtering. The lepton can- 123 didate is required to be contained within detector acceptance. In the case 124 of electrons the region η < 2.47 is considered (being the coverage of Inner | | 125 Detector) with the exception of 1.37 < η < 1.52 barrel - end-cap transition | | 126 regions in the Calorimeter. The acceptance region for muons is defined as 127 η < 2.4. In addition, the electron cluster transverse energy is required | | 128 to be ET > 20 GeV, and the muon transverse momentum needs to satisfy 129 pT > 20 GeV. 130 The selected samples are further refined by adding requirements on lep- 131 tons. The electrons selected for W eν analysis need to pass the tight → 132 identification criteria, whereas medium cuts are applied in Z ee selec- → 133 tion. In Z µµ and W µν selections, lepton isolation criteria are → → 134 exploited. The sum of transverse momenta of tracks surrounding the candi- 135 date in the cone of ∆R< 0.4 must not exceed 20% of the muon candidate 136 transverse momentum.
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