Electron reconstruction and identification efficiency measurements with the ATLAS detector using the 2011 LHC proton–proton collision data

The ATLAS Collaboration 0 0 CERN, 1211 Geneva 23, Switzerland Many of the interesting physics processes to be measured at the LHC have a signature involving one or more isolated electrons. The electron reconstruction and identification efficiencies of the ATLAS detector at the LHC have been evaluated using proton-proton collision data collected in 2011 at s = 7 TeV and corresponding to an integrated luminosity of 4.7 fb1. Tag-and-probe methods using events with leptonic decays of W and Z bosons and J / mesons are employed to benchmark these performance parameters. The combination of all measurements results in identification efficiencies determined with an accuracy at the few per mil level for electron transverse energy greater than 30 GeV. 1 Introduction The good performance of electron1 reconstruction and identification in the ATLAS experiment at the Large Hadron Collider (LHC) based at the CERN Laboratory has been an essential ingredient to its successful scientific programme. It has played a critical role in several analyses, as for instance in Standard Model measurements [14], the discovery of a Higgs boson [5], and the searches for new physics beyond the Standard Model [6]. Isolated electrons produced in many interesting physics processes can be subject to large backgrounds from misidentified hadrons, electrons from photon conversions, and non-isolated electrons originating from heavy-flavour decays. For this reason, it is important to efficiently reconstruct and identify electrons over the full acceptance of the detector, while at the same time to have a significant background rejection. In ATLAS, this is accomplished using a combination of powerful detector technologies: silicon detectors and a transition radiation tracker to identify 1 Throughout this paper, the term electron usually indicates both electrons and positrons. the track of the electron and a longitudinally layered electromagnetic calorimeter system with fine lateral segmentation to measure the electrons energy deposition, followed by hadronic calorimeters used to veto particles giving rise to significant hadronic activity. During the 2011 data-taking period at s = 7 TeV, the LHC steadily increased the instantaneous luminosity from 5 1032 cm2 s1 to 3.7 1033 cm2 s1, with an average superposition (pile-up) of approximately nine proton proton interactions per beam crossing. In contrast to the electron performance goals for the 2010 period [7], which focused on robustness for the first LHC running, the goals for the 2011 period aimed at substantially increasing the background rejection power in this much busier environment to keep the online output rate of events triggered by electron signatures within its allocated budget while at the same time preserving high reconstruction and identification efficiencies for electrons. During this period, ATLAS collected large samples of isolated electrons from W e, Z ee, and J / ee events, allowing precise measurements of the electron reconstruction and identification efficiencies over the range of transverse energies, ET, from 7 to 50 GeV. This paper reports on the methods used to perform these measurements, describes the improvements with respect to previous results [7], and benchmarks the performance of the 2011 electron reconstruction and identification used in various analyses performed with protonproton collisions. The structure of the paper is as follows. Section 2 provides a brief summary of the main components of the ATLAS detector. The electron trigger design, the algorithm for electron reconstruction and the electron identification criteria are described in Sect. 3. Section 4 focuses on the method used to compute the various efficiencies. The data and simulation samples used in this work are given in Sect. 5 together with the main triggers that enabled the event collection. Section 6 reports on the identification efficiency measurement, presenting the background evaluation and the results obtained with the tag-and-probe technique. A similar methodology, but using a subset of the samples available for the identification efficiency measurement, is used to extract the efficiency of the electron reconstruction described in Sect. 7. The study of the probability to mismeasure the charge of an electron is presented in Sect. 8. The summary of the work is given in Sect. 9. 2 The ATLAS detector The ATLAS detector is designed to observe particles produced in high-energy protonproton and heavy-ion collisions. It is composed of an inner tracking detector (ID) immersed in a 2 T axial magnetic field produced by a thin superconducting solenoid, electromagnetic (EM) and hadronic calorimeters outside the solenoid, and air-coretoroid muon spectrometers. A three-level triggering system reduces the total data-taking rate from a bunch-crossing frequency of approximately 20 MHz to several hundred Hz. A detailed description of the detector is provided elsewhere [8]. In the following, only an overview (...truncated)