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