Electron Reconstruction and Identification with the ATLAS Detector: Slides for SUSY09 conference (05-10/06/2009 in Northeastern University)

The understanding of the reconstruction of electrons will be one of the key issues at the start-up of data-taking with the ATLAS experiment at the LHC in 2009. The first signals from prompt electrons are expected in direct J/psi and Upsilon production and in semi-leptonic decays of heavy flavour. They will be accompanied of course by W to enu and Z to ee decays, which will be used for a detailed and complete understanding of the performance of the trigger and offline algorithms once the accumulated statistics will be adequate. The energy measurement of electrons is based on the electromagnetic calorimetry over most of the relevant energy range (10 GeV to a few TeV). The electromagnetic calorimeter cluster algorithm starting from electronically calibrated calorimeter cells will be described. Local position and energy variations are corrected for. A refined calibration procedure, developed and validated over years of test-beam data-taking and analysis, strives to identify all sources of energy losses upstream of the calorimeter and outside the cluster and corrects for them one by one (using Monte-Carlo). The electron reconstruction relies on two algorithms, one which is calorimeter-seeded and the second one which is track-seeded. The former is optimised mostly for high-pT isolated electrons whereas the latter is optimised more for low-pT electrons, including those which are not isolated. The electron identification is based on th e shower shapes in the calorimeter and relies on the tracker and combined tracker/calo information to achieve the required rejection of 10$^5$ against QCD jets for a reasonably clean inclusive electron spectrum in the moderate pT region of 10 to 50 GeV. The required rejection factor is closer to a thousand per jet to cleanly extract the signal expected from di-electron resonances in the TeV mass range. The electron identification methods and their performance will be discussed. Finally tag-and-probe methods, using Z to ee decays, have been developed in order to determine the efficiency of the electron trigger, reconstruction and identification from data alone. Good agreement was obtained in comparing the results of these methods with the estimations from Monte-Carlo truth.