Reconstruction and Identification of Photons

This note presents the description and performance of photon identification methods in ATLAS. The reconstruction of an electromagnetic object begins in the calorimeter, and the inner detector information determines whether the object is a photon - either converted or unconverted - or an electron. Three photon identification methods are presented: a simple cut-based method, a Loglikelihood- ratio-based method and a covariance-matrix-based method. The shower shape variables based on calorimeter information and track information used in all three methods are described. The efficiencies for single photons and for photons from the benchmark H !gg signal events, as well as the rejection of the background from jet samples, are presented. The performance of the cut-based method on high-pT photons from a graviton decay process G!gg is also discussed.