Photon Reconstruction and Identification with the ATLAS Detector

The understanding of the reconstruction of photons will be one of the key issues at the start-up of data-taking with the ATLAS experiment at the LHC in 2009. Large statistics of photons produced in association with jets are expected over a wide range of ET , from 20 GeV to several hundred GeV. These will be used as an important in situ calibration tool for the jet energy scale. The energy measurement of unconverted photons is based on the electromagnetic calorimetry over the full 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). Unconverted photons require a specific calibration depending on the conversion radius to reach the optimal linearity and resolution.