Photon/jet separation with DC1 data

Results on the photon identification and QCD-jet rejection capabilities of the {\sc Atlas} detector are presented. The simulation, reconstruction and analysis are done in the framework of Data Challenge 1. This study is based on fully simulated events at the initial low ($2\times 10^{33} {\rm cm^{-2}\ s^{-1}}$) and design high ($10^{34} {\rm cm^{-2}\ s^{-1}}$) luminosity, on which pile-up is superimposed. A jet rejection factor of about 5000-6000 for $p_T>25$ GeV can be obtained for a photon identification efficiency $\sim 80\%$. The application of this rejection to the fake background in the $H\rightarrow\gamma\gamma$ analysis is discussed.