Position resolution and particle identification with the ATLAS EM calorimeter

In the years between 2000 and 2002 several pre-series and series modules of the ATLAS EM barrel and end-cap calorimeter were exposed to electron, photon and pion beams. The performance of the calorimeter with respect to its finely segmented first sampling has been studied. The polar angle resolution has been found to be in the range 50–60 $(mrad)/\sqrt{E(GeV)}$. The $\pi^0$ rejection has been measured to be about 3.5 for 90\% photon selection efficiency at $\rho_T$=50GeV. $e–\pi$ separation studies have indicated that a pion fake rate of (0.07–0.5)\% can be achieved while maintaining 90\% electron identification efficiency for energies up to 40 GeV.