Nuclear counter effect and π-e misidentification

The e±/π± discrimination within the CMS(1) ECAL is investigated using GEANT simulations and the 1998 test beam results. If one takes into account the energy left in the ECAL crystals alone (i.e. without read-out effects), the probability that a π± leaves more than 95% of its initial energy decreases from about 0.01% for 10 GeV to about 0.001% for 50 GeV. The Nuclear Counter Effect within the Avalanche Photo-Diodes (APD) enhances the probability of an electron misidentification. With the expected value of this effect (≈100 MeV), this probability appears then to be between 0.2% and 0.01% for initial momenta varying, respectively, between 5 and 50 GeV. Important consequences of the pion-electron misidentification could appear in the form of new possible backgrounds for physics channels.