Calibration of the CMS Electromagnetic Calorimeter

Calibration is one of the factors that set limits on the ultimate performance of the CMS electromagnetic calorimeter (ECAL) at LHC. In situ calibration with physics events will be the main tool to reduce the constant term in the energy dependent resolution function to the design goal of 0.5 %. At the start of detector operation, a fast intercalibration method, based on phi symmetry in minimum bias events, will be used, exploiting the uniformity of energy deposition as a function of phi at a given eta. Energetic electrons from Z -> e^+e^- decays will be recorded at a sufficiently high rate, even at the initial low luminosity, to provide an intercalibration of calorimeter regions and to set the absolute energy scale. Once the Tracker has been commissioned, intercalibration of different crystals within a single module may be achieved using E/p measurement of isolated electrons. This cal ibration strategy will be described in detail and illustrated with simulated results.