Calibration of the electromagnetic calorimeter of the CMS experiment

The calibration of the relative response of the individual channels, or intercalibration, and of the energy scale for electrons and photons are severe technical challenges for the operation of the electromagnetic calorimeter of the CMS experiment. The channel response uniformity within ECAL and stability in time will directly contribute to the overall energy resolution. Intercalibrations performed during the commissioning phase of the detector provide an acceptable performance at startup and a handle for validation of in situ calibration procedures based on the use of events collected during LHC operation. A laser monitoring system will be used to track response variations with time, as in the case of changes in crystal transparency caused by irradiation. Triggers, selections and reconstruction procedures of in situ calibration signals and their projected performance are reviewed. The interplay with reconstruction algorithms of electrons and photons is also discussed. Calibration procedures offer sufficient redundancy and flexibility to fast improve on startup conditions, intercalibrate the response of the individual channels towards a target precision of 0.5% and calibrate the energy scale of electrons and photons.