The Calibration Stategy of CMS Electromagnetic Calorimeter

Calibration is one of the main factors that set limits on the ultimate performance of the CMS electromagnetic calorimeter (ECAL) at LHC. Crystals raw intercalibration from laboratory measurements during assembly will be available for all the crystals and has been demonstrated to be a good precalibration value at the start-up; some crystals will be also intercalibrated using an electron beam. In situ calibration with physics events will be the main tool to reduce the constant term of the emergy resolution to the design goal of 0.5%. In the following the calibration strategy will be described in detail.