CMS ECAL intercalibration of ECAL crystals using laboratory measurements

The CMS electromagnetic calorimeter is presently under construction at CERN. Inter-calibration of the channels is a crucial issue for physics performance. The main source of channel-to-channel response variation in the barrel is the crystal-to-crystal variation of scintillation light yield which has an RMS of 8%. An improved method of using laboratory measurements performed during the ECAL assembly phase to provide an initial intercalibration is presented. The intercalibration constants derived from these measurements are compared to reference coefficients measured using high energy electrons in a test-beam. It is found that the intercalibration coefficients obtained from the laboratory measurements have an RMS precision of about 4%.