Pre-calibration of the CMS electromagnetic calorimeter with cosmic rays

The electromagnetic calorimeter of the CMS experiment at the new CERN proton Collider (LHC), consisting in about 75000 scintillating PWO crystals, is in advanced construction phase. One necessary condition for optimal performance is an adequate inter-calibration of all channels. The possibility of using cosmic rays to pre-calibrate the ECAL calorimeter was investigated using real data collected with a full ECAL module exposed both to cosmic rays and electron test beams and simulated data based on the GEANT4 toolkit. The data collected are well described by the Monte Carlo simulation and confirm that an inter-calibration of the calorimeter channels after assembly in super-modules can be performed at a precision better than 3% following a procedure that is compatible with the CMS construction schedule. While the ultimate calibration of the detector is expected to rely on physics data, this procedure will definitely guarantee a reasonable performance already at the startup.