Feasibility of Intercalibration of CMS ECAL Supermodules with Cosmic Rays

The feasibility of using cosmic rays to make an intercalibration of the ECAL Supermodules before installation in CMS has been investigated. In a test with a single crystal a clear signal with a width of 15% rms was seen, with rates as expected. Simulations using a simplified detector geometry and a parameterisation of the vertical cosmic ray muon flux indicate that it is feasible to use the surrounding crystals as veto counters to ensure a longitudinal trajectory through the crystal, without introducing a large systematic error. The statistical error would be around 1% for one week of running. The best test of the reliability of such an intercalibration would be comparison with the results of a calibration with a high energy electron beam.