Calibration of the CMS Electromagnetic Calorimeter by Cosmic Ray Muons

The electromagnetic calorimeter of the CMS experiment consists in 75568 scintillating PbWO4 crystals. In order to fully exploit the potential of the calorimeter, an accurate inter-calibration of all the channels is necessary. While the final calibration at the 0.5% level will be obtained in situ with physics events, still a good equalization of the channels at the start-up is advisable. The exposure of the detector to cosmic rays allows the inter-calibration of all the channels before the final installation in CMS. The procedure for the inter-calibration with cosmic ray muons is described and the results are presented. It is shown that with an exposure of 10 days an overall precision better than 2% is reached.