Commissioning of the CMS ECAL calibration with muons from cosmic rays and beam dumps

The electromagnetic calorimeter of the CMS experiment is designed to reach excellent energy resolution, essential for the discovery of narrow electromagnetic resonances, such as the Standard Model decay H$\to \gamma \gamma$. The non uniformity of the individual channels directly contributes to the calorimeter energy resolution. While the final performance will be achieved with {\it in situ} calibration exploiting physics events, a good performance at start-up is already offered by the result of pre-calibration procedures involving test beam data, exposure to cosmic rays and laboratory measurements of the crystal light yield and photodetector gains. During the CMS commissioning phase, muons from cosmic rays and from beam dumps were used to validate the calibrations. In particular, the measurement of the muon momentum by the tracking system allow to check the validity of the energy scale defined on a 120 GeV electron beam down to the energy released by a minimum ionising particle. Muons from beam dump data are used to improve the intercalibration of the ECAL endcaps.