Calibration of the CMS electromagnetic calorimeter at the LHC start up

The electromagnetic calorimeter of the CMS experiment is designed to reach excellent energy resolution, essential for the Higgs search in the H → γ γ decay channel. The non-uniformity of the individual channel responses directly contributes to the energy resolution. The target precision of the detector calibration, both of the relative response of the channels (inter-calibration) and of the electron and photon energy scale, at the 0.5% level, will be reached employing the collision events collected at the LHC. arter of the barrel ECAL has been calibrated at this level of precision on electron beams. The entire barrel was calibrated with cosmic rays before installation in CMS with an accuracy at the 2% level, giving acceptable performance at the startup. During the commissioning phase in CMS about 600 million muon events have been recorded allowing to verify that the working conditions used during the pre-calibration were reproduced in CMS. fferent phases of the pre-calibration procedures are summarized and the techniques for the calibration with the first LHC data are presented.