Calibration of the CMS Electromagnetic Calorimeter with first LHC data

The electromagnetic calorimeter (ECAL) of the CMS experiment is an homogeneous, hermetic detector made of high granularity lead tungstate crystals. Its potential performances are outstanding in terms of energy resolution, dynamic range and noise level. However, the energy resolution depends crucially from the channel to channel intercalibration precision. Thus, intercalibrating ECAL in situ at the LHC to achieve the designed energy resolution approaching 0.5\% for high energy unconverted photons is going to be important to fully exploit the physics reach of the detector, particularly for the discovery of the Higgs boson in the two-photon decay channel. A detailed description of the strategies for in-situ calibration of the calorimeter in 2010 will be given, focusing on the crystal intercalibration using the phi symmetry and pizero methods. Preliminary results using first collision data will be presented.