Operation and performance of the CMS Electromagnetic Calorimeter

The CMS Electromagnetic Calorimeter (ECAL) is a high resolution, finely grained calorimeter devised to measure photons and electrons at LHC. Built of lead tungstate crystals, it plays a crucial role in the search for new physics as well as in precision measurements in the Standard Model. The operation and general performance of the CMS ECAL in proton-proton collisions at sqrt{s} = 7 TeV are described. The precision of the inter-channel synchronization and calibration has been verified and improved exploiting in-situ data. Di-electron and di-photon states have been also used to verify and tune the energy scale. The quality of the offline data reconstruction, from low level quantities to showers, has been investigated and improved using known physics processes. Collision data and thorough data/MC comparisons have been used to measure and tune the detector performance. First performance results are given.