Calibration and Monitoring of the CMS Electromagnetic Calorimeter

The CMS Electromagnetic Calorimeter (ECAL) is an homogeneous and hermetic calorimeter with high granularity and fast response, designed to provide high resolution measurements of electron and photon energy. Precise calibration of the ECAL must be performed in situ at the LHC, in order to achieve and maintain its design performance and to fully exploit the physics reach of CMS. Several techniques have been developed for the inter-calibration of ECAL using collision data. These methods are based on the reconstruction of the invariant mass peak of unconverted photons from low mass particle decays (pi0 and eta) and the azimuthal symmetry of the average energy deposition at a given pseudorapidity. Further inter-calibration is carried out by using isolated electrons from W and Z bosons to compare the energy measured in ECAL to the momentum of the reconstructed track. The absolute calibration of the energy scale is performed using Z decays into electron-positron pairs. Changes in the ECAL response due to crystal radiation damage and changes in photo-detector output must be monitored. A system based on the injection of laser light into each crystal is used to track and correct for these variations during LHC operations.