Relatore
Summary
The Electromagnetic Calorimeter (ECAL) of CMS is a hermetic homogeneous
calorimeter
made of 61200 lead-tungstate scintillating crystals readout by avalanche-photodiodes
in the barrel part, closed by 7324 crystals readout by vacuum-phototrieodes in each
of the two end-caps. The calibration of the relative response of the individual
channels, or intercalibration, and of the absolute energy scale for electrons and
photons are severe technical challenge for the operation of ECAL. The channel
response uniformity within ECAL and stability in time will contribute directly to the
overall energy resolution. Complex procedures based on the use of events collected
during LHC operation have been designed to achieve an ultimate intercalibration
precision of 0.5%. A laser monitoring system will be used to track response
variations with time, as in the case of changes in crystal transparency caused by
irradiation. Trigger, selection and reconstruction procedures of calibration signals
and their projected performance are discussed. Results of calibrations accomplished
with electron beams and cosmic rays during the commissioning phase of the detector
are also reviewed. These set the intercalibration precision at the startup of LHC
operation and provide a reference for validation and further development of the
procedures based on physics events. Reconstruction algorithms and effects affecting
the energy measurement of electron and photons, such as containment effects or
issues
related to conversions and bremmstrahlung radiation in the tracker material, are also
discussed.
