Optimizing the performance of the CMS Electromagnetic Calorimeter to measure Higgs properties during Phase I and Phase II of the LHC

The CMS Electromagnetic Calorimeter (ECAL), is a high granularity lead tungstate (PbWO4) crystal calorimeter operating at the CERN LHC. The ECAL performance has been crucial in the discovery and subsequent characterization of the Higgs boson. The original ECAL design considerations, and the improvements to the energy reconstruction and energy calibration algorithms to cope with the LHC Run II are described. For the High-Luminosity LHC (HL-LHC) upgrades to ECAL are necessary. The crystals in the barrel region will be retained, defining the HL-LHC CMS barrel electromagnetic calorimeter ECAL. The readout electronics will be upgraded and operating at lower temperatures, to maintain the required performance of ECAL from 2027 onwards. The new readout electronics, the timing resolution and the electron and photon reconstruction efficiencies and energy resolution expected for HL-LHC are presented. The performance relevant to a number of key Higgs decay channels is reported.