The CMS Electromagnetic Calorimeter its performance and role in the discovery of a Higgs boson and perspectives for the future

The Electromagnetic Calorimeter (ECAL) of the Compact Muon Solenoid (CMS) experiment at the LHC is a hermetic, fine grained, homogeneous calorimeter, comprising 75,848 lead tungstate scintillating crystals. We describe its construction and operation, and its role in the discovery and elaboration of the standard model Higgs boson. We discuss the challenges of operating a crystal calorimeter at a hadron collider, particularly in terms of calibration in the harsh radiation environment. The ECAL was designed to operate for a minimum of ten years at the LHC, with instantaneous/integrated luminosities of 10³⁴ cm^-2s^-1 and 500 fb^-1 respectively. The high luminosity LHC (HL-LHC) is expected to be operational from about 2024 to 2035 and provide instantaneous/integrated luminosities of around 5 × 10³⁴ cm^-2s^-1 and 3000 fb^-1. We give an overview of the evolution of the ECAL thought to be necessary to maintain its performance throughout LHC and HL-LHC operation.