Performance of the CMS Electromagnetic Calorimeter in LHC Run2

Many analyses using the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC) at CERN require accurate, high-energy resolution measurements for electrons and photons. The CMS electromagnetic calorimeter (ECAL) is presently operating at the LHC with proton-proton collisions at 13~TeV center-of-mass energy, 25~ns bunch spacing, and an unprecedented peak luminosity of $2\times10^{34}$~cm$^{-2}$s$^{-1}$. High pileup levels (simultaneous collisions per bunch crossing) and the cumulative radiation damage of lead tungstate crystals necessitate a retuning of the ECAL readout, trigger thresholds, and reconstruction algorithms to maintain the best possible performance. In addition, the energy response of the detector must be precisely calibrated and monitored. This talk presents the new reconstruction algorithm and calibration strategies that have been implemented, and the excellent performance achieved by the CMS ECAL with the ultimate calibration of Run II data, in terms of energy scale stability and energy resolution.