Electromagnetic calorimetry at the highest energy and intensity proton-proton collider: CMS ECAL performance at LHC Run II and prospects for high luminosity LHC

The electromagnetic calorimeter (ECAL) of the Compact Muon Solenoid Experiment (CMS) started operating at the Large Hadron Collider (LHC) in Spring 2015 with proton-proton collisions at 13 TeV center-of-mass energy and at a reduced bunch spacing of 25 ns. The instantaneous luminosity during LHC Run 2 is expected to exceed the levels achieved in LHC Run 1 significantly. In this talk we present new reconstruction algorithms and calibration strategies to maintain the superb performance of the CMS ECAL under these challenging conditions. We will show first performance results from the 2015 data taking period and give an outlook on the foreseen performance for Run 2. The current Run 2 of LHC is expected to accumulate a few hundred inverse femtobarn of proton-proton collisions before LHC will be upgraded to provide an even higher instantaneous luminosity, referred to as High-Luminosity LHC (HL-LHC). HL-LHC is expected to provide a few inverse attobarn of integrated luminosity. This will result in even higher rates of simultaneous proton-proton collisions, called pile-up (PU), up to and exceeding 140 PU collisions. Likewise the physics event rates will increase further which requires upgrades to the readout and trigger electronics of the existing ECAL. We will briefly discuss the upgrade plans and present in detail the plans to combat the PU under these conditions. One promising technique is based on the very good time resolution the CMS ECAL provides. We present the current status of timing performance of the CMS ECAL, plans to improve this for the future and examples of how this can be used for PU mitigation.