Precision crystal calorimetry at high energy and high luminosity: CMS ECAL performance at 13 TeV and upgrade test beam studies

The Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) is currently operating with proton-proton collisions at 13 TeV center-of-mass energy and at a bunch spacing of 25 ns. Achieving and maintaining excellent energy resolution for electrons and photons is of primary importance for the CMS physics program for Standard Model measurements and beyond Standard Model searches. In this paper we review the current performance of the electromagnetic calorimeter (ECAL) of CMS during the LHC Run II period, highlighting the improvements with respect to the previous period of data-taking. The running conditions in 2016 provide a preview of the challenges that will be faced during the high luminosity phase of LHC (HL-LHC). This phase will impose stringent performance requirements on radiation hardness and pileup resilience that will require an upgrade of the current ECAL detector. We review the design and R&D studies for the CMS ECAL crystal calorimeter upgrade, focusing mostly on the upgrade of the central (barrel) part of the detector. A prototype of the new detector (a matrix of the same scintillating crystals used in ECAL) equipped with new readout electronics, has been built and tested with high-energy electron beams to study the performance of the upgraded ECAL. The encouraging results of these first tests are reported here.