The upgrade of the CMS electromagnetic calorimeter: Future prospects for precision timing and energy measurements at the HL-LHC

The Compact Muon Solenoid (CMS) electromagnetic calorimeter (ECAL) is a high granularity lead tungstate crystal calorimeter operated at the CERN LHC. The future High Luminosity upgrade of the LHC will provide unprecedented instantaneous and integrated luminosity with up to 200 collisions per bunch-crossing [pileup (PU)]. The CMS detector is therefore undergoing an extensive upgrade program to prepare for these demanding conditions. In the ECAL barrel region, the crystals will continue to perform well. The Avalanche Photodiodes (APDs) detecting the scintillation light will also continue to be operational, with some increase in noise that will be mitigated by reducing the operational temperature. Nonetheless, the entire readout and trigger electronics will need to be replaced to cope with the harsh conditions and increased trigger latency requirements. We plan a 25-fold improved readout granularity and a factor 4 sampling rate increase. The timing precision is used in important physics measurements and it is speculated that further improved time information could be exploited for PU mitigation and for the photons assignment to the correct collision vertex. As presented in this paper the endcaps region calorimeter needs to be replaced due to irrecoverable radiation damage of the crystals in the forward region. The upgraded ECAL will preserve the energy resolution, and will improve on time resolution for photons and electrons. In this paper, the ongoing R&D; activities for the ECAL barrel readout electronics upgrade are presented.