Upgrade of the CMS electromagnetic calorimeter for precision timing and energy measurements at the High Luminosity LHC

The high luminosity upgrade of the LHC (HL-LHC) at CERN will provide unprecedented instantaneous and integrated luminosities of around $5\times {10}^{34}$ cm${}^{-2}$ s${}^{-1}$ and up to 4500 fb${}^{-1}$, respectively, from 2027 to 2035. We predict an average of 140 to 200 concurrent interactions per bunch crossing (pile-up). This poses a major challenge to the event reconstruction. The Compact Muon Solenoid (CMS) detector is therefore undergoing an extensive Phase II upgrade program to prepare for these challenging conditions. The upgrade of the barrel part of the CMS electromagnetic calorimeter (ECAL) for HL-LHC will extend the detector performance to be able to cope with these harsh conditions, particularly the increased trigger and latency requirements at the HL-LHC. The lead tungstate crystals in the ECAL barrel sub-detector will still perform well. The avalanche photodiodes which detect the scintillation light will also continue to be operational with some increase in noise due to radiation-induced dark currents. However the entire readout electronics will need to be replaced. The upgraded detector will have a 25 fold improved trigger granularity and a sampling rate increase by a factor of four. In addition, the upgraded detector will have the new capability to provide precision timing measurements for photons and electrons with energies greater than 10 GeV. This will significantly improve the performance under the predicted pile-up conditions. High precision time resolution will be exploited for pileup mitigation and to permit the assignment of physics objects to their correct vertices. In this report the status of the ongoing R&D activities for the ECAL barrel upgrade will be presented.