High precision electromagnetic calorimetry with 40 MHz readout: the CMS crystal ECAL for the High-Luminosity LHC

The electromagnetic calorimeter (ECAL) of the Compact Muon Solenoid Experiment (CMS) will be upgraded to meet the challenging running conditions expected after the High-Luminosity upgrade of the LHC (HL-LHC). Particular challenges at HL-LHC are the harsh radiation environment, the increasing data rates and the extreme level of pile-up events, with up to 200 simultaneous proton-proton collisions. The detector will have to sustain an instantaneous luminosity of above $5 \times 10^{34} cm^2 s^{-1}$, maintaining a performance similar to the one of LHC Run I for an integrated luminosity of 3 to 5 $ab^{-1}$. This poses stringent requirements on the radiation resistance of detector components, the readout and data transfer from the front end to the back end electronics, as well as the latency of the trigger system. The barrel region of the CMS ECAL will be able to retain the current lead tungstate crystals and avalanche photodiode detectors which will meet the energy measurement performance requirements throughout the operational lifetime of the HL-LHC. To improve the physics performance of CMS under severe pile-up conditions, the timing precision of the CMS ECAL will be improved to reach around 30 ps for energies down to 10 GeV. The very front end readout will utilize trans-impedance amplifiers to optimally utilize the excellent timing performance of the ECAL crystals. The detector will be fully read out without any noise suppression at the LHC collision rate of 40 MHz. A powerful back-end electronics will reconstruct the amplitude and time of each of the 60000 channels of the ECAL barrel in real time. We will present the status of the R and D of the readout and back-end electronics, report on the latest beam tests with pre-production prototypes, and describe the expected performance of the upgraded detector.