The CMS electromagnetic calorimeter barrel upgrade for High-Luminosity LHC

The High Luminosity LHC (HL-LHC) will provide unprecedented instantaneous and integrated luminosity. The lead tungstatecrystals forming the barrel part of the CMS Electromagnetic Calorimeter (ECAL) will still perform well, even after theexpected 3000$\,$fb$^{-1}$ at the end of HL-LHC. The scintillation light is measured with avalanche photodiodes (APDs).Although the APDs will continue to be operational, there will be some increase in noise due to radiation-induceddark-currents. Triggering on electromagnetic objects with $\sim$140 pileup events necessitates a change of the front-endelectronics. New developments in high-speed optical links will allow single-crystal readout at 40 MHz to upgradedoff-detector processors, allowing maximum flexibility and enhanced triggering possibilities. The very-front-end system willalso be upgraded, to provide improved rejection of anomalous signals in the APDs as well as to mitigate the increase in APDnoise. We are also considering lowering the ECAL barrel operating temperature from 18$^{\text{o}}$C to about8$~\sim$10$^{\text{o}}$C, in order to increase the scintillation light output and reduce the APD dark current.