Upgrade of the CMS Barrel Electromagnetic Calorimeter for the High Luminosity LHC

The high luminosity upgrade of the LHC (HL-LHC) at CERN will provide unprecedented instantaneous and integrated luminosities of up to $7.5\times10^{34}$~cm$^{-2}$s$^{-1}$ and 4500~fb$^{-1}$, respectively, from 2029 onwards. To cope with the extreme conditions of up to 200 collisions per bunch crossing, and increased data rates, the on- and off-detector electronics of the CMS electromagnetic calorimeter (ECAL) will be replaced. A dual gain trans-impedance amplifier and an ASIC providing two 160 MHz ADC channels, gain selection, and data compression will be used. The lead tungstate crystals and avalanche photodiodes (APDs) in the current ECAL will keep performing well and will therefore be maintained. The noise increase in the APDs, due to radiation-induced dark currents, will be minimised by reducing the ECAL operating temperature from 18 $^\circ$C to around 9~$^\circ$C. Prototype HL-LHC electronics have been tested and have shown promising results. In two test beam periods using the CERN SPS H4 beamline and an electron beam, the new electronics achieved the target energy resolution and a timing resolution consistent that is consistent with our requirements of 30~ps timing for energies greater than 50 GeV.