Longevity of the CMS ECAL and scintillator-based options for electromagnetic calorimetry at HL-LHC

The CMS lead tungstate (PbWO$_{4}$) electromagnetic calorimeter (ECAL) has successfully achieved its first goal, the Higgs discovery 2012. However, longevity studies show that part of the PbWO$_{4}$ ECAL does not maintain the required performance due to the radiation damaged incurred at the HL-LHC. The forward region of the detector will suffer the most from radiation damage, and the ECAL Endcaps (EE) will need to be replaced. A scintillator-based option for the EE replacement, Shashlik, is presented in the paper. The Shashlik EE is a sampling calorimeter. Tungsten absorber plates are interleaved with scintillator plates (LYSO or CeF$_3$), with quartz and wavelength-shifting (WLS) capillaries optically coupled to the scintillator plates for light output. The Shashlik EE maintains a high energy resolution, but compared to the current PbWO$_{4}$ EE, it is at least five times greater in radiation hardness and has a module size four times smaller allowing four times higher granularity laterally for pileup mitigation and particle identification. Irradiation tests and beam tests results have confirmed the high performance of the Shashlik EE.