Characterization of sensor modules for the CMS Barrel Timing Layer at HL-LHC.

The CMS detector is undergoing an extensive upgrade program to prepare for the challenging conditions of the High-Luminosity LHC starting in 2029. In particular, a new timing detector will measure minimum ionizing particles with a time resolution of $30\mbox{--}60 {ps}$. The technology selected for the central part of the detector, the Barrel Timing Layer, consists of scintillating crystals of lutetium yttrium oxyorthosilicate doped with cerium ($LYSO:Ce$) read out with silicon photo-multipliers. A detailed characterization of several sensor modules, consisting of a sixteen-channel crystal array coupled to a pair of SiPM arrays, will be presented. The results represent the first thorough investigation of the performance of modules made with crystals and SiPMs of different geometries characterized both with radioactive sources and laser excitation in laboratory as well as with test beam.