Characterization of the lutetium-yttrium orthosilicate scintillating crystals for the CMS experiment

The Compact Muon Solenoid (CMS) detector is undergoing an extensive Phase II upgrade program to prepare for the challenging conditions of the High-Luminosity LHC starting in 2027. In particular, a new timing detector, the Mip Timing Detector (MTD) will measure minimum ionizing particles with a time resolution of 30–50 ps. The technology selected for the central part of the detector, the Barrel Timing Layer, consists of scintillating crystals of lutetium yttrium orthosilicate doped with cerium (LYSO:Ce) read out with Silicon Photo-Multipliers. A study of the performances of LYSO samples from different producers has been performed last year in Rome; in this report the experimental setup is described and the optical measurements are presented.