CMS MTD Barrel Timing Layer: Optical Crosstalk Studies in LYSO Crystals

In preparation for the High-Luminosity LHC upgrade (HL-LHC), the Compact Muon Solenoid (CMS) detector is undergoing an extensive series of upgrades. With these upgrades, CMS will be able to maximize its physics discovery potential in Phase II through the end HL-LHC lifetime. To mitigate the challenging pileup environment of HL-LHC and expand the physics capabilities of the CMS detector, a high-precision timing detector sensitive to minimum-ionizing particles (MIPs) will be installed in the barrel and endcap regions. This new MIP Timing Detector (MTD) will provide precision timing information with hermetic coverage up to |η| less than 3. The barrel timing layer (BTL) of the MTD will be outfitted with Lutetium-Yttrium Orthosilicate (LYSO) scintillating crystals paired with silicon photomultipliers (SiPMs), comprising a detection unit that is well suited to the unique challenges of installing such a detector in the barrel. Fully characterizing the optical behavior of the LYSO-SiPM modules under development for BTL is essential to optimize the capabilities of MTD and achieve as small a timing resolution as possible. A study of optical crosstalk in these modules was conducted in 2022 at one of the BTL assembly centers using sodium-22 and 8-channel SiPM readout, offering a proof of principle for the setup planned for use in the assembly phase of MTD. The execution and results of this study will be presented along with a discussion of implications for further development of the BTL detection modules.