Investigating the Effect of Surface Material on Time Resolution of the Barrel Timing Layer Sensor Modules for the MIP Timing Detector*

To sufficiently disentangle the multiple tracks from all primary interactions at the HL-LHC, the CMS experiment will be upgraded to include the Minimum Ionizing Particle Timing Detector (MTD). The fundamental unit of the Barrel Timing Layer of the MTD is composed of a 57 mm long and 3 mm thick LYSO:Ce scintillator crystal with silicon photomultipliers (SiPMs) attached to each end. We constructed a Geant4 simulation of the LYSO:Ce crystal to determine the impact of the surface material properties of the LYSO:Ce crystals on the overall time resolution. The timing resolution and photon paths are compared for various implementations of diffuse and specular reflectors. Preliminary results show that for scintillation light from high energy muons, the majority of the first photons that reach the SiPMs reflect off the surface material. For specular reflectors, this suggests that the major contributor to time resolution is the photo- statistics of the scintillator and the photon paths through the crystal are less relevant. Thus, to improve the timing resolution for ESR it is best to try to optimize the light yield. We also found that high reflectivity specular reflectors and reflectors with an airgap layer of low refractive index generates the best time resolutions at relevant photon thresholds.