Precision Timing in the CMS MTD Barrel Timing Layer with Crystal Bars and SiPMs

The Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC) is undergoing an extensive Phase II upgrade program to prepare for the challenging conditions of the High-Luminosity LHC (HL-LHC). In particular, a new timing layer will measure minimum ionizing particles (MIPs) with a time resolution of 30-40 ps and hermetic coverage up to a pseudo-rapidity of 3. The precision time information from this detector will reduce the effects of the high levels of pile-up expected at the HL-LHC and will bring new and unique capabilities to the CMS detector. This MIP Timing Detector (MTD) will consist of a central barrel timing layer (BTL) based on L(Y)SO Ce crystals read out with SiPMs and two end-caps instrumented with radiation-tolerant Low Gain Avalanche Detectors. With the goal of maximizing the detector performance within the stringent constraints of space, cost, and channel count, the BTL exploits elongated crystal bars, each read out with two SiPMs. This unusual geometry enables the instrumentation of large surfaces while minimizing the active area of the photodetectors, and thus noise and power consumption. We will present the extensive studies carried out to optimize this crystal-based technology and key beam test results in which the target time resolution of 30 ps has been achieved.