Precision timing with LYSO:Ce crystals and SiPM sensors in the CMS MTD barrel timing layer

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 detector will measure minimum ionizing particles (MIPs) with a time resolution of $\sim$30-40 ps and hermetic coverage up to a pseudo-rapidity of $|\eta |$=3. The precision time information from this MIP Timing Detector (MTD) will reduce the effects of the high levels of pileup expected at the HL-LHC and will bring new and unique capabilities to the CMS detector. The central Barrel Timing Layer (BTL) will be based on LYSO:Ce crystals read out with silicon photomultipliers (SiPMs). The BTL will use elongated crystal bars, with double-sided read-out, with a SiPM on each end of the crystal, in order to maximize detector performance within the constraints of space, cost, and channel count. 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 an overview of the MTD BTL design and will detail the extensive R&D studies carried out to optimize the MTD BTL crystal-based technology and test beam results in which the goal of 30 ps timing resolution has been achieved.