Precision Timing with the CMS MTD Barrel Timing Layer for HL-LHC

The Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC) is undergoing an extensive Phase-2 upgrade program to prepare for the challenging conditions of the High-Luminosity LHC (HL-LHC). A new timing detector in CMS will measure minimum ionizing particles (MIPs) with a time resolution of 30-40 ps. 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, enhancing and expanding the physics reach of 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 readout (a SiPM on each end of the crystal), in order to maximize detector performance within the constraints of space, cost, and channel count. An overview of the MTD BTL design, highlighting some of the physics analyses impacted by the MTD is presented. Extensive R&D studies carried out to optimize the BTL design are summarized, with particular emphasis on the test beam results in which the goal of 30 ps timing resolution has been achieved.