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). A new timing detector in CMS will measure minimum
ionizing particles (MIPs) with a time resolution of 30-40 ps for MIP signals at a rate of 2.5 Mhit/s
per channel at the beginning of HL-LHC operation. 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, bringing new capabilities to the CMS detector. The barrel timing layer (BTL) of the
MTD will use sensors that are based on LYSO:Ce scintillation crystals coupled to SiPMs with
TOFHIR ASICs for the front-end readout. In this talk we will present motivations for precision
timing at the HL-LHC and an overview of the MTD BTL design, including ongoing R&D studies
targeting enhanced timing performance and radiation tolerance.