  The MIP Timing Detector will provide additional timing capabilities for
detection of minimum ionizing particles (MIPs) at CMS during the High
Luminosity LHC era, improving event reconstruction and pileup rejection. The
central portion of the detector, the Barrel Timing Layer (BTL), will be
instrumented with LYSO:Ce crystals and Silicon Photomultipliers (SiPMs)
providing a time resolution of about 30 ps at the beginning of operation, and
degrading to 50-60 ps at the end of the detector lifetime as a result of
radiation damage. In this work, we present the results obtained using a 120 GeV
proton beam at the Fermilab Test Beam Facility to measure the time resolution
of unirradiated sensors. A proof-of-concept of the sensor layout proposed for
the barrel region of the MTD, consisting of elongated crystal bars with
dimensions of about 3 x 3 x 57 mm$^3$ and with double-ended SiPM readout, is
demonstrated. This design provides a robust time measurement independent of the
impact point of the MIP along the crystal bar. We tested LYSO:Ce bars of
different thickness (2, 3, 4 mm) with a geometry close to the reference design
and coupled to SiPMs manufactured by Hamamatsu and Fondazione Bruno Kessler.
The various aspects influencing the timing performance such as the crystal
thickness, properties of the SiPMs (e.g. photon detection efficiency), and
impact angle of the MIP are studied. A time resolution of about 28 ps is
measured for MIPs crossing a 3 mm thick crystal bar, corresponding to an MPV
energy deposition of 2.6 MeV, and of 22 ps for the 4.2 MeV MPV energy
deposition expected in the BTL, matching the detector performance target for
unirradiated devices.
