The PhaseII Upgrades of CMS are being planned for the High Luminosity LHC (HL-LHC) era when the mean number of interactions per beam crossing (in-time pileup) is expected to reach ~140-200. The potential backgrounds arising from mis-associated jets and photon showers, for example, during event reconstruction could be reduced if physics objects are tagged with an event time. This tag is fully complementary to the event vertex which is already commonly used to reduce mis-reconstruction. Since the tracking vertex resolution is typically ~10^{-3} (50 micron/4.8cm) of the rms vertex distribution, whereas only ~10^{-1} (i.e. 20 vs.170 picoseconds (psec)) is demonstrated for timing, it is often assumed that only photon (i.e. EM calorimeter or shower-max) timing is of interest. We show that the optimal solution will likely be a single timing layer which measures both charged particle and photon time (a pre-shower layer).
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