Fast Beam Condition Monitor of the CMS experiment at HL-LHC

To achieve the challenging target of 1% precision for luminosity measurement at the high-luminosity LHC (HL-LHC) with instantaneous luminosity up to 7.5×10$^{34}$ cm$^{−2}$ s$^{−1}$, the CMS experiment will employ multiple luminometers with orthogonal systematics. A key component of the proposed system is a standalone luminometer, the Fast Beam Condition Monitor (FBCM), which is fully independent of the central trigger and data acquisition and hence is able to operate outside of CMS data taking periods at 40 MHz providing bunch-by-bunch luminosity measurement with 1 s time granularity. FBCM will be placed inside the cold volume of the tracker as it utilizes silicon-pad sensors with an asynchronous read-out. A zero-hit-counting algorithm will be used to measure luminosity. FBCM will also provide precise timing information with a few ns precision enabling the measurement of beam-induced background. We report on the optimization of the design and the expected performance of FBCM.