To fulfill the heightened requirements of real-time and precision bunch-by-bunch luminosity
determination and beam-induced background monitoring at the High-Luminosity LHC (HL-
LHC), the CMS Beam Radiation Instrumentation and Luminosity (BRIL) project is constructing a
stand-alone luminometer, the Fast Beam Condition Monitor (FBCM). It will be fully independent
from the CMS central timing, trigger and data acquisition services and able to operate at all times
with a fast triggerless readout. The FBCM device is based on a modular design, adapting several
electronics components from the CMS Tracker for power, control, and read-out functionalities.
The 𝐶𝑂 2 -cooled silicon-pad sensors will be connected to a dedicated front-end Application-
Specific Integrated Circuit (ASIC) to amplify and discriminate the signals and provide a few
nanosecond timing resolution on the arrival time. The 6-channel FBCM23 ASIC outputs a binary
high-speed asynchronous signal carrying the Time-of-Arrival (ToA) and Time-over-Threshold
(ToT) information. Careful simulation, verified with laboratory tests and beam tests, is required
to ensure a successful construction of the FBCM luminometer. The simulation was performed
to optimize the FBCM performance in HL-LHC conditions: choose sensor size, sensor position
and estimate statistical precision. Final simulation shows a maximum deviation from linearity of
0.5% at pileup 200 for the selected sensor location. The detector design, performance simulation
and the results of the first validation tests with proton beams are reported.