A new Beam Halo Monitor (BHM) detector system has been installed in the CMS
cavern to measure the machine-induced background (MIB) from the LHC. This back-
ground originates from interactions of the primary beam halo with the nal set of
collimators before the CMS experiment and from beam gas interactions. The BHM
detector uses the directional nature of Cherenkov radiation and event timing to select
particles coming from the direction of the beam and to suppress those originating from
the interaction point. It consists of 40 quartz cylinders, placed on each side of the CMS
detector, coupled to UV-sensitive PMTs. For each bunch crossing, the PMT signal is
digitized by a charge integrating ASIC and the arrival time of the signal is recorded.
The data are processed in real time to yield a precise measurement of per-bunch-crossing
MIB rate for each beam. This measurement is made available to CMS and the LHC,
in order to provide real-time feedback on the beam quality and improve the e ciency
of data taking. The BHM detector is now in the commissioning phase, and rst results
have been obtained.