  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
background originates from interactions of the LHC beam halo with the final 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
rods, 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 background rate.
This measurement is made available to CMS and the LHC, to provide real-time
feedback on the beam quality and to improve the efficiency of data taking.
Here, I present the detector system and first results obtained in Run II.
