Commissioning of the New Beam Halo Monitoring System for the CMS Experiment

Abstract

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.