The CMS Beam Halo Monitor Detector System

TL;DR

The CMS Beam Halo Monitor (BHM) detector system measures machine-induced background in real-time using Cherenkov radiation, aiding beam quality assessment and improving data collection efficiency at the LHC.

Contribution

This paper introduces a novel Cherenkov-based detector system for real-time background measurement in the CMS experiment at the LHC.

Findings

Successful installation and operation during Run II

Real-time background rate measurement per bunch crossing

Enhanced beam quality monitoring capabilities

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 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.