Tracking the LHC halo

In the LHC, beams of 25-ns-spaced proton bunches travel at almost the speed of light and pass through many different devices installed along the ring that monitor their properties. During their whirling motion, beam particles might interact with the collimation instrumentation or with residual gas in the vacuum chambers and this creates the beam halo – an annoying source of background for the physics data. Newly installed CMS sub-detectors are now able to monitor it.


The Beam Halo Monitors (BHM) are installed around the CMS rotating shielding. The BHM are designed and built by University of Minnesota, CERN, Princeton University, INFN Bologna and the National Technical University of Athens. (Image: Andrea Manna).

The Beam Halo Monitor (BHM) is a set of 20 Cherenkov radiators – 10-cm-long quartz crystals – installed at each end of the huge CMS detector. Their design goal is to measure the particles that can cause the so-called “machine-induced” background. “Thanks to these new sub-detectors, we will be able to measure these particles and study their impact on some of the physics final states, particularly the rare ones and those that are more sensitive to background,” explains Anne Dabrowski, CMS physicist and technical coordinator of the BRIL (Beam Radiation Instrumentation and Luminosity) project, which encompasses, amongst other detector developments, the BHM. Understanding the sources of background is key to constantly improving the LHC beam performance for CMS and interpreting the physics data, because it allows scientists to disentangle the signals coming from beam collisions from the beam halo particles.

The 40 BHM units are installed parallel to the beam pipe around the experiment’s rotating shielding, in a ‘golden position’ for this type of measurement, whose sensitivity is highly dependent on the direction of the incoming particles. When a charged halo particle crosses a Cherenkov radiator, it produces a light cone in the forward direction, which is collected and transformed into an electronic signal by the photomultiplier. “The direction-dependent signal coming from the Cherenkov radiator is the main feature that we use to distinguish the halo particles from the particles originating from the collision point, as they would hit our sub-detectors from different directions,” explains Anne. “To optimise directional sensitivity, great care has been taken in the optical properties of every surface. We have painted the end of the crystal opposite the photomultiplier black. In this way, photons created by particles coming from the other direction or photons reflected by the photomultiplier surface can be completely absorbed and do not contribute to the signal of the photomultiplier.” In addition, a very fast response from the whole system ensures excellent performance. This allows the CMS collaboration to exploit fully the BHM’s golden location, where beam background particles will arrive 12.5 ns before outgoing collision products.

In its initial implementation phase, the BHM will be used to give the LHC machine experts feedback about beam quality. After the commissioning phase, it will be included in the CMS trigger and its information will be exploited during the experiment’s data processing. Their excellent radiation hardness will allow the sub-detectors to remain in position even in the future HL-LHC environment.   

by Antonella Del Rosso