Directional \v{C}erenkov Detectors

It is sometimes useful to have a particle detector that determines not only the amount of energy deposited in the detector but also the direction from which it came. With a colliding beam machine, such as the LHC, at small angles a detector is exposed both to particles coming from the interaction region and to particles produced by incoming beam particles. A directional detector can identify and enable the elimination of the background from the incoming beam. A charged particle with the velocity of light passing through a medium with an index of refraction n emits (\v{C}erenkov) light at an angle $\theta_c$ with respect to its direction such that $cos\theta_c$ = 1/n. This angle is 45$^\circ$ for n = 1.414. Directional counters can be made by using the directional properties of the \v{C}erenkov light. A photomultiplier tube, by itself, acts as such a detector by responding to \v{C}erenkov light produced in the glass over the photocathode. Various counter configurations have been studied using cosmic- ray muons identified by cosmic-ray telescopes from the NSF-DOE QuarkNet program. These counters are candidates for Forward Shower Counters (FSC) for the CMS experiment at the LHC.