Studies of Photon Probability Distributions in the Belle II Particle Identification System

The Belle II experiment studies collisions of subatomic particles to better understand laws of nature. Interactions are observed by multiple detectors, and particle identification is provided by the imaging Time-of-Propagation detector, comprised of 16 modules with two 1.25m long quartz radiator bars. Charged particles passing through quartz with a relativistic velocity greater than light in that medium emit Cherenkov photons at angles characteristic for their mass. These photons can be internally reflected in the quartz before being detected by a series of multi-channel plate photomultiplier tubes. The charged particle can be identified by its photons. We compute the probability distribution functions of photons detected by channels and simulate each photon with GEANT4. We compare the probability distribution functions and GEANT4 simulations at select locations within the bar to determine any regions with systematic discrepancies. It was found that entry points closest to the readout plane yielded greater disagreement between probability distribution functions and simulations.