Parity Violation in High-Energy Electron Scattering: Data Analysis for the G Experiment

The focus of the G experiment at Jefferson Lab is the contribution of the strange quark to the character of the proton. The effects of different flavors of quarks can be disentangled by comparing the electromagnetic and weak contributions, since the electromagnetic and weak couplings of the various flavors differ. The weak interaction can be isolated for study because it violates parity symmetry, so there is a difference in the scattering rates of electrons with their spins aligned with their momentum and those with oppositely aligned spins. The electron beam being collided with the proton target, however, carries with it a halo: a non-Gaussian fringe caused by interactions with stray gas in the beam enclosure, the beamline, the beam itself, etc. The halo could adversely affect the results of the experiment, especially if it is helicity-correlated, since G is a parity violation experiment. It is therefore important to characterize the beam halo, which is monitored with a halo target (of two possible sizes) and several halo monitors. The halo monitors themselves are characterized to investigate the extent of their reliability and applicability. After establishing this, the halo rates are analyzed in several ways to determine what sort of beam halo the experiment usually has to contend with. Overall, it seems that the halo monitors perform as expected, and that the halo is within reasonable bounds. Having the halo monitors and target in place during experimental runs provides valuable insight into the quality of the beam.