Beam Modulation System for the Qweak Experiment at Jefferson Lab

Abstract: The Qweak experiment aims to measure the weak charge of the proton to 4% via the parity-violating asymmetry ($\sim$-200 ppb) in elastic e+p scattering. The scattering rate is largely influenced by five beam parameters: horizontal position (X), angle (X'), vertical position (Y), angle (Y'), and beam energy (E). Changes in these parameters when the beam polarization is reversed will create a false asymmetry. To measure the detector sensitivities, we modulate X, X', Y, Y' using four air-core dipoles in the Hall C beamline as well as beam energy with a superconducting RF cavity. Linear regression is then used to remove the false asymmetry. The beam modulation system has also proven valuable for tracking changes in the Hall-C beamline optics including dispersion at the target.