Beam Modulation System for the Qweak Experiment at Jefferson Lab

TL;DR

The paper describes a beam modulation system for the Qweak experiment that modulates beam parameters to measure detector sensitivities and correct false asymmetries, improving measurement accuracy of the proton's weak charge.

Contribution

It introduces a novel beam modulation system using air-core dipoles and RF cavities for precise control and correction of beam-related false asymmetries in the Qweak experiment.

Findings

Effective measurement of detector sensitivities achieved.

System helps track beamline optics changes.

Reduces false asymmetries in parity-violation measurements.

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.