A Measurement of the Parity-Violating Asymmetry in Aluminum and its Contribution to A Measurement of the Proton\u27s Weak Charge

Abstract

The Qweak experiment, which ran at the Thomas Je↵erson National Accelerator Facility, made a precision measurement of the proton’s weak charge, Q_p^W . The weak charge is extracted via a measurement of the parity-violating asymmetry in elastic electron-proton scattering from hydrogen at low momentum transfer (Q^2=0.025 GeV^2). This result is directly related to the electroweak mixing angle, sin2 (theta_W ), a fundamental parameter in the Standard Model of particle physics. This provides a precision test sensitive to new, as yet unknown, fundamental physics. This dissertation focuses on two central corrections to the Qweak measurement: the target window contribution and sub-percent determination of the electron beam polarization. The aluminum target windows contribute approximately 30% of the measured asymmetry. Removal of this background requires precise measurements of both the elastic electron-aluminum scattering rate and its parity-violating asymmetry. The results reported here are the most precise measurement of the Qweak target dilution and asymmetry to date. The parity-violating asymmetry for the aluminum alloy was found to be 1.6174 ± 0.0704 (stat.) ± 0.0113 (sys.) parts-per-million. The first sub-percent precision polarization measurements made from the Hall C Møller polarimeter are also reported, with systematic uncertainties of 0.84%