Measuring the Weak Charge of the Proton via Elastic Electron-Proton Scattering

TL;DR

The Qweak experiment measured the proton's weak charge through elastic electron-proton scattering, providing a precise test of the Standard Model and the running of the weak mixing angle at low energies.

Contribution

This work presents new techniques for reducing systematic errors in parity-violating electron scattering experiments, especially in beam polarization measurement.

Findings

Measured weak charge of the proton: Q_W^p=0.064±0.012

Agreement with Standard Model prediction

Enhanced precision in parity-violation measurements

Abstract

The Qweak experiment which ran at Jefferson Lab in Newport News, VA, measured the weak charge of the proton $Q_W^p$ via elastic electron-proton scattering. Longitudinally polarized electrons were scattered from an unpolarized liquid hydrogen target. The Standard Model predicts a small parity-violating asymmetry of scattering rates between electron right and left helicity states due to the weak interaction. An initial result using 4% of the data was published in October 2013 with a measured parity-violating asymmetry of $-279\pm 35(\text{stat})\pm 31$ (syst) parts per billion (ppb). This asymmetry, along with other data from parity-violating electron scattering experiments, provided the world's first determination of the weak charge of the proton. The weak charge of the proton was found to be $Q_W^p=0.064\pm0.012$, in agreement with the Standard Model prediction of $Q_W^p(SM)=0.0708\pm0.0003$. The results of the full dataset are expected to decrease the statistical error from the initial publication by a factor of 4-5. The level of precision of the final result makes it a useful test of Standard Model predictions and particularly of the "running" of $\sin^2\theta_W$ from the Z-mass to low energies. This thesis focuses on reduction of systematic error in two key systematics for the Qweak experiment. First, techniques for measuring and removing false asymmetries arising from helicity-correlated electron beam properties at the few ppb level are discussed. Second, as a parity-violating experiment, Qweak relies on accurate knowledge of electron beam polarimetry. To help address the requirement of accurate polarimetry, a Compton polarimeter built specifically for Qweak. Compton polarimetry requires accurate knowledge of laser polarization inside a Fabry-Perot cavity enclosed in the electron beam pipe. A new technique was developed for Qweak that nearly eliminates this systematic error.