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

The Qweak experiment which ran in Hall C at Jefferson Lab in Newport News, VA, and completed data taking in May 2012, 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 helicity of the electron beam was flipped at approximately 1~kHz between left and right spin states. The Standard Model predicts a small parity-violating asymmetry of scattering rates between 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 $-279\pm 35(stat)\pm 31$ (syst)~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 good agreement with the Standard Model prediction of $Q_W^p(SM)=0.0708\pm0.0003$. The results of the full dataset are expected to be published in early 2016 with an expected decrease in 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. However, this level of statistical precision is not useful unless the systematic uncertainties also fall proportionately. This thesis focuses on reduction of error in two key systematics for the Qweak experiment. First, false asymmetries arising from helicity-correlated electron beam properties must be measured and removed. Techniques for determining these false asymmetries and removing them 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 reduces this uncertainty to near zero.

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