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

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### 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.

### Most cited references33

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### A Model of Leptons

(1967)
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### Big-Bang Nucleosynthesis

(2015)
A critical review is given of the current status of cosmological nucleosynthesis. In the framework of the Standard Model with 3 types of relativistic neutrinos, the baryon-to-photon ratio, $$\eta$$, corresponding to the inferred primordial abundances of deuterium and helium-4 is consistent with the independent determination of $$\eta$$ from observations of anisotropies in the cosmic microwave background. However the primordial abundance of lithium-7 inferred from observations is significantly below its expected value. Taking systematic uncertainties in the abundance estimates into account, there is overall concordance in the range $$\eta = (5.7-6.7)\times 10^{-10}$$ at 95% CL (corresponding to a cosmological baryon density $$\Omega_B h^2 = 0.021 - 0.025$$). The D and He-4 abundances, when combined with the CMB determination of $$\eta$$, provide the bound $$N_\nu=3.28 \pm 0.28$$ on the effective number of neutrino species. Other constraints on new physics are discussed briefly.
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### The Quantum Theory of the Electron

(1928)
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### Author and article information

###### Journal
1601.07172

Technical & Applied physics, Nuclear physics