A Precision Test of the Standard Model via Parity-Violating Electron Scattering in the Q_weak Experiment 580 views

The $Q_\text{weak}$ Collaboration has completed a challenging measurement of the parity-violating asymmetry in elastic $\vec{e}p$ scattering of longitudinally polarized electrons from protons at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The experiment released an early result for the parity-violating asymmetry $A_{ep} = -279 \pm 35~\text{(stat)} \pm 31~\text{(syst)}$ parts-per-billion (ppb) from the commissioning period, constituting about 4\% of the full data set. This result allowed the first determination of the weak charge of the proton $Q_w^p$ from a global fit of parity-violating elastic scattering (PVES) results from nuclear targets, where earlier data at higher $Q^2$ constrain uncertainties of hadronic structure. The value extracted from the global fit is $Q_w^p \text{(PVES)} = 0.064 \pm 0.012$, in agreement with the standard model prediction $Q_w^p \text{(SM)} = 0.0710 \pm 0.0007$. The analysis of the full $Q_\text{weak}$ data is ongoing and expected to yield a value for the asymmetry within 10 ppb of precision. Because of the suppression of $Q_w^p$, such a high precision measurement will allow the most precise extraction of the weak mixing angle below the $Z^0$ pole and place significant constraints to models of physics beyond the standard model. The coupling of beam parameters to the electron spin direction can give rise to false asymmetries and bias the experimental measurement. These effects must be controlled at the 1-2 ppb level to achieve the challenging precision goals of $Q_\text{weak}$. We describe our efforts to suppress helicity-correlated beam parameters by optimizing the Jefferson Lab polarized source and arranging for cancellations. The correction methods to remove false asymmetries and their shortcomings are presented. Backgrounds generated at the beamline are considered to be connected to higher moments of helicity-correlated beam parameters and contributed the largest systematic uncertainty in the early result. An algorithm is developed to estimate the false asymmetry associated with this background source, improving the precision of this important systematic correction by an order of magnitude. We extract from a subset of the full $Q_\text{weak}$ dataset a preliminary and blinded result $A_{ep} =-226.95 \pm 9.44~\text{(stat)} \pm 6.64~\text{(syst)}$ ppb, translating to a value for the weak charge $Q_w^p = 0.0716 \pm 0.0053$ from a global fit of PVES asymmetries. In the absence of a blinding term this result would be in excellent agreement with the standard model prediction and constrain new physics at the multi-TeV scale. The status of ongoing analysis for important systematic corrections in the $Q_\text{weak}$ measurement will be presented.

PHD (Doctor of Philosophy)

Physics; Nuclear Physics; Particle Physics; Intensity Frontier; Standard Model; Qweak; Parity violation; weak interactions

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2015-12-11