The Weak Charge of the Proton. A Search For Physics Beyond the Standard Model

Abstract

The Q weak experiment will provide the most precise determination of the proton's weak charge Q p W by measuring the parity-violating asymmetry in elastic electron-proton scattering at low momentum transfer Q 2 ∼ 0.0250 (GeV/c) 2. Q p W is related to the weak mixing angle sin 2 θ W , a fundamental parameter of the standard model. A final measurement of the weak charge at the proposed 4% relative uncertainty is sensitive to certain types of new parity-violating physics beyond the standard model at the TeV mass-scale. Data were taken over a two year period beginning in 2010, using a custom apparatus installed in Hall-C at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. A 180 µA beam of longitudinally polarized electrons was accelerated to 1.165 GeV and then scattered from unpolarized protons in a liquid hydrogen target. The scattered electrons were then collimated and steered using a magnetic spectrometer onto a set of azimuthally symmetric quartz barČerenkov detectors. The performance of this main detector subsystem will be described in detail in this dissertation. A blinded analysis of Run-II, roughly 2/3 of the entire Q weak data set, resulted in an elastic ep asymmetry of −235.6 ± 8.7 (Stat) ± 9.3 (Syst.) ± 39.3 (Blind) ppb. Using this value, the proton's weak charge was calculated to be Q p W = 0.0714 ± 0.0093. This constitutes a 17% relative measurement, that will reduce to < 9% upon unblinding of the asymmetry. This dissertation will provide the details that went into extracting both the parity-violating asymmetry and the weak charge of the proton. i Subedi, Katherine Myers, and Nurruzaman. Great thanks are warranted to Rakitha Beminiwatha and Buddhini Waidyawansa who were great office-mates as well as friends to my family. Thank you to those students who are still working hard to bring Q weak to a conclusion:

Determination of the Proton's Weak Charge via Parity Violating Electron Scattering

“…V encompasses contributions from charge symmetry violation [94]. Rewriting the asymmetry in this way, the global fit becomes a function of five independent parameters: the weak vector couplings (C 1u and C 1d ), the strange charge radius ρ s , the strange magnetic moment µ s , and the axial form factors G…”