Speaker
Prof.
Paul King
(Ohio University)
Description
The Qweak experiment, which completed a two-year data taking phase in May 2012 at Jefferson Lab, has made the first determination of the weak charge of the proton. We access the weak charge by measuring the parity-violating asymmetry in the elastic scattering of polarized electrons from protons at a small squared four-momentum transfer of Q^2 = 0.025 (GeV/c)^2. Due to the interference of the photon and Z-boson exchange processes, this asymmetry is proportional to the weak charge of the proton at low momentum transfers. We determined a value for the asymmetry of Aep = -279 +/- 35 (statistics) +/- 31 (systematics) ppb, which is the smallest and most precise asymmetry ever measured in polarized electron-proton scattering. To achieve the precision necessary to measure this small asymmetry, we integrated events from eight fused silica detectors by scattering the high-current polarized electron beam on a 35 cm long liquid hydrogen target. Based on only 4% of the total data collected in the experiment, and by incorporating earlier results from earlier parity-violating electron scattering experiments, we find a value for the weak charge of the proton of QpW = 0.064 +/- 0.012 in agreement with predictions of the Standard Model of particle physics. The projected precision for the full data set will result in sensitivity to new physics at the TeV scale, complementing searches at the Large Hadron Collider. In addition to the search for physics beyond the Standard Model, we will measure parity-violating and parity-conserving observables using longitudinally and transversely polarized electron beam, in elastic and inelastic scattering channels, on proton and aluminum targets by observing both electrons and pions in the final state. These precise measurements will allow us to constrain various relevant models in hadronic and nuclear physics.
Primary author
Prof.
Paul King
(Ohio University)
