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. Based on only 4% of the total data collected by the experiment, and by incorporating earlier results from other experiments, we find a value in agreement with predictions of the Standard Model of particle physics. The projected precision for the full set of data will result in sensitivity to new physics at the TeV scale, complementing searches at the Large Hadron Collider.
We access the weak charge by measuring the parity-violating asymmetry in the elastic scattering of polarized electrons from protons. Due to the interference of the photon and Z-boson exchange processes, this asymmetry is proportional to the weak charge at low momentum transfers. To achieve the precision necessary to measure the small asymmetry of less than one part per million, we integrated events from eight fused silica detectors by scattering the high-current polarized electron beam on a 35 cm long liquid hydrogen target.