The Qweak experiment: First determination of the weak charge of the proton

The Qweak Collaboration has completed a challenging measurement of the parity-violating asymmetry in elastic electron-proton (ep) scattering at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The initial result reported here is extracted from the commissioning part of the experiment, constituting about 4% of the full data set. The parity-violating asymmetry at a low momentum transfer Q2 = 0.025 GeV2 is Aep = −279 ± 35 (stat) ± 31 (syst) ppb, which is the smallest and most precise asymmetry ever measured in ep scattering. This result allowed the first determination of the weak charge of the proton Qp W from a global fit of parity-violating elastic scattering (PVES) results from nuclear targets, where earlier data at higher Q2 constrain uncertainties of hadronic structure. The value extracted from the global fit is Qp W (PVES) = 0.064 ± 0.012, in agreement with the standard model prediction Qp W (SM) = 0.0710 ± 0.0007. The neutral weak charges of up and down quarks are extracted from a combined fit of the PVES results with a previous atomic parity violation (APV) measurement on 133Cs. The analysis of the full Qweak data is ongoing and expected to yield a value for the weak charge to about 5% precision. Because of the suppression of Qp W , such a high-precision measurement will place significant constraints to models of physics beyond the standard model