Strange Quark Contributions to Parity-Violating Asymmetries in the Backward Angle G0 Electron Scattering Experiment

We have measured parity-violating asymmetries in elastic electron-proton and quasielastic electron-deuteron scattering at $Q^2=0.22$ and $0.63{\mathrm{GeV}}^2$. They are sensitive to strange quark contributions to currents in the nucleon and the nucleon axial-vector current. The results indicate strange quark contributions of $\lesssim 10\%$ of the charge and magnetic nucleon form factors at these four-momentum transfers. We also present the first measurement of anapole moment effects in the axial-vector current at these four-momentum transfers.

Figure 1

Example of counting rates—${\mathrm{LH}}_2$, 0.684 GeV—for various CED-FPD combinations (FPDs 1 and 2 not used). Electrons from elastic (inelastic) scattering are in the upper right (lower left).

Figure 2

The form factors (a) $G_E^s$, (b) $G_M^s$, and (c) $G_A^e$ determined by the G0 experiment forward- and backward-angle measurements. Error bars show statistical and statistical plus point-to-point systematic uncertainties (added in quadrature); shaded bars below the corresponding points show global systematic uncertainties (for G0 points). For $G_E^s$ and $G_M^s$, the extraction from Ref. [5] as well as the results of the PVA4 (Mainz) experiment [3] are shown. Recent calculations from Adelaide [29] and Kentucky [30] groups are also shown; for the former the uncertainties are smaller than the symbols. For $G_A^{e,T=1}$, results from the SAMPLE experiment [1] are shown together with the calculation of Zhu et al. [12].