Parity-Violating Electron Deuteron Scattering and the Proton’s Neutral Weak Axial Vector Form Factor

We report on a new measurement of the parity-violating asymmetry in quasielastic electron scattering from the deuteron at backward angles at $Q^2=0.038$ $(\mathrm{GeV}/c{)}^2$. This quantity provides a determination of the neutral weak axial vector form factor of the nucleon, which can potentially receive large electroweak corrections. The measured asymmetry $A=-3.51\pm 0.57(\mathrm{stat})\pm 0.58(\mathrm{syst})\mathrm{ppm}$ is consistent with theoretical predictions. We also report on updated results of the previous experiment at $Q^2=0.091$ $(\mathrm{GeV}/c{)}^2$, which are also consistent with theoretical predictions.

Figure 1

Results for the detector yield asymmetry (average of ten detectors). Each data point represents the average between two half-wave plate state changes. The out data are plotted as filled circles and the in data as open circles. The sign of the in data is reversed to give the correct sign.

Figure 2

The physics asymmetries measured in SAMPLE II (updated results) and SAMPLE III are plotted as a function of $Q^2$ (solid circles). Also plotted (with offset $Q^2$ for visibility) are the theoretical predictions with the value of $G_A^e$ taken from Ref. [8], and $G_M^s=0.15$ (open circles). The height of the gray rectangles represents the change in the physics asymmetry corresponding to a 0.6 change in $G_M^s$.