Testing the Standard Model by Precision Measurement of the Weak Charges of Quarks

In a global analysis of the latest parity-violating electron scattering measurements on nuclear targets, we demonstrate a significant improvement in the experimental knowledge of the weak neutral-current lepton-quark interactions at low energy. The precision of this new result, combined with earlier atomic parity-violation measurements, places tight constraints on the size of possible contributions from physics beyond the standard model. Consequently, this result improves the lower-bound on the scale of relevant new physics to $\sim 1\mathrm{TeV}$.

Figure 1

Normalized, parity-violating asymmetry measurements on a proton target, extrapolated to the forward-angle limit using our analysis of all world data on PVES (see text). The extrapolation to $Q^2=0$ measures the proton’s weak charge, where the previous experimental knowledge (within uncertainties on the neutron weak charge) is shown by the triangular data point, and the standard model prediction by the star. The solid curve and shaded region indicate, respectively, the best fit and 1-$\sigma$ bound, based upon our global fit to all electroweak data. The dotted curve shows the resulting fit if one incorporates the theoretical estimates [19] of the anapole form factors of the nucleon.

Figure 2

Knowledge of the neutral-weak effective couplings of Eq. (1). The dotted contour displays the previous experimental limits (95% CL) reported in the PDG [14] together with the prediction of the standard model (black star). The filled ellipse denotes the new constraint provided by recent high-precision PVES scattering measurements on hydrogen, deuterium, and helium targets (at 1 standard deviation), while the solid contour (95% CL) indicates the full constraint obtained by combining all results. All other experimental limits shown are displayed at 1 standard deviation.

Figure 3

Constraints on new physics beyond the standard model in terms of the mass scale versus the flavor mixing angle. New physics is ruled out at the 95% CL below the curves. The long-dashed curve shows the limits before this work, corresponding to the dotted ellipse of Fig. 2. The solid curve shows the new limits based on the solid contour in Fig. 2. The short-dashed curve displays the reach of the future proton weak charge measurement to be performed at Jefferson Lab, assuming agreement with the standard model is observed.