New Precision Limit on the Strange Vector Form Factors of the Proton

The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons has been measured at a four-momentum transfer squared $Q^2=0.624{\mathrm{GeV}}^2$ and beam energy $E_b=3.48\mathrm{GeV}$ to be $A_{\mathrm{PV}}=-23.80\pm 0.78(\mathrm{stat})\pm 0.36(\mathrm{syst})$ parts per million. This result is consistent with zero contribution of strange quarks to the combination of electric and magnetic form factors $G_E^s+0.517G_M^s=0.003\pm 0.010(\mathrm{stat})\pm 0.004(\mathrm{syst})\pm 0.009(\mathrm{ff})$, where the third error is due to the limits of precision on the electromagnetic form factors and radiative corrections. With this measurement, the world data on strange contributions to nucleon form factors are seen to be consistent with zero and not more than a few percent of the proton form factors.

Figure 1

Constraints on $G_E^s$ and $G_M^s$ at $Q^2\sim 0.62{\mathrm{GeV}}^2$. The experimental bands are from the results presented in this Letter (HAPPEX-III) and the G0 measurements [7, 21].

Figure 2

Results of strange-quark vector form factors for all measurements of forward-angle scattering from the proton. The solid curve represents a 3% contribution to the comparable linear combination of proton form factors.