Measurement of the neutral weak form factors of the proton

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The kinematic point [ $〈{\theta }_{\mathrm{lab}}〉=12.3\mathrm{}°$ and $〈Q^2〉=0.48\mathrm{}(\mathrm{GeV}/c{)}^2$] is chosen to provide sensitivity, at a level that is of theoretical interest, to the strange electric form factor $G_E^s$. The result, $A=-14.5\mathrm{}\pm 2.2\mathrm{ppm}$, is consistent with the electroweak standard model and no additional contributions from strange quarks. In particular, the measurement implies $G_E^s{+0.39G}_M^s=0.023\mathrm{}\pm 0.034\left(\mathrm{stat}\right)\pm 0.022\left(\mathrm{syst}\right)\pm 0.026\left(\delta G_E^n\right)$, where the last uncertainty arises from the estimated uncertainty in the neutron electric form factor.