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 [ $〈{\ensuremath{\theta}}_{\mathrm{lab}}〉\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}12.3\ifmmode^\circ\else\textdegree\fi{}$ and $〈{Q}^{2}〉\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.48(\mathrm{GeV}/c{)}^{2}$] is chosen to provide sensitivity, at a level that is of theoretical interest, to the strange electric form factor ${G}_{E}^{s}$. The result, $A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}14.5\ifmmode\pm\else\textpm\fi{}2.2\mathrm{ppm}$, is consistent with the electroweak standard model and no additional contributions from strange quarks. In particular, the measurement implies ${G}_{E}^{s}{+0.39G}_{M}^{s}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.023\ifmmode\pm\else\textpm\fi{}0.034(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.022(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}0.026(\ensuremath{\delta}{G}_{E}^{n})$, where the last uncertainty arises from the estimated uncertainty in the neutron electric form factor.