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 [ ⟨θlab⟩=12.3° and ⟨Q2⟩=0.48(GeV/c)2] is chosen to provide sensitivity, at a level that is of theoretical interest, to the strange electric form factor GsE. The result, A=−14.5±2.2ppm, is consistent with the electroweak standard model and no additional contributions from strange quarks. In particular, the measurement implies GsE+0.39GsM=0.023±0.034(stat)±0.022(syst)±0.026(δGnE), where the last uncertainty arises from the estimated uncertainty in the neutron electric form factor.