[en] Measuring the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from protons is a practical technique for determining the contribution of strange quarks to the charge radius and magnetic moment of the nucleon. First results have recently been published by JLab and MIT-Bates

[en] We report the most precise measurement to date of a parity-violating asymmetry in elastic electron-proton scattering. The measurement was carried out with a beam energy of 3.03 GeV and a scattering angle <θ_lab> = 6.0 degrees, with the result A_PV = -1.14 ± 0.24 (stat) ± 0.06 (syst) parts per million. From this we extract, at Q² = 0.099 GeV², the strange form factor combination G_E^s + 0.080 G_M^s = 0.030 ± 0.025 (stat) ± 0.006 (syst) ± 0.012 (FF) where the first two errors are experimental and the last error is due to the uncertainty in the neutron electromagnetic form factor. The measurement significantly improves existing constraints on G_E^s and G_M^s at Q² ∼0.1 GeV². A consistent picture emerges from all measurements at this Q². A combined fit shows that G_E^s is consistent with zero while G_M^s prefers positive values though G_E^s = G_M^s = 0 is compatible with the data at 95% C.L