Constraints on the nucleon strange form factors at Q2∼0.1 GeV2

Abstract 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 ○ , with the result A PV = ( − 1.14 ± 0.24 ( stat ) ± 0.06 ( syst ) ) × 10 −6 . From this we extract, at Q 2 = 0.099 GeV 2 , 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. This result significantly improves current knowledge of G E s and G M s at Q 2 ∼ 0.1 GeV 2 . A consistent picture emerges when several measurements at about the same Q 2 value are combined: G E s is consistent with zero while positive values are favored for G M s , though G E s = G M s = 0 is compatible with the data at 95% C.L.