Constraints on the Nucleon Strange Form Factors at Q2 ∼ 0.1 GeV2

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 〈θlab〉 = 6.0◦, with the result APV = (−1.14±0.24(stat)±0.06(syst))×10−6. From this we extract, at Q2 = 0.099 GeV2, the strange form factor combination G E +0.080Gs M = 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 and G M at Q2 ∼ 0.1 GeV2. A consistent picture emerges when several measurements at about the same Q2 value are combined: G E is consistent with zero while positive values are favored for G M , though G E =Gs M = 0 is compatible with the data at 95% C.L.  2006 Elsevier B.V. PACS: 13.60.Fz; 11.30.Er; 13.40.Gp; 14.20.Dh