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 〈 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 {Mathematical expression} 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 GEs and GMs at Q2 ∼ 0.1   GeV2. A consistent picture emerges when several measurements at about the same Q2 value are combined: GEs is consistent with zero while positive values are favored for GMs, though GEs = GMs = 0 is compatible with the data at 95% C.L. © 2006 Elsevier B.V. All rights reserved.