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 hlabi = 6.0◦, with the result APV = (−1.14 ± 0.24 (stat) ± 0.06 (syst)) × 10−6.
From this we extract, at Q2 = 0.099GeV2, the strange form factor combination Gs
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. The measurement
significantly improves existing constraints on Gs
E and Gs
M at Q2 ∼ 0.1GeV2. A consistent picture
emerges from all measurements at this Q2. A combined fit shows that Gs
E is consistent with zero
while Gs
M prefers positive values though Gs
E = Gs
M = 0 is compatible with the data at 95% C.L. |
