New measurement of parity violation in elastic electron-proton scattering and implications for strange form factors

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The result is A = -15.05 +/- 0.98(stat) +/- 0.56(syst) ppm at the kinematic point (theta (lab)) = 1 2.3 degrees and (Q(2)) = 0.477 (GeV/c)(2). Both errors are a factor of two smaller than those of the result reported previously. The value for the str ange form factor extracted from the data is (G(E)(s) + 0.392 G(M)(s)) = 0.0 25 +/- 0.020 +/- 0.014. where the first error is experimental and the secon d arises from the uncertainties in electromagnetic form factors. This measu rement is the first fixed-target parity violation experiment that used eith er a "strained" GaAs photocathode to produce highly polarized electrons or a Compton polarimeter to continuously monitor the electron beam polarizatio n. (C) 2001 Published by Elsevier Science B.V.