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

Abstract 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 〈 θ lab 〉=12.3° 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 strange form factor extracted from the data is (G s E +0.392 G s M )=0.025±0.020±0.014 , where the first error is experimental and the second arises from the uncertainties in electromagnetic form factors. This measurement is the first fixed-target parity violation experiment that used either a “strained” GaAs photocathode to produce highly polarized electrons or a Compton polarimeter to continuously monitor the electron beam polarization.