| The parity-violating asymmetry of the elastic scattering of highly polarized 3.84 GeV electrons from unpolarized protons at Q2 = 0.624 GeV2 has been measured to high precision. The measurement was carried out by the HAPPEX collaboration in Hall A of Jefferson Lab's Continuous Electron Beam Accelerator Facility. This precision measurement required careful control of any potential systematic effects, as well as a very precise determination of the absolute electron-beam polarization. In order to obtain the required precision on the electron-beam polarization measurement, an upgrade of the Hall A Compton polarimeter, and, in particular, the polarimeter's photon-arm detector and data acquisition system, was completed.;A parity-violating asymmetry of APV = --23.80 +/- 0.78(stat) +/- 0.36(syst) ppm was measured. The predicted parity-violating asymmetry in the absence of strange quarks is ANS = --24.062 +/- 0.734 ppm. This allows for the extraction of the linear combination of proton strange-quark form factors GsE + 0.517 GsM = 0.003 +/- 0.010(stat) +/- 0.004(syst) +/- 0.009( ANS), where the third error is due to uncertainties in the nucleon electromagnetic form factors and radiative corrections. This measurement is consistent with zero strange contribution to the proton form factors at Q2 = 0.624 GeV2. |
