Measurement of Strange-Quark Contributions to the Nucleon's Form Factors atQ2=0.230(GeV/c)2

We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a $Q^2$ of $0.230(\mathrm{G}\mathrm{e}\mathrm{V}/c{)}^2$ and a scattering angle of ${\theta }_e=30°–40°$. Using a large acceptance fast ${\mathrm{P}\mathrm{b}\mathrm{F}}_2$ calorimeter with a solid angle of $\Delta \Omega =0.62\mathrm{s}\mathrm{r}$, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is $A_{\mathrm{p}\mathrm{h}\mathrm{y}\mathrm{s}}=(-5.44\pm {0.54}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {0.26}_{\mathrm{s}\mathrm{y}\mathrm{s}})\times {10}^{-6}$. The standard model expectation assuming no strangeness contributions to the vector form factors is $A_0=(-6.30\pm 0.43)\times {10}^{-6}$. The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is $G_E^s+0.225G_M^s=0.039\pm 0.034$ or $F_1^s+0.130F_2^s=0.032\pm 0.028$.