Elastic Scattering of Polarized Electrons by Polarized Protons

We report on a new type of high-energy electron-proton scattering experiment in which longitudinally polarized electrons are scattered from longitudinally polarized protons. The asymmetry in elastic scattering at ${Q}^{2}=0.765$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ was measured; our result agrees with the theoretical asymmetry and determines the sign of $\frac{{G}_{E}}{{G}_{M}}$ to be positive.