Measurements of the electromagnetic form factor of the Proton at JLab

The ratio of the proton’s elastic electromagnetic form factors $G_{Ep}\mathrm{/}{G}_{Mp}$ was obtained by measuring $P_t$ and $P_{\mathrm{\ell }}$, the transverse and longitudinal recoil proton polarization, respectively. For the elastic reaction $\vec{e}p\to e\vec{p}$, $G_{Ep}\mathrm{/}{G}_{Mp}$ is proportional to $P_t\mathrm{/}{P}_{\mathrm{\ell }}$. The simultaneous measurement of $P_t$ and $P_{\mathrm{\ell }}$ in a polarimeter reduces systematic uncertainties. The results for the ratio $G_{Ep}\mathrm{/}{G}_{Mp}$ measured in Hall A so far show a systematic decrease with increasing $Q^2$, indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. Together these experiments cover the Q${}^2$- range of 0.5 to 5.6 GeV${}^2$. A new experiment is currently being prepared, to extend the Q${}^2$-range to 9 GeV${}^2$ in Hall C.