New proton form factor ratio measurements in Jefferson Lab Hall C

Experiments E04-108 and E04-019 in Hall C at Jefferson Lab measure the ratio of the proton's electric ($G_E$) and magnetic ($G_M$) form factors using the recoil polarization technique. E04-108 will extend the $Q^2$ reach of this technique to approximately 8.5 GeV$^2$, while E04-019 will measure the form factor ratio with high precision at a fixed $Q^2$ of 2.5 GeV$^2$ and three different values of $\epsilon$ in the range $0.1 \leq \epsilon \leq 0.8$. Measuring the $\epsilon$ dependence of the form factor ratio from polarization transfer at fixed $Q^2$ will establish or significantly constrain the size of two-photon effects in elastic electron-proton scattering. Such effects have been proposed to explain the different values of $G_E/G_M$ obtained from polarization transfer and cross section (Rosenbluth separation) measurements at Q$^2 > 1$ GeV$^2$. E04-019 was completed in January 2008, and E04-108 will be completed in April-June of 2008. Form factor ratios extracted from a preliminary analysis of the data taken so far will be reported and discussed.