The GEP-5 Proton Electric Form Factor Experiment in Hall A at Jefferson Lab

Measurements of the elastic electromagnetic form factors of the proton and neutron have been a central component of Jefferson Lab’s scientific program for almost four decades. and have been important, among other things, in constraining Generalized Parton Distributions and testing the validity of the onset of perturbative quantum chromodynamics. The ratio of $G_E^p/G_M^p$ measured in the late 1990s at Jefferson Lab via the polarization transfer technique, showed that $G_E^p$ falls off more rapidly than $G_M^p$ at $Q^2>1$~GeV$^2$ in contradiction of results from cross section measurements using the Rosenbluth separation method. This led to the measurement of $G_E^p/G_M^p$ by the recoil polarization method up to 8.5~GeV$^2$ in 2008. A new measurement of $G_E^p$ is under preparation with plans to reach as high as 12~GeV$^2$. A precision, high-$Q^2$ measurement such as this will test the log scaling of the ratio of the Sachs proton form factors F2/F1 proposed 2 decades ago by Belitsky et al. (Belitsky, Ji, Yuan PRL 2003) and will be useful in finding the high $Q^2$ behavior of the individual quark flavor form factors. Furthermore, this measurement will provide a clearer picture of the orbital momentum contributions of the quarks to the overall proton spin via the Ji sum rule. With the form factors dropping as $Q^{-4}$ and the cross section as $Q^{-12}$, going to high $Q^2$ is a challenge, requiring a large acceptance for statistics and the ability to discriminate elastic events from the backgrounds at very high background rates. I will discuss the motivation for this new measurement and how the challenges will be met.