Measurements of GEp/GMp to high Q2 and search for 2{gamma} contribution in elastic ep at Jefferson Lab

The ratio, μpGEp/GMp, where μp is the proton magnetic moment, has been measured extensively over the last decade at the Jefferson Laboratory, using the polarization transfer method. This ratio is extracted directly from the measured ratio of transverse to longitudinal polarizations components of the recoiling proton in elastic electron-proton scattering. The polarization transfer results are of unprecedented high precision and accuracy, due in large part to the small systematic uncertainties associated with the experimental technique. Prior to these measurements, the form factors were empirically observed to exhibit dipole forms, such that μpGEp/GMp ≈ 1 over all regions of momentum transfer studied. With the Hall A results confirming that the ratio μpGEp/GMp shows a steady decrease below unity as a function of Q2, beginning around Q2 ≈ 1 GeV2, discussions revolving around the implication of this deviation from dipole behavior for the structure of the proton have been accompanied by renewed experimental interest in these elastic form factors. Starting in the fall of 2007, two new experiments, GEp-III and GEp-2γ in Hall C at JLab, measured the form factor ratio, GEp/GMp; the GEp-III experiment pushed the highest Q2 limit from 5.6 to 8.49 GeV2, with intermediate points at 5.2 and 6.8more » GeV2, and the GEp-2γ experiment measured the ratio in three different kinematics at the constant value Q2=2.5 GeV2, by changing beam energy and detector angles. Preliminary results from both experiments are reported.« less