The proton form factor measurements at Jefferson Lab, past and future

Use of the double-polarization technique to obtain the elastic nucleon form factors has resulted in a dramatic improvement of the quality of two of the four nucleon electromagnetic form factors, G Ep and G En . It has also changed our understanding of the proton structure, having resulted in a distinctly different Q 2-dependence for both G Ep and G Mp , contradicting the prevailing wisdom of the 1990’s based on cross section measurements, namely that G Ep and G Mp obey a “scaling” relation μG Ep ∼ G Mp . A related consequence of the faster decrease of G Ep revealed by the Jefferson Lab (JLab) polarization results was the disappearance of the early scaling F 2/F 1 ∼ 1/Q 2 predicted by perturbative QCD. In three experiments, GEp(1), GEp(2) and GEp(3), in Halls A and C at JLab, the ratio of the proton’s electromagnetic elastic form factors, G Ep /G Mp , was measured up to four momentum transfer Q 2 of 8.5 GeV2 with high precision, using the recoil polarization technique. The initial discovery that the proton form factor ratio measured in these three experiments decreases approximately linearly with four-momentum transfer, Q 2, for values above ∼ 1 GeV2, was modified by the GEp(3) results, which suggests a slowing down of this decrease. There is an approved experiment, GEp(5), to continue these measurements to 15 GeV2. A dedicated experimental setup, the super bigbite spectrometer (SBS), will be built for this purpose. It will be equipped with a new focal plane polarimeter to measure the polarization of the recoil protons. In this presentation, I will review the status of the proton elastic electromagnetic form factors, mention succinctly a number of theoretical approaches to describe results and show some features required for the future GEp(5) experiment.