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

Abstract

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, GEp and GEn. It has also changed our understanding of the proton structure, having resulted in a distinctly different Q2-dependence for both GEp and GMp, contradicting the prevailing wisdom of the 1990’s based on cross section measurements, namely that GEp and GMp obey a “scaling” relation μGEp ∼ GMp. A related consequence of the faster decrease of GEp revealed by the Jefferson Lab (JLab) polarization results was the disappearance of the early scaling F2/F1 ∼ 1/Q2 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, GEp/GMp, was measured up to four momentum transfer Q2 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, Q2, 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.