The proton form factor ratio measurements at Jefferson Lab

The ratio of the proton form factors, G_Ep/G_Mp, has been measured from Q² of 0.5 GeV² to 8.5 GeV², at the Jefferson Laboratory, using the polarization transfer method. This ratio is extracted directly from the measured ratio of the transverse and longitudinal polarization components of the recoiling proton in elastic electron-proton scattering. The discovery that the proton form factor ratio measured in these experiments decreases approximately linearly with four-momentum transfer, Q², for values above ∼ 1 GeV², is one of the most significant results to come out of JLab. These results have had a large impact on progress in hadronic physics; and have required a significant re-thinking of nucleon structure. There is an approved experiment at JLab, GEp(5), to continue the ratio measurements to 12 GeV². A dedicated experimental setup, the Super Bigbite Spectrometer (SBS), will be built for this purpose. In this paper, the present status of the proton elastic electromagnetic form factors and a number of theoretical approaches to describe nucleon form factors will be discussed. (authors)