The Proton Form Factor Measurements at Jefferson Lab, Past and Future

摘要

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, G_(Ep)(1), G_(Ep)(2) and G_(Ep)(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 GeV~2 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 GeV~2, was modified by the G_(Ep)(3) results, which suggests a slowing down of this decrease. There is an approved experiment, G_(Ep)(5), to continue these measurements to 15 GeV~2. 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 G_(Ep)(5) experiment.