The electromagnetic form factors of the nucleon describe charge and current distributions inside the nucleon, and therefore are intimately connected with its structure.
The ratio of the electric and magnetic proton form factors, G_Ep/G_Mp, has been obtained in Hall A at JLab in two experiments, from measurements of the longitudinal and transverse polarization of the recoil proton in elastic ep scattering, P_l and P_t, respectively, with a polarized electron beam. Together, these experiments cover the Q^2-range 0.5 to 5.6 GeV^2. The results of the second experiment are discussed here; they confirm and extend the finding of the first experiment, that the ratio mu_pG_Ep/G_Mp, which is close to 1 for Q^2<0.8 GeV^2, decreases approximately linearly with Q^2 above this momentum transfer squared.
A third step in this investigation has recently been approved by the JLab PAC; experiment 01-109 will extend the Q^2-range to 9 GeV^2; it will take advantage of the higher momentum capability of the HMS in Hall C. It requires installation of a polarimeter in the focal plane of the HMS, and the construction of a large size calorimeter to detect the electron.
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