Measurement of the proton’s G[sub Ep]/G[sub Mp] ratio to Q[sup 2]=5.6 GeV[sup 2] at JLab

Abstract

The ratio of the electric and magnetic proton form factors, GEp/GMp, has been studied in the Thomas Jefferson National Accelerator Facility (Hall) A at JLab in two experiments, from measurements of the longitudinal and transverse polarization of the recoil proton, Pl and Pt, respectively, in the elastic scattering of polarized electrons, e⃗p→ep⃗. Together, these experiments cover the Q2-range 0.5 to 5.6 GeV2. The results of the second experiment are discussed here; although preliminary, they confirm and extend the finding of the first experiment, that the ratio μpGEp/GMp, which is close to 1 for Q2<0.8 GeV2, decreases approximately linearly with Q2 above this momentum transfer squared, and reaches a value ∼0.19 at Q2=5.6 GeV2. The new results presented here have been obtained by measuring the recoil proton polarization in e⃗p→ep⃗, which provides a better way to obtain GEp, even in the presence of a dominant magnetic form factor. Precision measurements of polarization observables have become possible with the advent of polarized electron beams with ≈100% duty factor, high current and high polarization.