Measurement of the Proton Form Factor Ratio GE/GM from Double Spin Asymmetries

Detailed study of the nucleons is very important to understand the nucleus. Electron-Proton scattering is the best tool to probe the nucleus and hence deep into it. In the One-Photon exchange (Born) approximation, the structure of the nucleon characterized by electric and magnetic form factors, GE(Q ) and GM(Q ), which depend only on momentum transfer squared, Q. At Q=0, the proton form factors are defined by GE(0)=1 and G p M=μp where μp = 2.79 is the proton’s magnetic moment and the neutron form factors are defined by GE(0)=0 and GM= μn where μn = -1.91 is the neutron’s magnetic moment. The Rosenbluth separation technique which measures the scattering cross sections by detecting the elastically scattered electrons from the proton target at different beam energies and different electron scattering angles at constant Q is the first method developed worldwide to extract the proton form factors. In addition, experiments that measure the proton and neutron electromagnetic form factors by measuring the polarization observables in elastic electron-nucleon scattering reached high demand over a large Q range with the advent of high current and highly polarized electron beam accelerators such as Jefferson Lab. The proton GE/GM ratio have been extracted from measurement of recoil polarization components of the scattered protons in elastic scattering of polarized electron from an unpolarized proton target. Both longitudinal, Pz and transverse, Px polarization components of the scattered proton’s recoil polarization are depend on GE/GM. By simultaneously measuring both components one can extract the GE/GM from the ratio of polarization measurements, Px/Pz, which cancel the systematic errors of the experiment.