Proton Form Factor Ratio, μpGEp/GMpμp GEp /GMp from Double Spin Asymmetry

The ratio of the electric and magnetic form factor of the proton, μpGpE/GpM, has been measured for elastic electron-proton scattering with polarized beam and target up to four-momentum transfer squared, Q2 = 5.66 (GeV/c)2 using the double spin asymmetry for target spin orientation aligned nearly perpendicular to the beam momentum direction.This measurement of μpGpE/GpM agrees with the Q2 dependence of previous recoil polarization data and reconfirms the discrepancy at high Q2 between the Rosenbluth and the polarization- transfer method with a different measurement technique and systematic uncertainties uncorrelated to those of the recoil-polarization measurements. The form factor ratio at Q2=2.06 (GeV/c)2 has been measured as μpGpE/GpM = 0.720 ± 0.176stat ± 0.039sys, which is in agreement with an earlier measurement with the polarized target technique at similar kinematics. The form factor ratio at Q2=5.66 (GeV/c)2 has been determined as μpGpE/GpM = 0.244 ± 0.353stat ± 0.013sys, which represents the highest Q2 reach with the double spin asymmetry with polarized target to date.