Proton Form Factor Ratio, $\mu_p G_E^p/G_M^p$ from Double Spin Asymmetry

The ratio of the electric to magnetic form factors of the proton, mu(p)G(E)(p)/G(M)(p), has been measured for elastic electron-proton scattering with polarized beam and target up to four-momentum transfer squared Q(2) = 5.66 (GeV/c)(2) using double spin asymmetry for target spin orientation aligned nearly perpendicular to the beam momentum direction. This measurement of mu(p)G(E)(p)/G(M)(p) agrees with the Q(2) dependence of previous recoil polarization data and reconfirms the discrepancy at high Q(2) 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 Q(2) = 2.06 (GeV/c)(2) has been measured as mu(p)G(E)(p)/G(M)(p) = 0.720 +/- 0.176(stat) +/- 0.039(sys), which is in agreement with an earlier measurement using the polarized target technique at similar kinematics. The form factor ratio at Q(2) = 5.66 (GeV/c)(2) has been determined as mu(p)G(E)(p)/G(M) = 0.244 +/- 0.353(stat) +/- 0.013(sys), which represents the highest Q(2) measurement reached using double spin asymmetries with polarized target to date.