The ratio of the proton's electric to magnetic form factor, $G_E/G_M$, can be extracted in elastic electron-proton scattering by measuring cross sections, beam-target asymmetry, or recoil polarization. Separate determinations of $G_E/G_M$ by cross sections and recoil polarization observables disagree for $Q^2>1$ (GeV/$c){}^2$. Measurement by a third technique might uncover an unknown systematic error in either of the previous measurements. The beam-target asymmetry has been measured for elastic electron-proton scattering at $Q^2$ = 1.51 (GeV/$c){}^2$ for target spin orientation aligned perpendicular to the beam momentum direction. This is the largest $Q^2$ at which $G_E/G_M$ has been determined by a beam-target asymmetry experiment. The result, $\mu G_E/G_M=0.884\pm 0.027\pm 0.029$, is compared to previous world data.

• Received 2 June 2006

DOI:https://doi.org/10.1103/PhysRevC.74.035201