Single-spin polarization effects and the determination of timelike proton form factors

We show that measurements of the proton’s polarization in $e^{+}{e}^{-}\to p\overline{p}$ strongly discriminate between analytic forms of models which fit the proton form factors in the spacelike region. In particular, the single-spin asymmetry normal to the scattering plane measures the relative phase difference between the timelike $G_E$ and $G_M$ form factors. The expected proton polarization in the timelike region is large, of the order of several tens of percent.