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

We show that measurements of the proton’s polarization in [Formula Presented] 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 [Formula Presented] and [Formula Presented] form factors. The expected proton polarization in the timelike region is large, of the order of several tens of percent.