Single-Spin Polarization Effects and the Determination of Timelike Proton Form Factors∗

We show that measurements of the proton’s polarization in e+e− → pp̄ 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 GE and GM form factors. The expected proton polarization in the timelike region is large, of order of several tens of percent.