Polarization transfer measurements of proton form factors: deformation by initial collinear photons

It is demonstrated that an emission of collinear photons by the polarized initial electron in elastic electron-proton polarization transfer scattering leads to an apparent shifting of real events with small momentum transfer into the data sample with large momentum transfer. Effectively this shows a fictive enhancement of the cross section at large momentum transfer. However, the enhancement is different for transverse and longitudinal polarizations of the recoil proton. The former is responsible for a deformation of results when extracting the proton electromagnetic form factors ratio from the data on electron-proton polarization transfer scattering. Nevertheless, this effect does not explain the suppression of the Dirac form factor at large momentum transfer completely.