| World data of elastic and inelastic scattering of electrons and positrons on nucleon and nuclei are
compiled and discussed. The evidence of an experimental charge asymmetry different from zero,
being a pure quantum effect, would be a signature of contributions beyond the Born approximation.
After reviewing the published results, we compare the elastic data to a calculation which includes
the box diagram due to two-photon exchange. We show that all the data on the cross section ratio,
in the limit of their precision, do not show evidence of sizable hard two-photon contribution, which
however should be present if it constitutes the explanation for the difference between Rosenbluth
and polarization measurements of the proton electromagnetic form factors. |
