Two-photon exchange in the space-like and time-like region

Julia Guttmann (Mainz)

In the extraction of electromagnetic form factors from electron-proton scattering the discrepancy between data of unpolarized Rosenbluth measurements and of polarization experiments has triggered a whole new field studying the influence of two-photon exchange corrections.  Using the available cross section and polarization data for elastic electron-proton scattering, we provide an extraction of the two-photon exchange amplitudes and present predictions for forthcoming experiments.  We also discuss the influence of two-photon exchange in the time-like region.  An estimate of the two-photon contribution at large momentum transfer, which involves the proton distribution amplitudes, will be discussed at leading order in perturbative QCD.

(Seminarraum II)