Abstract
We investigate the corrections to electron-proton scattering and proton-antiproton annihilation into an electron-positron pair due to the two-photon intermediate state in a dispersion-theoretic treatment in terms of the proton Compton scattering amplitude. The possible resonances in the annihilation channel which might enhance this contribution are discussed. It turns out in the case of negligible electron mass that up to spin 2 only a 2+ particle can contribute via real Compton scattering, which could give a 10% effect in the cross-section with reasonable choice of the coupling constants. Even if we relax the restriction that the two-photon intermediate state should be real, scalar and pseudoscalar particles do not contribute; however, a 1+ particle might appear.
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