The two photon exchange amplitude is investigated in frame of analytic
properties of the virtual Compton scattering amplitude as a function of
the invariant mass squared of the intermediate hadronic state. A sum
rule is built, based on arguments from analyticity. It relates the
differential cross section of elastic electron-proton scattering including
form factors, and the cross section of inelastic scattering channel,
with a contribution of nucleon anti-nucleon pair production arising
from the Fermi statistics. The last term is calculated in frame of
a simple model of nucleon-pion interaction. |
