Gauge-Invariant Approach to Meson Photoproduction Including the   Final-State Interaction
  A gauge-invariant formalism is presented for the practical treatment of photo- and electroproduction of pseudoscalar mesons off nucleons that allows an explicit incorporation of hadronic final-state interactions. The semi-phenomenological approach is based on a field theory developed by one of the authors. It generalizes an earlier approach by allowing for systematic improvement of approximations in a controlled manner. The practical feasibility is illustrated by applying the lowest-order result to the photoproduction of both neutral and charged pions. 
H. Haberzettl, K. Nakayama, and S. Krewald