Unitary, gauge invariant, relativistic resonance model for pion photoproduction

Pion photoproduction up to 770 MeV photon laboratory energy is described by a manifestly covariant wave equation, which includes a treatment of the final state πN interactions consistent with the covariant, unitary, resonance model of pi-N scattering previously developed. The kernel of the equation includes nucleon (N), Roper (N*), Delta, and D13 poles and their crossed poles, as well as π, μ, and w exchange terms. The Kroll-Ruderman term and other interaction currents insure that the model is exactly gauge invariant to all orders in the strong coupling, gπNN, and that the low energy theorem is satisfied. Unitarity is maintained up to first order in the charge e (Watson theorem). The complete development of this model, which gives a good fit to all the data up to 770 MeV, is presented