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 pi N 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 (Delta), and D-13 poles and their crossed poles, as well as pi, rho, and omega exchange terms. The Kroll-Rudermann term and other interaction currents ensure that the model is exactly gauge invariant to all orders in the strong coupling g(pi NN). The threshold value of the E(0+) amplitude is in good agreement with recent estimates obtained from chiral perturbation theory. Elastic unitarity to first order in the charge e (Watson theorem) is maintained up to the two-pion production threshold. The complete development of this model, which gives a good fit to all L less than or equal to 2 multiples up to 770 MeV, is presented.