Pion photoproduction on nucleons in a covariant hadron-exchange model

We present a relativistic dynamical model of pion photoproduction on the nucleon in the resonance region. It offers several advances over the existing approaches. The model is obtained by extending our πN-scattering description to the electromagnetic channels. The resulting photopion amplitude is thus unitary in the πN, γN channel space; Watson’s theorem is exactly satisfied. At this stage we have included the pion, nucleon, Δ(1232) resonance degrees of freedom. The ρ and ω meson exchanges are also included, but play a minor role in the considered energy domain (up to √s=1.5 GeV). In this energy range the model provides a good description of all the important multipoles. We have allowed for only two free parameters—the photocouplings of the Δ resonance. These couplings are adjusted to reproduce the strength of corresponding resonant-multipoles M₁₊ and E₁₊ at the resonance position.