Simple dynamical model for γN → π+-Δ

An elementary phenomenological extension of the gauge-invariance- constrained dynamical amplitudes for γN → π+-Δ is considered. The extended amplitudes are parameterized with a real (minimal gauge-invariant extension of one-pion-exchange) background term and simple Regge-pole forms. The two free parameters which both appear in the unnatural-parity-exchange component are determined by comparison with dsigma''/dt data for γp → π-Δ++. The remainder of the natural-parity exchange is represented by rho and A2 exchange. Predictions are made for dsigma/dt and Ψ, for the various charge states and the unnatural-parity-exchange components are well described in each case