POINCARE-INVARIANT COUPLED-CHANNEL MODELS FOR MESON PHOTOPRODUCTION

A general procedure for constructing Poincare-invariant mass operators in a helicity basis is presented. The procedure is developed in the f ramework of the instant form of relativistic quantum mechanics, but it can be easily extended to other forms. The method is used to extend a previously developed Poincare-invariant coupled-channel model for the pion-nucleon system to include a photon-nucleon channel. This makes i t possible to carry out calculations on photoproduction from nucleons that satisfy exactly the requirements of special relativity. Methods a re given for deriving potentials that couple the photon-nucleon channe l to the pion-nucleon channel. These potentials are invariant under ga uge transformations of the photon's polarization vector. Amplitudes ob tained by solving the Lippmann-Schwinger equations that arise from the Poincare-invariant mass operators satisfy unitarity, and hence Watson 's theorem for photoproduction amplitudes. The methods presented can a lso be used to develop models for the photoproduction of eta and K mes ons, as well as vector mesons.