Neutral Photomeson Production and Nucleon Isobars

The magnitude of the neutral photomeson cross section, which is comparable with the cross section for charged mesons, is not given correctly by weak coupling perturbation theory. In fact, the phenomenon appears to depend on a nonlinear behavior of the meson fields corresponding to intermediate or strong coupling. Accordingly, the effect of the existence of nucleon isobars, which is one of the most characteristic features of strong coupling theory, has been investigated through the use of a classical model and is found to give the general features and magnitude of the neutral cross section correctly. The isobars are predicted to be unstable with an excitation energy of three or four hundred Mev.