Interference Effects of the Retardation Term in Pion Photoproduction

It is shown that the difference in behavior of the high-energy (>450 Mev c.m.) total photoproduction cross sections for π⁺ and π⁰ can be explained by the presence of the retardation term in the case of the π⁺ production. The analogy with the behavior at the (3/2, 3/2) resonance is noted. The discussion naturally provides an explanation for the difference in center-of-mass energies of the lower high-energy peak found in π^--p scattering and the corresponding peak in the π⁺ photoproduction. It is felt that the discussion contributes some evidence for the resonance nature of the peaks.