Photoproduction of Positive Pions in Hydrogen in the Angular Range7°<θc.m.<27°and Photon Energy Range220 Mev<k<390 Mev

Relative measurements of the cross sections for the production of positive pions by photons on protons have been carried out in the range of c.m. angles $7°<{\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}<27\mathrm{}°$ for four photon energies from 220 to 390 Mev. Positive pions were detected via the decay positron from the $\pi -\mu -e$ chain which was observed in a series of gates following a pulse of photons and electrons from the Mark III linear accelerator. The results have been compared with new dispersion theory calculations using a range of values of the coupling constant and resonance energy. The experimental data at these small angles are in good agreement with the calculations; however, the fit of the calculations with available data at larger angles is not satisfactory. It is shown that the fit in either case is substantially poorer if the so-called "retardation term," i.e., the diagram in which the photon is absorbed by a virtual meson, is omitted.