Current algebra calculation of low-energy pion photoproduction for physical pions

Abstract

Current and field commutators are used to derive Ward identities for the non-pole part of the pion photoproduction amplitude. The assumption that this part of the amplitude is as smoothly varying in the four-momenta as the Ward identities allow then leads directly to predictions for the low-energy photoproduction amplitudes for physical pions. The threshold values of the multipoles thus predicted are compared with the available data. The present calculation reduces to the earlier soft-pion result in the usual limit, but includes also explicit forms and magnitudes for the corrections to the soft-pion limit.