Discussion of Isotensor Terms andTViolation in Pion Photoproduction near the First Resonance

Abstract

An analysis is made of the presently available experimental and theoretical information for pion photoproduction and ${\ensuremath{\pi}}^{\ensuremath{-}}$ radiative capture near the first resonance with regard to the possibility of a violation of the $\ensuremath{\Delta}I\ensuremath{\le}1$ rule, and the possibility of a $T$-violating electromagnetic current. An isotensor term is only one of the several possible explanations of the recent ${\ensuremath{\pi}}^{\ensuremath{-}}$ photoproduction data. The radiative-capture data near the first resonance support a substantial $T$ violation in the magnetic dipole amplitude. However, it is not established that this $T$ violation may be ascribed to a single isospin part of the amplitude. Some predictions for experimental tests are given which could clarify the situation.