Low-Energy Theorem in the Radiative Decays of Charged Pions

Abstract

A low-energy theorem is derived for the structure-dependent axial-vector form factor in the radiative decay $\ensuremath{\pi}\ensuremath{\rightarrow}l\ensuremath{\nu}\ensuremath{\gamma}$ using current-algebra techniques. By saturation of the sum rule with a few low-lying resonances, an estimate is made which is compared with experiments.