Strong violations of the conserved-vector-current hypothesis in pion radiative decay

We study the question whether large isospin-breaking effects due to the quark mass matrix may be expected in the decay π → eν/sub e/γ. This has been predicted previously on the basis of quantum-chromodynamic perturbation theory. However, perturbation theory may be rather misleading in this context because it does not take into account the correct γ5 structure of the vacuum. We show quite generally that isospin-breaking effects vanish in the chiral limit, independently of xi=m/sub d//m/sub u/. An illustration and quantitative estimate is given in the framework of the sigma model