Chiral Loops

Abstract

The correct Feynman rules for chiral-invariant theories differ from the naive rules. The difference is such that the soft-pion theorems, including the vanishing of the pion mass, are preserved to each order of peturbation theory. In particular, there are special definitions of the pion field for which all higher-order corrections to the off-shell pion-pion scattering amplitude cancel at low momenta.