The role of massive states in chiral perturbation theory

Abstract

I review some basic facts about the chiral limit of QCD. This allows to formulate an effective field theory below the chiral symmetry breaking scale, chiral perturbation theory (CHPT). I show that for threshold reactions, the spectrum of QCD is most economically encoded in a set of coupling constants of operators of higher chiral dimension. A consistent scheme to incorporate the $Δ(1232)$ is also discussed and some examples are given. It is stressed that more precise low--energy data are needed to further test and sharpen the resonance saturation hypothesis in the presence of baryons.