Energy dependence of the delta resonance: chiral dynamics in action

There is an important connection between the low-energy theorems of QCD and the energy dependence of the Δ resonance in π-N scattering, as well as the closely related γ* N → N π reaction. The resonance shape is due not only to the strong π-N interaction in the p wave but the small interaction in the s wave; the latter is due to spontaneous chiral symmetry breaking in QCD (i.e., the Nambu-Goldstone nature of the pion). A brief overview of experimental tests of chiral perturbation theory and chiral-based models is presented. (author)