Pion scattering and isobars in nuclei

General features of pion scattering by systems with equal numbers of neutrons and protons are discussed. To begin with, it is pointed out that most phenomena can be understood within the framework of a simple formalism in terms of an index of refraction for pions traversing nuclear matter. The pion-nucleon scattering amplitude is of resonance type, the resonance being identified with the Δ (1230 MeV) resonance. The Lorentz-Lorenz correction to the scattering, arising from short-range nucleon-nucleon correlations, is first discussed in classical terms, and then later in terms of isobar dynamics. It is found that there are only small differences in descriptions of pion-nucleon scattering in nucleon medium: a) in which the isobar is considered as a composite system of nucleon and pion, b) in which the isobar is considered to be an elementary particle. Self-energy interactions of the isobar with the nuclear medium are estimated, and found to be nearly as attractive as those of a nucleon. In the final section we outline a dynamical approach towards pion scattering from finite nuclei and give instruction on "How to know an isobar when you see one".