Delta-nucleon and pion-nucleus interactions

The effect of the $\Delta -N$ interaction in elastic and inelastic pion scattering is discussed. In elastic scattering, the two-body $\Delta -N$ interaction generates both $\Delta$ self-energy and $\Delta -h$ vertex corrections. The relative importance of these two kinds of medium corrections is discussed analytically. Numerical calculations are presented which show in detail the role of the $\Delta -N$ interaction in elastic scattering. In pion-nucleus inelastic scattering, a new reaction mechanism appears due to the two-body nature of the $\Delta -N$ interaction. The qualitative characteristics of these processes induced by $\Delta -N$ collisions are discussed. Deviations from quasifree isospin ratios in knockout reactions and excitations of specific nuclear levels are signatures of the interference between processes generated by the $\pi -N$ and $\Delta -N$ interaction, respectively. In the formalism presented, the order of magnitude of these modifications can be related to the size of the experimentally observed true pion absorption cross sections.