Abstract
The behaviour of nucleon resonances in nuclei excited by electromagnetic and hadronic probes is discussed. Special emphasis is placed on the Δ(1232) isobar propagation which is explained in terms of an interplay of mesonic, nuclear, and Δ dynamics. Inclusive cross section data of the (p,n) and ( 3He,t) charge exchange reactions show that there is a substantial downward energy shift of the Δ resonance in nuclei compared to the Δ excitation of the proton target. A large part of this shift is caused by a nuclear medium effect on the spin-longitudinal response function. This response function is shifted down in energy by 60 MeV relative to the spin-transverse response function measured in photon scattering and in inelastic electron scattering experiments. The shift is caused by the attractive π-exchange interaction between Δ-particle nucleon-hole tates leading to a collective pionic mode in the nucleus. The various damping mechanisms of this mode are discussed. The methods used for the description of Δ excitations in nuclei can also be applied to the study of other N ∗ resonances in the nucleus.
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