The interplay between nucleonic and nuclear spin-isospin excitations

Following a brief review of pion-nucleon coupling in relativistic quark models and the introduction of the Δ(1232) at the quark level, we develop the nuclear spin-isospin response function in a framework of nucleons and Δ′s interacting through exchange of mesons. The spin-isospin dependent particle-hole interaction will be elaborated and various different aspects of the Landau-Migdal parameter g′ for nucleons and Δ′s will be discussed. We summarize the evidence for broad Δ-hole states at high excitation energies (200-400 MeV). Then the impact of the existence of such Δ-hole excitations on the quenching of Gamow-Teller and magnetic isovector spin transitions at low energy is discussed in combination with conventional nuclear mechanisms like core polarization and ground state correlations.