Δ excitations in nuclei and the coherent pion decay

The Δ excitations in nuclei induced by intermediate energy charge exchange reactions are analyzed within the isobar-hole model. It is shown that the isovector spin-longidutinal (S.qT) response function of nuclei in the Δ resonance region is shifted down in energy by ∼60 MeV in comparison to the spin-transverse (SxqT) response function. This shift is consistent with pion and photon total cross section data. The coherent pion decay of the Δ resonance is studied by means of the 12C(p, nπ+)12C(g.s) coincidence reaction. The calculations show that the coherent pion production cross section provides a unique signature on the nuclear pionic mode. The coherent pions have a peak energy of E=250 MeV and a strongly forward peaked angular distribution. (orig.)