Δ excitations in nuclei and the coherent pion decay

The (p,n) and (3He, t) reactions at intermediate energies have shown that there is a substantial downward energy shift in the excitation spectra of the Δ resonance in nuclei compared with the corresponding spectra in the Proton target. Part of this shift, is caused by a coherent medium effect on the spin-longitudinal response function. The coherent, effect is produced by the attractive π exchange interaction between Δ-hole states in the medium. Evidence of such a coherent mode has recently been obtained in the 12C(p, nπ) and 12C(3He, π) reaction. In this paper, we report results of our calculations or the coherent pion production cross sections. It is shown that the coherent pions provide a unique signature on the nuclear pionic mode; they have a peak energy of Eπ = 250 MeV and a strongly forward peaks angular distribution. It is now important that these results are confirmed and checked by other experiments, such as the 12C(n, nπ+) spinflip transfer experiments at LANPF and 12C(e, e'π0) experiments at CEBAF