DELTA EXCITATIONS IN NUCLEI AND THEIR DECAY PROPERTIES

A formalism for the calculation of intermediate energy charge exchange reactions exciting the DELTA resonance region in nuclei is presented. The nuclear structure part of the formalism is based on the isobar-ho le model and the nuclear reaction part is treated within the distorted -wave impulse approximation (DWIA). In the nuclear structure part, all important nuclear medium effects are included, such as nucleon and is obar binding, two-body DELTA isobar-nucleon hole correlations, and int ermediate coupling to multiparticle-multihole channels. The latter cou pling is treated phenomenologically through a DELTA spreading potentia l. Explicit account is alw taken of the nucleon knockout mode and the related nucleon particle-nucleon hole correlations. In order to perfor m the calculations, we first set up coupled-channel equations for the excited nucleon and DELTA, which are slightly, but importantly, transf ormed into equations for localized functions only. We solve these equa tions by using the Lanczos method. The resultant formalism allows us t o calculate cross sections for DELTA excitation, quasielastic scatteri ng, and low-lying Gamow-Teller excitations on the same footing. In thi s way a detailed study of the nuclear medium effects on the DELTA isob ar can be done. Particular attention is paid to the DELTA-hole correla tions in the spin-longitudinal (S . q T) channel. It is shown that the coherent pion production events in the exclusive C-12(p, npi+)C-12(g. s.) and C-12(He-3, tpi+)C-12(g.s.) reactions provide a unique signatur e on the nuclear pionic mode.