RPA-APPROACH TO THE EXCITATIONS OF THE NUCLEON PART-II - PHENOMENOLOGY

The tensor-RPA approach developed previously in part I is applied to t he Nambu-Jona-Lasinio (NJL) model, As a first step we investigate the structure of Dirac-Hartree-Fock solutions for a rotationally and isosp in invariant ground-state density. Whereas vacuum properties can be re produced, no solitonic configuration for a system with unit baryon num ber is found. We then solve the tensor-RPA equation employing simple m odels of the nucleon ground state. In general the ph interaction tends to decrease the excitation energies. Due to an enhanced level density at low energies the obtained spectra cannot be matched with the exper imental data when a standard MIT-bag configuration is used, However, w hen the size of the nucleon quark core is reduced to approximate to 0. 35 fm a fair description of the baryon spectrum in the positive-parity channel is achieved. For this purpose the residual interaction turns out to be crucial and leads to a significant improvement compared with the mean-field spectra. (C) 1997 Elsevier Science B.V.