THE DELTA-POTENTIAL IN NUCLEAR-MATTER FROM A REALISTIC INTERACTION

Previous studies on the nuclear matter properties have shown the relev ance of the DELTA isobar degrees of freedom, as well as the need of an accurate estimate of the corresponding DELTA potential. Motivated by this result, we have calculated the potential energy of the DELTA isob ar in nuclear matter with the same model nucleon-nucleon interaction, the Argonne upsilon28, which was previously used in the study of nucle ar matter. This potential includes all possible transitions from NN st ates to NDELTA and DELTADELTA ones, and give an excellent fit to nucle on-nucleon phenomenological phase shifts and deuteron properties. All diagrams with one nucleon hole-line are summed up in the framework of a G-matrix approach. The DELTA potential so obtained was found to be w eakly attractive at low density, becoming repulsive at increasing dens ity with a value of 50 MeV at saturation. The repulsive potential rece ives the main contribution from the isotopic spin T = 2 channels and i t appears to be in contradiction with the phenomenology on DELTA produ ction in nuclei. The sensitivity to possible simple modifications of t he potential is explored and it is concluded that complete refitting o f the parameters would be necessary in order to remove the contradicti on with phenomenology, while keeping the agreement with nucleon-nucleo n data. This result indicates the strong constraints that the phenomen ology on the DELTA isobar imposes on any realistic nucleon-nucleon pot ential, to be used in self-consistent calculations of nuclear matter p roperties, which include DELTA degrees of freedom. Possible improvemen ts of the interaction are briefly indicated.