SELF-CONSISTENT RING APPROXIMATION IN QUANTUM HADRODYNAMICS

Within the framework of quantum hadrodynamics including DELTA-baryons we sum up all ring diagrams in a self-consistent method which is developed as a generalization of the Hartree-Fock approximation. We can demonstrate analytically by using the Kallen-Lehmann representation for the polarization propagator that the ring-energy contributions are purely real under assumptions which we show to be well fulfilled. The resulting equation of state becomes soft compared to the Hartree-Fock result with an incompressibility of kappa almost-equal-to 200-350 MeV, and the effective mass increases to M*/M almost-equal-to 0.8. Following the practice in mean-field and Hartree-Fock calculations so far, we regard the theory as an effective one, and drop all divergent parts of the loop diagrams.