Interplay of Three-Body Interactions in the EOS of Nuclear Matter

The equation of state of symmetric nuclear matter has been investigated within Brueckner approach adopting the charge-dependent Argonne V18 twobody force plus a microscopic three-body force based on a meson-exchange model. The effects on the equation of state of the individual processes giving rise to the three-body force are explored up to high baryonic density. It is found that the major role is played by the competition between the strongly repulsive (σ, ω) exchange term with virtual nucleon-antinucleon excitation and the large attractive contribution due to (σ, ω) exchange with N∗(1440) resonance excitation. The net result is a repulsive term which shifts the saturation density corresponding to the only two-body force much closer to the empirical value, while keeping constant the saturation energy per particle. The contribution from (π, ρ) exchange 3BF is shown to be attractive and rather small. The analysis of the separate three-body force contributions allows to make a comparison with the prediction of Dirac-Brueckner approach which is supposed to incorporate via the dressed Dirac spinors the same virtual nucleon-antinucleon excitations as in the present three-body force. The numerical results suggest that the three-body force components missing from the Dirac-Brueckner approach are not negligible, especially at high density. The calculation of the nuclear mean field and the effective mass shows that the three-body force affects to a limited extent such properties. PACS: 25.70.-z; 13.75.Cs; 21.65.+f; 24.10.Cn