Three-body forces in nuclear matter from intermediate Δ-states in three-nucleon clusters

Those three-body force contributions in nuclear matter usually generated through a πN scattering amplitude dominated by the Δ(1236) resonance, are here treated as a three-nucleon cluster, in which one of the nucleons becomes, in an intermediate state, a Δ-resonance. All exchange diagrams are calculated and found to significantly reduce the energy per particle from the direct graph. This is contrary to earlier estimates of the exchanges, using more approximate approaches. The resulting attractive contribution is rather small, −1.1 MeV at κ_F = 1.4 fm⁻¹, but the roughly linear density dependence has a crucial effect on the saturation properties. The sensitivity of the results to the correlations used, and to the two-body force spin structure, is displayed. The energy per particle from clusters with three intermediate Δ-resonances is also estimated.