Unitary-model-operator approach to nuclear effective interaction. ii: Effects of three-body cluster

The problem of the short-range correlations in nuclei is studied in the framework of the unitary-model-operator approach. A unitary-transformed Hamiltonian is introduced and given in a cluster expansion form. A general theory of treating the effects of the three-body cluster teim is proposed. It is emphasized that the one-body average field representing the dispersive effect of medium plays an important role in the evaluation of the contributions of the three-body cluster term to the one- and two-body effective interactions acting among valence particles. It is shown that the core-polarization term with rather high 3P-1h intermediate states is derived naturally from the analysis of the three-body cluster term and it appears as the leading correction term to the one- and two-body effective interactions. The relation is discussed between the present approach and the usual linked-diagram theory based on the G matrix