BOSON MAPPINGS AND 4-PARTICLE CORRELATIONS IN ALGEBRAIC NEUTRON-PROTON PAIRING MODELS

Neutron-proton pairing correlations arts studied within the context of two solvable models, one based on the algebra SO(5) and the other on the algebra SO(8). In both of these models, particles interact in L = 0 pairs only. Boson-mapping techniques are applied to these models and shown to provide a convenient methodological tool both for solving su ch problems and for gaining useful insight into general features of pa iring. We first focus on the SO(5) model, which involves generalized T =1 pairing. Neither boson mean-field methods nor fermion-pair approxim ations an able to describe in detail neutron-proton pairing in this mo del. The analysis suggests, however, that the boson Hamiltonian obtain ed from a mapping of the fermion Hamiltonian contains a pairing force between bosons, pointing to the importance of boson-boson (or equivale ntly four-fermion) correlations with isospin T=0 and spin S=0. These c orrelations are investigated by carrying out a second boson mapping. C losed forms for the fermion wave functions are given in terms of the f ermion-pair operators. Similar techniques are applied-albeit in less d etail-to the SO(8) model, involving a competition between T=1 and T=0 pairing. Conclusions similar to those of the SO(5) analysis are reache d regarding the importance of four-particle correlations in systems in volving neutron-proton pairing.