NEUTRON-PROTON CORRELATIONS IN AN EXACTLY SOLVABLE MODEL

We examine isovector and isoscalar neutron-proton correlations in an e xactly solvable model based on the algebra SO(8). We look particularly closely at Gamow-Teller strength and double beta decay, both to isola te the effects of the two kinds of pairing: and to test two approximat ion schemes: the renormalized neutron-proton quasiparticle random phas e approximation (QRPA) and generalized BCS theory. When isoscalar pair ing correlations become strong enough a phase transition occurs and th e dependence of the Gamow-Teller beta(+) strength on isospin changes i n a dramatic and unfamiliar way, actually increasing as neutrons are a dded to an N=Z core. Renormalization eliminates the well-known instabi lities that plague the QRPA as the phase transition is approached, but only by unnaturally suppressing the isoscalar correlations. Generaliz ed BCS theory, on the other hand, reproduces the Gamow-Teller strength more accurately in the isoscalar phase than in the usual isovector ph ase, even though its predictions for energies are equally good everywh ere. It also mixes T=0 and T=1 pairing, but only on the isoscalar side of the phase transition.