Theory of double beta decay

The nuclear matrix elements for 2 nu double beta decay is calculated. It is shown that the decay matrix element passes through zero as a function of the coupling constant, g/sup pp/, of the particle-particle component of the spin-isospin polarization force, neglected previously. The analysis of EC/β+ decay rates for semi-magic nuclei suggests values for g/sup pp/ in the very vicinity of this zero, thereby yielding long lifetimes and providing a possible resolution of the longstanding discrepancy between calculated and measured 2 nu double beta decay rates. 8 refs., 2 figs