The nuclear matrix element for 2ν double-beta decay is calculated within the quasiparticle random-phase approximation. It is shown that the decay matrix element passes through zero as a function of the strength $g^{\mathrm{pp}}$ of the particle-particle component of the spin-isospin polarization force, neglected previously. The analysis of electron capture/${\beta }^{+}$ decay rates for semimagic neutron-deficient nuclei suggests values for $g^{\mathrm{pp}}$ in the very vicinity of this zero, which gives rise to long lifetimes. The qualitative features of nuclear ββ decay are illustrated with the example of an exactly soluble model.

• Received 14 July 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.3148