2-VACUA RPA AND THE 2-NEUTRINO DOUBLE-BETA DECAY

A new method ''two-vacua random-phase approximation'' (TVRPA) for the description of the two-neutrino nuclear double-beta decay is developed . The nuclear wave functions of the intermediate odd-odd nucleus are c onstructed as linear combinations of proton-particle neutronhole excit ations from the ground state of the initial even-even (N, Z) nucleus a nd, at the same time, as neutron-particle proton-hole excitations from the ground state of the final (N - 2, Z + 2) nucleus. The Ritz variat ional principle is used to obtain RPA-like equations. The ground state in the initial and final nucleus is approximated with HFB wave functi ons including proton-neutron pairing. A projection on numbers of proto ns and neutrons is performed additionally on the wave functions in bot h nuclei to restore the exact number of particles, broken by the quasi -particle transformation. The method is applied, as a preliminary test , to the 2nu double-beta decay in 48(20)Ca28. More realistic B(GT-) an d B(GT+) Gamow-Teller strength distributions than QRPA and PQRPA are o btained. No dependence is found on the particle-particle strength para meter g(pp), renormalizing Brueckner reaction matrix elements of the B onn potential.