DOUBLE-BETA DECAY IN PN-QRPA MODEL WITH ISOSPIN AND SU(4) SYMMETRY CONSTRAINTS

The transition matrix elements for the 0+ --> 0+ double-beta decays ar e calculated for 48Ca, Ge-76, Se-82, Mo-100, Te-128 and Te-130 nuclei, using a delta-interaction. As a guide, to fix the particle-particle i nteraction strengths, we exploit the fact that the missing symmetries of the mean-field approximation are restored in the random phase appro ximation by the residual interaction. Thus, the T = 1, S = 0 and T = 0 , S = 1 coupling strengths have been estimated by invoking the partial restoration of the isospin and Wigner SU(4) symmetries, respectively. When this recipe is strictly applied, the calculation is consistent w ith the experimental limit for the 2v lifetime of 48Ca and it also cor rectly reproduces the 2nu lifetime of 8 Se. In this way, however, the two-neutrino matrix elements for the remaining nuclei are either under estimated (for 76Ge and Mo-100) or overestimated (for Te-128 and Te-13 0) approximately by a factor of 3. With a comparatively small variatio n (< 10%) of the spin-triplet parameter, near the value suggested by t he SU(4) symmetry, it is possible to reproduce the measured T1/2(2nu) in all the cases. The upper limit for the effective neutrino mass, as obtained from the theoretical estimates of Ov matrix elements, is [m(n u)] congruent-to 1 eV. The dependence of the nuclear matrix elements o n the size of the configuration space has been also analyzed.