Extracting information on the 0νββ decays from the 2νββ decays

We have analyzed the relation between the two-neutrino (2νββ) and neutrinoless (0νββ) double beta decays of 76Ge, 82Se, 100Mo, and 116Cd. The relevant nuclear matrix elements have been calculated by using the proton-neutron quasiparticle random-phase approximation (pn-QRPA) with realistic two-body interactions. The dependence of the calculated matrix elements on the strength gpp of the particle-particle part of the proton-neutron two-body interaction is investigated. Recently a procedure was proposed where data on 2νββ-decay half-lives could be used to derive appropriate values of gpp for calculating the 0νββ-decay matrix elements. Following this procedure, we have determined the allowed values of gpp by including experimental errors of the measured 2νββ-decay half-lives and the uncertainties in the axial-vector coupling constant gA. This set of gpp values is used to predict single-beta and 0νββ observables. Careful study of these observables points to serious shortcomings in the adopted procedure.