Nuclear matrix elements of ββ decay from β-decay data

The evaluation of the nuclear matrix elements (NME) of the two-neutrino double beta (2νββ) decay and neutrinoless double beta (0νββ) decay using the proton-neutron quasiparticle random-phase approximation (pnQRPA) is addressed. In particular, the extraction of a proper value of the proton-neutron particle-particle interaction parameter, gpp, of this theory is analyzed in detail. Evidence is shown, that it can be misleading to use the experimental half-life of the 2νββ decay to extract a value for gpp. Rather, arguments are given in favour of using the available data on single beta decay for this purpose.