Nuclear matrix elements of double beta decay from beta decay

Abstract: 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, $g_{\rm pp}$, 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 $g_{\rm pp}$. Rather, arguments are given in favour of using the available data on single beta decay for this purpose.