Neutrinoless $\beta\beta$-decay nuclear matrix elements from two-neutrino $\beta\beta$-decay data

We study two-neutrino ($2\nu\beta\beta$) and neutrinoless double-beta ($0\nu\beta\beta$) decays in the nuclear shell model and proton-neutron quasiparticle random-phase approximation (pnQRPA) frameworks. Calculating the decay of several dozens of nuclei ranging from calcium to xenon with the shell model, and of $\beta\beta$ emitters with a wide range of proton-neutron pairing strengths in the pnQRPA, we observe good linear correlations between $2\nu\beta\beta$- and $0\nu\beta\beta$-decay nuclear matrix elements for both methods. We then combine the correlations with measured $2\nu\beta\beta$-decay half-lives to predict $0\nu\beta\beta$-decay matrix elements with theoretical uncertainties based on our systematic calculations. Our results include two-body currents and the short-range $0\nu\beta\beta$-decay operator.