Ab Initio Neutrinoless Double-Beta Decay Matrix Elements for Ca48, Ge76, and Se82

We calculate basis-space converged neutrinoless ββ-decay nuclear matrix elements for the lightest candidates: 48Ca, 76Ge, and 82Se. Starting from initial two- and three-nucleon forces, we apply the ab initio in-medium similarity renormalization group to construct valence-space Hamiltonians and consistently transformed ββ-decay operators. Here, we find that the tensor component is non-negligible in 76Ge and 82Se, and the resulting nuclear matrix elements are overall 25%–45% smaller than those obtained from the phenomenological shell model. While a final matrix element with uncertainties still requires substantial developments, this work nevertheless opens a path toward a true first-principles calculation of neutrinoless ββ decay in all nuclei relevant for ongoing large-scale searches.