Speaker
Description
The search for neutrinoless double-beta decay ($0\nu\beta\beta$) is among the most promising probes of physics beyond the Standard Model. Its observation would establish lepton-number violation, confirm the Majorana nature of neutrinos, and probe the absolute neutrino mass scale. As upcoming experiments aim to extend half-life sensitivities by up to two orders of magnitude, reliable nuclear matrix elements (NMEs) are critical: without them, neutrino masses and the underlying decay mechanisms cannot be meaningfully constrained. Since $0\nu\beta\beta$ is intrinsically a beyond-Standard-Model process, multiple mechanisms may contribute. While the standard light-neutrino exchange channel has been widely studied, exotic mechanisms—particularly those mediated by heavy particles—remain comparatively unexplored within nuclear theory. Heavy sterile Majorana neutrinos, in particular, are strongly motivated in many extensions of the Standard Model, where they may play a significant or even dominant role in $0\nu\beta\beta$ decay.
We present the first ab initio calculations of the short-range NMEs required to describe exotic $0\nu\beta\beta$ exchange mechanisms in four experimentally relevant isotopes. Starting from two- and three-nucleon interactions derived from chiral effective field theory, we employ the in-medium similarity renormalization group to construct effective valence-space Hamiltonians and consistently evolved decay operators. Because heavy-particle exchange probes short distances, the resulting operators show strong sensitivity to the renormalization procedure. By varying chiral interactions and operator-renormalization schemes, we obtain NME ranges that are consistent with—but generally smaller than—those from phenomenological approaches. Finally, we apply our results with current experimental limits to the 3+1 model, assuming heavy-neutrino-exchange dominates the decay, and obtain constraints in the sterile-neutrino mixing-mass parameter space.
| Keyword-1 | Neutrinoless doube-beta decay |
|---|---|
| Keyword-2 | Nuclear matrix element |
| Keyword-3 | Ab initio nuclear theory |