Neutrinoless double-beta (0νββ) decay is a hypothetical nuclear process where two neutrons transmute into two protons, with only two electrons being emitted with no accompanying antineutrinos. The measurement of such a process would imply that neutrinos are Majorana particles (their own antiparticle) and, since lepton number would not be conserved, this would point to an event beyond the Standard Model of particle physics .
The 0νββ decay rate is governed by the nuclear matrix element . Since no measurements are available for this process, we resort to methods of nuclear structure to calculate these magnitudes. In this case, our frame of work is the nuclear shell model, one of the most successful models for nuclear structure.
Using this model as our frame of work, we evaluate for the first time both the leading long-range and the newly acknowledged short-range contributions to the matrix element for the 0νββ decay of the nuclei most relevant for experiments .
In addition, we use shell model results to carry out, for the first time, more accurate calculations when combining them with ab initio quantum Monte Carlo results, which are able to capture additional correlations. We combine the nuclear shell model and quantum Monte Carlo approaches using the generalized contact formalism , and obtain improved results with respect to the standard shell model matrix elements.
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