Ab initio benchmarks of neutrinoless double beta decay in light nuclei with a chiral Hamiltonian

Abstract

We report ab initio benchmark calculations of nuclear matrix elements (NMEs) for neutrinoless double-beta ($0\hat{I}½{\hat{I}}^2{\hat{I}}^2$) decays in light nuclei with mass number ranging from $A=6$ to $A=22$. We use the transition operator derived from light-Majorana neutrino exchange and evaluate the NME with three different methods: two variants of in-medium similarity renormalization group (IMSRG) and importance-truncated no-core shell model (IT-NCSM). The same two-plus-three-nucleon interaction from chiral effective field theory is employed, and both isospin-conserving ($\hat{I}”T=0$) and isospin-changing ($\hat{I}”T=2$) transitions are studied. We compare our resulting ground-state energies and NMEs to those of recent ab initio no-core shell model and coupled-cluster calculations, also with the same inputs. We show that the NMEs of $\hat{I}”T=0$ transitions are in good agreement among all calculations, at the level of 10%. For $\hat{I}”T=2$, relative deviations are more significant in some nuclei. The comparison with the exact IT-NCSM result allows us to analyze these cases in detail, and indicates the next steps towards improving the IMSRG-based approaches. The present study clearly demonstrates the power of consistent cross-checks that are made possible by ab initio methodology. This capability is crucial for providing meaningful many-body uncertainties in the NMEs for the $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decays in heavier candidate nuclei, where quasi-exact benchmarks are not available.

16 pages with 15 figures, PRC in press