Large-Scale Shell-Model Analysis of the Neutrinoless ββ Decay of 48Ca

We present the nuclear matrix element for the neutrinoless double-beta decay of Ca-48 based on large-scale shell-model calculations including two harmonic oscillator shells (sd and pf shells). The excitation spectra of Ca-48 and Ti-48, and the two-neutrino double-beta decay of Ca-48 are reproduced in good agreement to the experimental data. We find that the neutrinoless double-beta decay nuclear matrix element is enhanced by about 30% compared to pf-shell calculations. This reduces the decay lifetime by almost a factor of 2. The matrix-element increase is mostly due to pairing correlations associated with cross-shell sd-pf excitations. We also investigate possible implications for heavier neutrinoless double-beta decay candidates.