Skyrme SV density-functional analysis of the $2\nu\beta\beta$ decay in $^{76}$Ge

We present a theoretical study of the two-neutrino $0^+ \rightarrow 0^+$ double beta decay of $^{76}$Ge within the No-Core Configuration-Interaction framework based on the Skyrme SV density functional. We analyze three allowed decay scenarios distinguished by the $[n,m] \equiv [(\nu g_{9/2})^n, (\pi g_{9/2})^m]$ occupancy of the $0g_{9/2}$ intruder orbital, which remains conserved to high precision, as well as by the triaxiality of the daughter nucleus. The resulting $2\nu\beta\beta$ nuclear matrix element is found to depend strongly on the scenario. For the energetically favored $[4,2]$ occupancy, we obtain $|\mathcal{M}^{2\nu}| = 0.069(7)$~MeV$^{-1}$. For the $[6,0]$ occupancy, the matrix element further depends on the triaxiality parameter $\gamma$ of the two coexisting, closely lying minima in $^{76}$Se, yielding $|\mathcal{M}^{2\nu}| = 0.040(4)$~MeV$^{-1}$ at $\gamma = 17.7^\circ$ and $|\mathcal{M}^{2\nu}| = 0.22(2)$~MeV$^{-1}$ at $\gamma = 41.9^\circ$. The latter result is consistent with the empirical value reported by A. S. Barabash, $|\mathcal{M}^{2\nu}| = 0.204(14)$~MeV$^{-1}$, while the two former results are comparable to existing calculations based on energy-density-functional frameworks. Our calculations reveal challenges in the precise determination of the $|\mathcal{M}^{2\nu}|$ for the $^{76}$Ge decay. The structural complexity, triaxiality, and shape coexistence identified in the analyzed nuclei imply a strong sensitivity to fine details of the interaction and configuration mixing. This, in turn, explains the difficulties in theoretical modeling of the $|\mathcal{M}^{2\nu}|$ matrix elements for the $^{76}$Ge decay, which vary by almost an order of magnitude in the available literature.