Two neutrino 0+ → 0+ double beta decay of 48Ca within the DFT-NCCI framework

Two-neutrino double beta decay ($2\nu\beta\beta$) is a second-order weak-interaction process. Consequently, it is among the rarest radioactive processes observed in nature.\\ The $2\nu\beta\beta$ decay has recently attracted significant attention due to substantial investments in the search for the yet unobserved neutrinoless double beta decay ($0\nu\beta\beta$), a process considered a potential gateway to new physics beyond the Standard Model. Current efforts focus on high-precision half-life measurements and, on the theoretical side, on high-precision calculations of the nuclear matrix elements using various theoretical models.\\ In this talk, we present the results of our seminal calculation of the nuclear matrix element for the $2\nu\beta\beta$ decay 48Ca $\rightarrow$ 48Ti, performed using a post-Hartree-Fock (HF) Density Functional Theory-based No-Core Configuration-Interaction (DFT-NCCI) framework developed by our group. The preliminary value we have obtained for the nuclear matrix element describing this process, $|\mathcal{M}^{2\nu}|$ = 0.056(6) $\text{MeV}^{-1}$, is in excellent agreement with the results of the shell-model study by Horoi et al., which yielded 0.054 (0.064) $\text{MeV}^{-1}$ for the GXPF1A (GXPF1) interactions, respectively.\\ The consistency of our prediction with the shell-model results strengthens our confidence in the nuclear modeling of this extremely rare process, which is of paramount importance for the further modeling of the $0\nu\beta\beta$ decay.