Two-neutrino \(0^+ \rightarrow 0^+\) Double-Beta Decay of \(^{48}\mathrm {Ca}\) in the DFT-NCCI Framework

We present the results of our calculation of the nuclear matrix element for the \(2\nu \beta \beta \) decay \(^{48}\mathrm {Ca} \rightarrow {^{48}\mathrm {Ti}}\), 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, \(|\mathcal {M}^{2\nu }| = 0.056(6)\) MeV\(^{-1}\), is in excellent agreement with the results of the shell-model study by Horoi <span class="it">et al.</span> , which yielded 0.054 (0.064) MeV\(^{-1}\) for the GXPF1A (GXPF1) interactions, respectively. It is also in reasonable agreement with the most recent experimental estimate from the review by Barabash, which is 0.068(6) MeV\(^{-1}\), assuming a quenching factor \(qg_{\mathrm {A}} \approx 1\). The consistency of our prediction with the shell-model results strengthens our confidence in the nuclear modeling of this second-order, extremely rare process, which is of paramount importance for the further modeling of the \(0\nu \beta \beta \) decay. Abstract Published by the Jagiellonian University 2026 authors