Double Beta Decay of

The calculation of nuclear matrix elements (NMEs) for neutrinoless double-beta decay (0vßß) remains a major theoretical challenge in nuclear physics, as NMEs for this process cannot be accessed directly through experiment. Cur­rent nuclear structure models yield significantly different predictions for 0 vßß NMEs, making it essential to validate theoretical approaches using observables that are experimentally known. In contrast, two-neutrino double-beta decay (2vßß) has been measured in eleven isotopes, providing a valuable benchmark for testing the reliability of nuclear many-body methods. In this work, we compute the 2vßß decay half-life of 48Ca using the Second Tamm-Dancoff Approximation (STDA) and the Equation of Motion Phonon Method (EMPM). By extending EMPM up to two-phonon configurations, we confirm that the two approaches yield practically identical numerical results, providing a strong internal consistency check. In addition, we study the de­pendence of the 2 vßß NME on the Gamow-Teller and Fermi contributions, on the particle-hole interaction strength parameter gph. Accurately reproducing the experimental 2 vßß half-life requires a substantial quenching of the axial-vector coupling constant gA. Furthermore, our theoretical framework outlines a strat­egy to mitigate this quenching issue by systematically including more complex configurations in the nuclear wave function.