Double Beta Decay of 48Ca within EMPM and STDA methods

The calculation of nuclear matrix elements (NMEs) for neutrinoless double-beta decay (0νββ) 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 0νββ NMEs, making it essential to validate theoretical approaches using observables that are experimentally known. In contrast, two-neutrino double-beta decay (2νββ) 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 2νββ 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 2νββ NME on the Gamow-Teller and Fermi contributions, on the particle-hole interaction strength parameter gph. Accurately reproducing the experimental 2νββ 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.