The first large-scale shell-model calculation of the two-neutrino double beta decay of $^{76}$Ge to the excited states in $^{76}$Se

TL;DR

This paper presents the first large-scale shell-model calculation of the two-neutrino double beta decay of $^{76}$Ge to excited states in $^{76}$Se, providing detailed nuclear matrix elements and decay half-life predictions.

Contribution

It introduces a comprehensive shell-model approach with over 10,000 intermediate states, enabling accurate calculation of decay properties to excited states in $^{76}$Se.

Findings

Calculated nuclear matrix elements for multiple states

Derived branching ratios and half-life predictions

Estimated quenched axial-vector coupling constant g_A

Abstract

Large-scale shell-model calculations were carried out for the half-lives and branching ratios of the $2\nu\beta\beta$ decay of $^{76}$Ge to the ground state and the lowest three excited states $2_1^+$, $0_2^+$ and $2_2^+$ in $^{76}$Se. In total, the wave functions of more than 10,000 intermediate $1^+$ states in $^{76}$As were calculated in a three-step procedure allowing an efficient use of the available computer resources. In the first step, 250 lowest states, below some 5 MeV of excitation energy, were calculated without truncations within a full major shell $0f_{5/2}-1p-0g_{9/2}$ for both protons and neutrons. The wave functions of the rest of the states, up to some 30 MeV, were computed in two more steps by introducing two consecutive stages of truncation. The computed magnitudes of the $2\nu\beta\beta$ nuclear matrix elements (including the value of the axial-vector coupling $g_{\rm A}$), $\vert M_{2\nu}\vert g_{\rm A}^2$, converged to the values 0.168$g_{\rm A}^2$, $1.2\times10^{-3}$$g_{\rm A}^2$, 0.121$g_{\rm A}^2$, and $3.1\times10^{-3}$$g_{\rm A}^2$ for the $0^+_{\rm g.s.}$, $2^+_1$, $0^+_2$, and $2^+_2$ states, respectively. Using up-to-date phase-space integrals, the corresponding branching ratios were derived to be 99.926\%, 4.4$\times10^{-5}$\%, 0.074\% and 2.5$\times10^{-7}$\%. The experimental half-life $(1.926\pm0.094)\times10^{21}$ yr of the ground-state transition was used to derive the value $g_{\rm A}=0.80\pm0.01$ for the axial-vector coupling, which is consistent with other shell-model calculations suggesting a quenched value of $g_{\rm A}$. Using this value of $g_{\rm A}$, predictions for the transition half-lives were derived.