Researchers performed first-principles calculations of double-beta decay for the nucleus calcium-48 (48Ca) and found that enlarging the model space changes the results in an important way. Using a microscopic many-body method, they showed that the usual model space underestimates the measured two-neutrino double-beta (2νββ) decay strength. When they added one more orbital (the 0d3/2) to make a d3/2+pf valence space, the calculation matched the experimental number without any tuning.

The team used the valence-space in-medium similarity renormalization group (VS-IMSRG), a method that builds an effective Hamiltonian for a chosen set of active orbitals. Their inputs were nuclear forces and weak interaction currents derived from chiral effective field theory (χEFT). They tested two different χEFT interactions, called 1.8/2.0(EM) and ΔN2LOGO(394), and included both one-body currents (1BC) and the leading two-body currents (2BC) that arise in χEFT. The calculations tracked contributions from up to about 250 intermediate 1+ states in the odd-odd nucleus scandium-48 (48Sc), because the two-neutrino decay is sensitive to these virtual transitions.

In the smaller pf shell the computed nuclear matrix element (NME) for 2νββ was much smaller than the experimental value. The enlarged d3/2+pf space changed the distribution of Gamow–Teller (GT) transition strength in 48Sc. Gamow–Teller transitions are the main type of spin-isospin transitions involved in beta decay. The improved GT strength distribution in the larger space brought the running sum of contributions into very good agreement with experiment for both interactions, especially when the leading two-body currents were included.

The change in the GT strength also affects the hypothetical neutrinoless double-beta (0νββ) decay, which is of interest because its observation would tell us whether neutrinos are their own antiparticles. In the enlarged valence space the calculated 0νββ NMEs for 48Ca roughly doubled compared to the pf-shell result. A larger NME corresponds to a shorter predicted half-life for 0νββ decay; in this case the half-life would be about four times shorter than the pf-shell prediction. This outcome suggests that the choice of valence space and the resulting GT strengths are important when estimating NMEs for heavier candidate nuclei.