Researchers developed a new first‑principles calculation that connects two types of double‑beta decay in the nucleus 48Ca. The method, called IM‑NCCI, combines two modern many‑body tools to compute both the ordinary two‑neutrino decay and the hypothetical neutrinoless decay in a single framework. Using this approach they find a strong relation between the measurable two‑neutrino rate and the neutrinoless nuclear matrix element (NME), and use it to give a tighter prediction for the neutrinoless case: M0ν = 1.30–1.65 (95% confidence level) for 48Ca, where M0ν is the number that controls how the decay rate maps to neutrino mass in simple models of the decay mechanism.

The IM‑NCCI approach mixes the in‑medium similarity renormalization group (IMSRG), which simplifies the starting nucleus by decoupling its ground state from excited states, with a no‑core configuration‑interaction (NCCI) method that builds the final nucleus from many particle–hole excitations. The authors use nuclear forces derived from chiral effective field theory, including the widely used EM1.8–2.0 interaction and a family of 34 sampled interactions around a ΔN2LO form. To keep the calculation tractable they truncate operators at the normal‑ordered two‑body level and use an importance sampling threshold (κmin = 1.0×10−5) and extrapolations to estimate truncation uncertainties.

As a test, the calculation reproduces the main peaks in the measured Gamow–Teller (GT) strength distribution for the charge‑exchange reaction 48Ca → 48Sc. GT transitions are a particular kind of spin‑flip transition that show up in (p,n) experiments and are closely related to the two‑neutrino decay. The calculation matches peaks around excitation energies near 3, 8 and 11 MeV but tends to overestimate strength in the 12–16 MeV region. The total computed GT strength below 25 MeV is larger than the experimental value. The authors interpret that excess as missing contributions from two‑body weak currents and introduce an effective “quenching” factor q ≃ 0.84. Applying that quenching brings the calculated two‑neutrino NME into excellent agreement with experiment.