Precise 113Cd beta decay spectral shape measurement and interpretation in terms of possible gA quenching

Highly forbidden decays provide a sensitive test to nuclear models in a regime in which the decay goes through high spin-multipole states, similar to the neutrinoless double-beta decay process. There are only 3 nuclei (50V, 113Cd, 115In) which undergo a forbidden non-unique beta decay. In this work, we compare the experimental 113Cd spectrum to theoretical spectral shapes in the framework of the spectrum-shape method. We measured with high precision, with the lowest energy threshold and the best energy resolution ever, the spectrum of 113Cd embedded in a 0.43 kg crystal, operated over 26 days as a bolometer at low temperature in the Canfranc underground laboratory (Spain). We performed a Bayesian fit of the experimental data to three nuclear models (IBFM-2, MQPM and NSM) allowing the reconstruction of the spectral shape as well as the half-life. The fit has two free parameters, one of which is the effective weak axial-vector coupling constant, gA, which resulted in gA between 1.0 and 1.2, compatible with a possible quenching. Based on the fit, we measured the half-life of the 113Cd beta decay including systematic uncertainties as 8.04 × 10^15 yr, in agreement with the previous experiments. These results represent a significant step towards a better understanding of low-energy nuclear processes.