2025

Suhonen, Jouni

There is a vastly expanding interest in computing the values of the nuclear matrix elements (NMEs) of the neutrinoless double beta (ββ) decay since this is essential for extraction of precise neutrino properties from the potential results of the running and future ββ-decay experiments. Not only reliable computed NMEs are needed, but also one needs to know the effective value of the the weak axial-vector coupling constant gA since its value affects strongly the ββ half-lives. In order to gain knowledge of the possible quenching of gA in finite nuclei one can study, e.g., allowed Gamow-Teller β decays, first- and higher-forbidden unique and non-unique β decays, and two-neutrino ββ decays. All these studies probe the value of gA at low momentum exchanges. For high momentum exchanges, in the range of 100 MeV/c, relevant for the neutrinoless ββ decay, a new promising tool, the ordinary muon capture (OMC), was proposed some two decades ago. Since those days the experimental muon facilities have been improving fast and presently interesting measurements of OMC properties can be carried out. In addition to the indirect (anti)neutrino-mass studies by ββ decays, direct (anti)neutrino-mass information can be gained by using β decays and electron-capture (EC) decays. In this article the above-mentioned methods of probing the value of gA and the (anti)neutrino mass are reviewed.