The effective value of the axial-vector coupling constant gA in nuclear media represents one of the dominant sources of uncertainty in the interpretation of neutrinoless double-beta decay (0νββ) experiments. Its quenching relative to the free-nucleon value significantly impacts nuclear matrix element calculations and, consequently, the inferred values (limits) of the Majorana neutrino mass.
The GAIAS (GAxIal Analysis with Scintillators) project proposes a crystal-scintillator-based strategy to probe the gA through high-precision measurements of forbidden non-unique beta decays. These transitions exhibit strong sensitivity of the beta spectral shape to the axial coupling constant, particularly in the low-energy region.
The method exploits key properties of scintillating crystals: a high light yield, low energy threshold (<5–20 keV), good energy resolution, high radiopurity, stable operation over a long time to accumulate a large enough statistic and the possibility of incorporating beta-emitting isotopes directly into the crystal bulk. Materials under investigation include CdWO4, 106CdWO4, CsI(Na,Rb), NaI(Tl,Tc), and CeCl3, hosting isotopes such as 113Cd, 113mCd, 87Rb, and 99Tc either intrinsically or as controlled dopants.
The experiment is being installed in the low-background environment of the INFN Gran Sasso National Laboratory (LNGS), enabling high-statistics beta spectroscopy under stable and well-controlled conditions. Particular emphasis is placed on crystal growth strategies, isotope incorporation, control of scintillation non-proportionality below 100 keV, and long-term energy-scale stability.
By combining optimized crystal engineering, optimized electronic read-outs, precision spectroscopy, Monte Carlo simulations, and nuclear-structure modeling, GAIAS aims to extract gA with high accuracy, strengthening the role of functional crystalline materials in fundamental physics.
