Axial coupling constant measurement with crystal scintillators
The search for neutrinoless double beta decay (0νββ) is one of the most fascinating challenges in contemporary physics. Its experimental observation would confirm that the neutrino and the antineutrino are the same particle, as postulated by the Majorana hypothesis. Such a result would revolutionize our understanding of the Universe, opening new perspectives in particle physics and on the origin of matter.
Correctly interpreting 0νββ data requires an accurate understanding of the physics that governs the decay rate. A crucial factor is the axial coupling constant gA, whose value in vacuum is gA,0=1.276. In 0νββ decay, its impact is extremely strong, as the decay rate is proportional to gA4. In nuclei, however, its effective value appears to be reduced, a phenomenon known as gA quenching. The uncertainty in the theoretical modeling of this effect limits the precision with which we can extract an upper bound on the Majorana neutrino mass from experimental data. In the event of an observation of the decay, precise knowledge of gA will be essential to translate the signal into reliable physical information on the neutrino mass.
The GAIAS experiment (GAxIal Analysis with Scintillators) aims to address this problem by studying the quenching of gA through high-precision measurements of the spectral shape of non-unique forbidden beta decays, which are particularly sensitive to this coupling constant. To achieve very high precision, GAIAS will employ low-threshold scintillating crystals installed at the Gran Sasso underground laboratories (LNGS), where cosmic radiation is strongly suppressed, allowing measurements with extremely low background.
Figure 1 shows a schematic image of the GAIAS experiment, which in its initial phases will focus on the isotopes of 113Cd and 113mCd present in CdWO₄ crystal scintillators, and in later phases, also on the isotopes of 87Rb and 99Tc, which can be incorporated into other crystal scintillators such as CsI(Tl) and NaI(Tl). These measurements will allow the extraction of the effective value of gA in certain nuclei, reducing the currently existing theoretical uncertainties, and will provide important information for next-generation experiments on the neutrino mass.