F. Iachello: Quenching of g_A and its Impact in Double Beta Decay
The half-life for processes not included in the standard model giving rise to neutrinoless double beta decay can be written as the project of a phase space factor depending on atomic physics, a matrix element squared for nuclear physics effects, and a factor enshrining the physics beyond the standard model, belonging to the realm of particle physics. The nuclear matrix element in turn contains a factor g_A^2.
The g_A factor used in most results for calculations of the neutrinoless double beta decay is 1.269. But g_A is renormalized in models of nuclei, so one can define an effective value of this parameter. This turns out to be in the range 0.5-0.6 in one analysis, or 0.7-0.8 in another.
Since the axial vector coupling constant g_A appears to the second power in the nuclear matrix element, the half-life depends on it with the fourth power. Therefore, the results for the expected half-lifes should be multiplied by a factor ranging from 6 to 34, and hence the limits on the average neutrino mass should be increased by a factor of 2.5 to 6, making it impossible to reach in the near future even the inverted hierarchy region with experimental measurements.
Possibilities to escape this negative conclusions may be:
1) if neutrino masses are degenerate and large – but this comes in tension with the cosmological bound on the sum of neutrino masses, which is of 0.23 eV from 2015 Planck data.
2) if both light and heavy neutrino exchange contribute simultaneously and are of the same order of magnitude, interfering constructively. This possibility, however, requires a fine tuning which looks quite unlikely.
3) Other scenarios (like Majoron emission or sterile neutrinos) must then be considered. The speaker discussed these in some detail -interested readers should refer to his slides, available in the conference web site.