Overconstrained estimates of neutrinoless double beta decay within the QRPA

TL;DR

This paper refines QRPA estimates of neutrinoless double beta decay nuclear matrix elements by overconstraining model parameters using multiple experimental data, including the possibility of strong quenching of the axial vector coupling.

Contribution

It introduces a method to reduce uncertainties in QRPA calculations by simultaneously fitting g_pp and g_A using diverse experimental data, resolving previous model disagreements.

Findings

Disagreement with data vanishes when g_A<1 is allowed.

Two free parameters (g_pp, g_A) accurately reproduce all lifetime data.

The sign of the 2nu2beta matrix element is unambiguously determined.

Abstract

Estimates of nuclear matrix elements for neutrinoless double beta decay (0nu2beta) based on the quasiparticle random phase approximations (QRPA) are affected by theoretical uncertainties, which can be substantially reduced by fixing the unknown strength parameter g_pp of the residual particle-particle interaction through one experimental constraint - most notably through the two-neutrino double beta decay (2nu2beta) lifetime. However, it has been noted that the g_pp adjustment via 2\nu2\beta data may bring QRPA models in disagreement with independent data on electron capture (EC) and single beta decay (beta^-) lifetimes. Actually, in two nuclei of interest for 0nu2beta decay (Mo-100 and Cd-116), for which all such data are available, we show that the disagreement vanishes, provided that the axial vector coupling g_A is treated as a free parameter, with allowance for g_A<1 (``strong quenching''). Three independent lifetime data (2nu2beta, EC, \beta^-) are then accurately reproduced by means of two free parameters (g_pp, g_A), resulting in an overconstrained parameter space. In addition, the sign of the 2nu2beta matrix element M^2nu is unambiguously selected (M^2nu>0) by the combination of all data. We discuss quantitatively, in each of the two nuclei, these phenomenological constraints and their consequences for QRPA estimates of the 0nu2beta matrix elements and of their uncertainties.