The $10^{-3}$ eV Frontier in Neutrinoless Double Beta Decay

TL;DR

This paper investigates the conditions under which the effective Majorana mass in neutrinoless double beta decay exceeds 10^{-3} eV, considering recent neutrino data and different CP phase scenarios, especially for normal mass ordering.

Contribution

It provides a detailed analysis of the parameter space where the neutrinoless double beta decay effective mass surpasses 10^{-3} eV, incorporating updated oscillation data and various CP phase assumptions.

Findings

If the sum of neutrino masses exceeds 0.10 eV, the effective mass is above 5×10^{-3} eV regardless of CP phases.

Normal ordering can still produce a significant effective mass under certain mass sum conditions.

CP conserving and symmetry-based phase scenarios are also considered for their impact on the effective mass.

Abstract

The observation of neutrinoless double beta decay would allow to establish lepton number violation and the Majorana nature of neutrinos. The rate of this process in the case of 3-neutrino mixing is controlled by the neutrinoless double beta decay effective Majorana mass $|\langle m \rangle|$. For a neutrino mass spectrum with normal ordering, which is favoured over the spectrum with inverted ordering by recent global fits, $|\langle m \rangle|$ can be significantly suppressed. Taking into account updated data on the neutrino oscillation parameters, we investigate the conditions under which $|\langle m \rangle|$ in the case of spectrum with normal ordering exceeds $10^{-3}~(5\times 10^{-3})$ eV: $|\langle m \rangle|_\text{NO} > 10^{-3}~(5\times 10^{-3})$ eV. We analyse first the generic case with unconstrained leptonic CP violation Majorana phases. We show, in particular, that if the sum of neutrino masses is found to satisfy $\Sigma > 0.10$ eV, then $|\langle m \rangle|_\text{NO} > 5\times 10^{-3}$ eV for any values of the Majorana phases. We consider also cases where the values for these phases are either CP conserving or are in line with predictive schemes combining flavour and generalised CP symmetries.