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

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 $\mathrm{\mid }⟨m⟩\mathrm{\mid }$. For a neutrino mass spectrum with normal ordering, which is favoured over the spectrum with inverted ordering by recent global fits, $\mathrm{\mid }⟨m⟩\mathrm{\mid }$ can be significantly suppressed. Taking into account updated data on the neutrino oscillation parameters, we investigate the conditions under which $|\langle m \rangle|$inthecaseofspectrumwithnormalorderingexceeds$10^{-3}~(5\times 10^{-3})$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 $\mathrm{\Sigma }>0.10$ eV, then $\mathrm{\mid }⟨m⟩{\mathrm{\mid }}_{\text{NO}}>5\times {10}^{-3}$ 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.