The meV frontier of neutrinoless double beta decay in the JUNO era

Abstract

Observing neutrinoless double beta decay would establish lepton number violation and the Majorana nature of neutrinos. Within the standard 3-flavour paradigm, the rate of this process is controlled by the effective Majorana mass $|\langle m \rangle|$, which may be severely suppressed if the neutrino mass spectrum presents normal ordering. Taking into account the first JUNO results, which significantly reduce the uncertainties on solar neutrino oscillation parameters, we provide updated conditions under which $|\langle m \rangle|_\text{NO}$ is guaranteed to exceed the $10^{-3}$ eV ($5\times 10^{-3}$ eV) threshold. We analyse both the generic case, as well as scenarios where the two Majorana phases either take CP conserving values, or at least one of them takes a CP-violating value, that are in line with predictive schemes combining flavour and generalised CP symmetries.

Figures (6)

• Figure 1: Ranges of the lightest mass $m_\text{min}= m_1$ (NO spectrum) for which different conditions apply: in green, a) ${|\langle m \rangle|_\text{NO}} > {|\langle m \rangle|}_0$, for all values of parameters and phases; in light grey, b) there exist values of parameters and phases for which ${|\langle m \rangle|_\text{NO}} < {|\langle m \rangle|}_0$; in red, c) for all values of parameters, there exist phases such that ${|\langle m \rangle|_\text{NO}} < {|\langle m \rangle|}_0$; in dark grey, d) ${|\langle m \rangle|_\text{NO}} < {|\langle m \rangle|}_0$, for all values of parameters and phases. See text for further details. Oscillation parameters are varied within their $n\sigma$ ($n=1,2,3$) ranges, cf. \ref{['tab:dataNO']}.
• Figure 2: Regions in the $m_\text{min}$-- $\alpha_{21}$ plane (left) and in the $m_\text{min}$-- $\alpha_{31}'$ plane (right) where the different conditions on ${|\langle m \rangle|_\text{NO}}$ apply, for a reference value ${|\langle m \rangle|}_0 = 5\times 10^{-3}$ eV and $3\sigma$ variations of oscillation parameters. Colours match those in \ref{['fig:th0']}.
• Figure 3: The effective Majorana mass ${|\langle m \rangle|}$ as a function of $m_\text{min}$, for both orderings and all possible values of the CPV phases $\alpha_{21}$ and $\alpha_{31}'=\alpha_{31}-2\delta$, at the $2\sigma$ CL. Green (blue) bands correspond to CP-conserving values of the phases, for a spectrum with NO (IO), with $k=0,1$. Within the red regions, at least one of the phases takes a CP-violating value. The KamLAND-Zen bounds of \ref{['eq:meffKZ', 'eq:meffKZ-sr']}, from KamLAND-Zen:2024eml, as well as the discussed KATRIN KATRIN:2024cdt and cosmology Capozzi:2025wyn constraints on $m_\text{min}$, are shown.
• Figure 4: The effective Majorana mass ${|\langle m \rangle|}$ as a function of $m_\text{min}$, for a spectrum with IO and all possible values of the CPV phases, at the $2\sigma$ CL. Blue bands correspond to CP-conserving values of phases (cf. \ref{['fig:CP']}), while yellow bands refer to pairs of phases that are gCP-compatible but not fully CP-conserving ($k=0,1,2,3$). Hatching indicates the overlap of blue and yellow regions, occurring for the cases $(0,\pi/2)$ and $(\pi,\pi/2)$.
• Figure 5: The effective Majorana mass ${|\langle m \rangle|}$ as a function of $m_\text{min}$, for a spectrum with NO and all possible values of the CPV phases, at the $2\sigma$ CL. Green bands correspond to CP-conserving values of phases (cf. \ref{['fig:CP']}), while yellow bands refer to pairs of gCP-compatible phases, with exactly one of them being CP-conserving. Hatching indicates region overlap.