Global constraints on absolute neutrino masses and their ordering

TL;DR

This work performs a global analysis of oscillation and nonoscillation data within the standard three-neutrino framework to constrain absolute neutrino masses and their ordering. By combining updated oscillation results, KamLAND-Zen $0\nu\beta\beta$ bounds, and cosmological measurements (Planck and companions) across multiple dataset combinations, it derives joint constraints in the $(\Sigma, m_{\beta\beta})$ plane and assesses implications for $m_\beta$. The study finds a mild preference for normal ordering at about $2\sigma$, with robust upper limits on the sum of neutrino masses that tighten when cosmology and $0\nu\beta\beta$ information are jointly considered; it also discusses the resulting expectations for upcoming beta-decay experiments. Overall, the results demonstrate meaningful cross-dataset synergy in probing the absolute neutrino mass scale and highlight the ongoing potential for future data to decisively determine the mass ordering and individual mass observables.

Abstract

Within the standard three-neutrino framework, the absolute neutrino masses and their ordering (either normal, NO, or inverted, IO) are currently unknown. However, the combination of current data coming from oscillation experiments, neutrinoless double beta decay searches, and cosmological surveys, can provide interesting constraints for such unknowns in the sub-eV mass range, down to O(0.1) eV in some cases. We discuss current limits on absolute neutrino mass observables by performing a global data analysis, that includes the latest results from oscillation experiments, neutrinoless double beta decay bounds from the KamLAND-Zen experiment, and constraints from representative combinations of Planck measurements and other cosmological data sets. In general, NO appears to be somewhat favored with respect to IO at the level of ~2 sigma, mainly by neutrino oscillation data (especially atmospheric), corroborated by cosmological data in some cases. Detailed constraints are obtained via the chi^2 method, by expanding the parameter space either around separate minima in NO and IO, or around the absolute minimum in any ordering. Implications for upcoming oscillation and non-oscillation neutrino experiments, including beta-decay searches, are also discussed.

Figures (11)

• Figure 1: Global $3\nu$ oscillation analysis. Projections of the $\chi^2$ function onto the parameters $\delta m^2$, $|\Delta m^2|$, $\sin^2\theta_{ij}$, and $\delta$, for NO (blue) and IO (red). In each panel, all the undisplayed parameters are marginalized, and the offset $\Delta \chi^2_{\mathrm{IO}-\mathrm{NO}}= 3.6$ is included.
• Figure 2: Constraints from $0\nu\beta\beta$ decay, in terms of the function $\chi^2(m_{\beta\beta})$ derived from KamLAND-Zen data KamZ16KLchi2 and from an estimate of the ${}^{136}$Xe nuclear matrix elements and its uncertainties based on Rotu15. The same constraints apply to both NO and IO. See the text for details.
• Figure 3: Constraints on the sum of neutrino masses from cosmological data. The $\chi^2(\Sigma)$ function is shown in NO (blue) and IO (red) for four representative cases, numbered as #10, #1, #9, and #6 in Table II, and including the corresponding $\Delta\chi^2_\mathrm{IO-NO}$ offset. In each case, the NO and IO curves diverge as $\Sigma$ approaches the extrema in Eq. (\ref{['lightest2']}), while they tend to converge for large $\Sigma$, as the mass ordering sensitivity vanishes.
• Figure 4: Global analysis in the $(\Sigma,\,m_{\beta\beta})$ plane, including only oscillation data. Constraints are shown in terms of $2\sigma$ (solid) and $3\sigma$ (dotted) allowed regions for NO (blue) and IO (red). In the left panel, the $\chi^2$ minimization is separately performed in each mass ordering, and the allowed regions should be separately considered for NO and IO. In the right panel, the $\chi^2$ is further minimized over the mass ordering, and the allowed regions (for any ordering) are given by the union of the NO and IO ones.
• Figure 5: As in Fig. 4, but including the $\chi^2({m_{\beta\beta}})$ function from Fig. 2.