Neutrinoless Double Beta Decay in Light of JUNO First Data

TL;DR

This paper analyzes how JUNO's first data release refines neutrino oscillation parameters, especially the solar angle $\theta_s$ and solar mass-squared difference $Δm^2_s$, and propagates these improvements to neutrinoless double beta decay phenomenology. By expressing the effective Majorana mass $m_{ee}$ as a sum of three vector contributions and examining NO and IO cases, the authors quantify how reduced oscillation uncertainties tighten the allowed ranges of $m_{ee}$ and the corresponding half-life predictions, including a tightened IO lower bound that has practical implications for next-generation experiments. The study also explores the Majorana triangle and the two Majorana CP phases, showing that JUNO's precision improves prospects for determining these phases in the NO scenario, particularly in regions where $m_{ee}$ could vanish. Cosmological constraints on the neutrino-mass sum, notably DESI+CMB bounds, further shape the allowed parameter space, enhance the funnel region probability for $m_{ee}$, and bolster the case for a combined approach using oscillation data, $0νββ$ experiments, and cosmology to fully characterize neutrino masses and CP violation.

Abstract

The first results from the JUNO reactor neutrino oscillation experiment improve our knowledge of neutrino masses and mixing parameters, especially the solar angle $θ_s \equiv θ_{12}$ and the solar mass squared difference $Δm^2_s \equiv Δm^2_{21}$. We discuss the implications of these results on neutrinoless double beta decay by itself and in combination with the global fit of neutrino oscillation experiments, the JUNO first data, and cosmological constraints on the neutrino mass sum. For the effective mass $\langle m_{ee} \rangle$, the uncertainties in its lower limits for both mass orderings and upper limits for the normal ordering are largely reduced. Since the cosmological CMB and DESI BAO data put a stringent constraint on the neutrino mass scale, we also show how the probability distribution of both the real and imaginary parts of the effective mass $\langle m_{ee} \rangle$ on the complex plane is affected. Especially, the funnel region with $|\langle m_{ee} \rangle| \lesssim 1$\,meV receives larger chance to happen. Correspondingly, the chance of determining the two Majorana CP phases simultaneously in this region also increases with reduced uncertainty.

Figures (7)

• Figure 1: The particle physics uncertainties for the $0 \nu 2 \beta$ decay effective mass $\langle m_{ee} \rangle$. For comparison and easy viewing, three panels are shown: (Left) overall features for both NO and IO; (Middle) zoomed-in linear plot for IO; (Right) zoomed-in linear plot of the low-mass horizontal branch for NO. The allowed regions of the effective mass $\langle m_{ee} \rangle$ have been demonstrated with four different uncertainty configurations: (1) the current $3\,\sigma$ range from NuFIT 6.0 global fit (orange); (2) the updated $3\,\sigma$ range with the JUNO first data release (blue); (3) the expected $3\,\sigma$ range with full JUNO (green); (4) the ultimate scenario with negligible uncertainty (red). The current best limit on the effective mass $\langle m_{ee} \rangle$ from KamLAND-Zen with 800 kg Xenon mass has been added as hashed region with solid boundaries while the expected sensitivities from CUPID, LEGEND1000 and nEXO are shown in the left plot as dashed line without uncertainties of nuclear matrix elements. The middel plot includes the sensitivity of LEGEND1000 including uncertainties of nuclear matrix elements.
• Figure 2: One-dimensional $\Delta \chi^2$ for the $0 \nu 2 \beta$ effective mas limits $\langle m_{ee} \rangle^\text{IO}_\text{min}$ (left), $\langle m_{ee} \rangle^\text{NO}_\text{min}$ (center), and $\langle m_{ee} \rangle^\text{NO}_\text{max}$ (right) according to the neutrino oscillation global fit (blue) NuFIT6 or the combined results with both global fit and JUNO (red) JUNO1st.
• Figure 3: The $0 \nu 2 \beta$ decay half-life time $T^{0 \nu}_{1/2}$ and effective Majorana mass $|\langle m_{ee} \rangle|$ as well as their uncertainties from the nuclear and particle physics sides. Different panels are for different isotopes. The light-blue band indicates the nuclear physics uncertainty when translating the measured half-life $T^{0 \nu}_{1/2}$ into a sensitivity on the $0 \nu 2 \beta$ effective mass $|\langle m_{ee} \rangle|$. Current experimental limits on $T^{0 \nu}_{1/2}$ (90% C.L.) are shown as solid vertical lines with the region to the left of each solid line is excluded while the future projections are shown as vertical dashed lines. The horizontal bands show the effective mass limits for the vanishing lightest neutrino mass ($m_\text{lightest}\to0$): $\langle m_{ee} \rangle^{\rm IO}_{\rm min}$, $\langle m_{ee} \rangle^{\rm NO}_{\rm min}$, and $\langle m_{ee} \rangle^{\rm NO}_{\rm max}$, with central values indicated by the red solid lines. For comparison, the orange band shows the $3\,\sigma$ ranges from the current global fit while the blue band is for the combined result of global fit plus JUNO.
• Figure 4: (Left) Cosmological constraints on the neutrino mass sum $\sum_i m_i$ at 95% C.L. with vertical line for the exiting observations from CMB (solid) and DESI BAO (dashed). For comparison, the projected sensitivities at the future EUCLID is shown as dash-dotted line. (Right) The current KATRIN (solid) and its design (dashed) sensitivities on the beta decay effective mass $m_\beta$. For comparison, the expected PROJECT 8 sensitivity is also shown as a vertical dash-dotted line. The uncertainties of the effective Majorana mass $\langle m_{ee} \rangle$ from the particle physics side are also shown according to the the color scheme of Fig. 1.
• Figure 5: Allowed regions in the complex plane of the effective mass $\langle m_{ee} \rangle$ (real vs. imaginary parts) for NO. Three scenarios are presented: (Left)$m_{\text{lightest}} \in [1,50]$ meV with a flat prior and no cosmological bounds on $\sum_i m_i$; (Center)$m_{\text{lightest}} \in [1,50]$ meV with a prior from the pre-DESI CMB cosmological bounds on $\sum_i m_i$; and (Right)$m_{\text{lightest}} \in [1,50]$ meV with a prior from DESI BAO + CMB cosmological bounds on $\sum_i m_i$. The colored contours represent different confidence levels: red (68% C.L.), purple (95% C.L.), and cyan ($99\%$ C.L.). The black circle around the origin indicates the funnel region with $|\langle m_{ee} \rangle| < 1$ meV.