Enhancing the sensitivity to neutrino oscillation parameters using synergy between T2K, NO$ν$A and JUNO

TL;DR

This work analyzes how JUNO’s strong neutrino-hierarchy sensitivity can lift the hierarchy–$\delta_{CP}$ degeneracy plaguing NO$\nu$A and T2K, thereby enhancing the global determination of the mass ordering and CP phase. Using GLoBES-based simulations and JUNO-like ensembles, the study shows that JUNO fixes the hierarchy and reduces degeneracies, with the extent of tension between NO$\nu$A and T2K depending on the true ordering. It also demonstrates that JUNO’s precise measurement of $|\Delta_{31}|$ modestly sharpens $\sin^2\theta_{23}$ and $\delta_{CP}$ constraints for NH, and that updated JUNO solar parameters further improve precision. Overall, JUNO acts as a crucial synergistic component, improving both hierarchy resolution and CP sensitivity when combined with long-baseline data, and clarifying existing tensions in the neutrino-oscillation programme.

Abstract

We study the impact of combining the present NO$ν$A and T2K data with simulated data from the JUNO experiment on the determination of the leptonic CP phase and the neutrino mass hierarchy. The current NO$ν$A data exhibit a hierarchy--$δ_{\rm CP}$ degeneracy, admitting both normal hierarchy (NH) with $δ_{\rm CP} \in [0,180^\circ]$, and inverted hierarchy (IH) with $δ_{\rm CP} \in [180^\circ,360^\circ]$ solutions at comparable significance, while T2K prefers $δ_{\rm CP}\simeq 270^\circ$ for both hierarchies, leading to a $2σ$ tension between the two experiments for normal hierarchy. Using detailed GLoBES simulations, we show that future JUNO data with excellent hierarchy sensitivity, can lift the hierarchy--$δ_{\rm CP}$ degeneracy in NO$ν$A and strengthen the hierarchy reach of T2K in spite of having no $δ_{\rm CP}$ sensitivity. Allowing the hierarchy to be a free parameter in the fit, if the true ordering is IH, JUNO aligns the NO$ν$A and T2K allowed regions and resolves their present tension; if NH is true, the tension continues to persist. We also show that JUNO's precise measurement of $|Δ_{31}|$ leads to improved constraints on $\sin^2θ_{23}$ and $δ_{\rm CP}$ for normal mass hierarchy in NO$ν$A even though JUNO itself is insensitive to these parameters. Finally, updated solar parameter measurements from JUNO's first data release further enhance the combined precision. Our results demonstrate that JUNO plays a crucial synergistic role in the global neutrino oscillation programme, enabling a more robust determination of the mass ordering and improving the sensitivity to the CP phase when combined with long-baseline data.

Figures (17)

• Figure 1: The electron antineutrino survival probability as a function of antineutrino energy for the JUNO experiment. The red (green) colour corresponds to NH (IH) of neutrino masses. Here, the values of oscillation parameters are adopted from NuFIT 6.0 as presented in Table \ref{['tab:t1']}.
• Figure 2: $P_{\mu e}$ (left panel) and $P_{\bar{\mu} \bar{e}}$ (right panel) oscillation probabilities as a function of neutrino energy depicting the hierarchy-$\delta_{\rm CP}$ degeneracy for NO$\nu$A (upper panel) and T2K (lower panel) experiments. The blue (red) band in all the plots represents the variation of $\delta_{\rm CP}$ when the neutrino mass hierarchy is NH (IH). The value of $\theta_{23}$ is taken to be maximal (i.e. $45\degree$), and the remaining oscillation parameters are adopted from NuFIT 6.0 given in Table \ref{['tab:t1']}.
• Figure 3: Allowed region in the $\sin^2{\theta_{23}}-\delta_{\rm CP}$ plane after complete analysis of NO$\nu$A 2024 and T2K 2020 data.
• Figure 4: Allowed regions in the $\sin^2{\theta_{23}}-\delta_{\rm CP}$ plane including T2K, NO$\nu$A data and JUNO simulation. The true events of JUNO have been simulated with the best-fit values from Ref. Esteban:2018azc as the true values of oscillation parameters. The top (bottom) panels present the cases for known (unknown) mass hierarchy and the left (right) panels present the cases when the true hierarchy for JUNO is NH (IH).
• Figure 5: $1\,\sigma$ allowed region in the $\Delta_{31}-\sin^2{\theta_{23}}$ (${\Delta_{31}}-\delta_{CP}$) plane after complete analysis of NO$\nu$A 2024 and T2K 2020 data in the top (bottom) panel. The true events of JUNO have been simulated with the best-fit values from Ref. Esteban:2018azc as the true values of oscillation parameters.