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

We study the impact of combining the present NO$\nu$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$\nu$A data exhibit a hierarchy--$\delta_{\rm CP}$ degeneracy, admitting both normal hierarchy (NH) with $\delta_{\rm CP} \in [0,180^\circ]$, and inverted hierarchy (IH) with $\delta_{\rm CP} \in [180^\circ,360^\circ]$ solutions at comparable significance, while T2K prefers $\delta_{\rm CP}\simeq 270^\circ$ for both hierarchies, leading to a $2\sigma$ 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--$\delta_{\rm CP}$ degeneracy in NO$\nu$A and strengthen the hierarchy reach of T2K in spite of having no $\delta_{\rm CP}$ sensitivity. Allowing the hierarchy to be a free parameter in the fit, if the true ordering is IH, JUNO aligns the NO$\nu$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 $|\Delta_{31}|$ leads to improved constraints on $\sin^2\theta_{23}$ and $\delta_{\rm CP}$ for normal mass hierarchy in NO$\nu$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.