Speaker
Description
A meticulous and high-fidelity determination of the atmospheric oscillation parameters, $\Delta m^2_{31}$ and $\theta_{23}$, emerges as an indispensable prerequisite for an accurate characterization of terrestrial matter effect in long-baseline neutrino experiments that is intrinsically entwined with the resolution of the neutrino mass ordering conundrum and a robust determination of the leptonic CP-violating phase within the canonical three-flavour neutrino oscillation framework. Through a comprehensive assessment of the empirical legacy of past and ongoing experiments, and a forward-looking appraisal of the prospective sensitivities anticipated from the imminent IceCube Upgrade and KM3NeT/ORCA, we embark on an extensive exploration of the potential gains in the precision for the 2–3 oscillation sector that the forthcoming flagship long-baseline ventures—DUNE and T2HK—are poised to deliver, both in isolation and through their synergistic interplay. We accentuate the profound complementarity between these two experiments, demonstrating how their confluence substantially augments the sensitivity to deviations from maximal mixing, facilitates the resolution of the octant degeneracy associated with $\theta_{23}$, and enhances the overall precision in constraining atmospheric parameters far beyond the scope of their individual capabilities. Assuming current global best-fit values and a normal mass ordering, our findings reveal that the combined configuration of DUNE and T2HK can decisively establish the non-maximal character of $\theta_{23}$ and exclude the incorrect octant with a statistical significance approaching 7$\sigma$, even with their nominal exposures. Furthermore, we discern that this tandem setup can surpass the prevailing 1$\sigma$ relative precision on $\sin^2\theta_{23}$ and $\Delta m^2_{31}$ by remarkable factors of 7 and 5, respectively, under the same mass ordering assumption. In light of the protracted decade-long duration required for DUNE and T2HK individually to reach their full exposure along with the associated financial constraints such endeavors entail, we observe that these heightened sensitivities can be attained with substantially reduced exposures — less than half of the nominal — by harnessing the intrinsic synergy between DUNE and T2HK, a synergy that further manifests in the form of markedly tighter constraints in the ($\sin^2\theta_{23}$, $\delta_{\mathrm{CP}}$) parameter space, effectively breaking the degeneracies between them and thus surpassing the individual reach of either experiment in isolation.
