Improved precision on 2–3 oscillation parameters using the synergy between DUNE and T2HK

A high-precision measurement of \( \Delta {m}_{31}^2 \) and θ₂₃ is inevitable to estimate the Earth’s matter effect in long-baseline experiments which in turn plays an important role in addressing the issue of neutrino mass ordering and to measure the value of CP phase in 3ν framework. After reviewing the results from the past and present experiments, and discussing the near-future sensitivities from the IceCube Upgrade and KM3NeT/ORCA, we study the expected improvements in the precision of 2–3 oscillation parameters that the next-generation long-baseline experiments, DUNE and T2HK, can bring either in isolation or combination. We highlight the relevance of the possible complementarities between these two experiments in obtaining the improved sensitivities in determining the deviation from maximal mixing of θ₂₃, excluding the wrong-octant solution of θ₂₃, and obtaining high precision on 2–3 oscillation parameters, as compared to their individual performances. We observe that for the current best-fit values of the oscillation parameters and assuming normal mass ordering (NMO), DUNE + T2HK can establish the non-maximal θ₂₃ and exclude the wrong octant solution of θ₂₃ at around 7σ C.L. with their nominal exposures. We find that DUNE + T2HK can improve the current relative 1σ precision on sin² θ₂₃ \( \left(\Delta {m}_{31}^2\right) \) by a factor of 7 (5) assuming NMO. Also, we notice that with less than half of their nominal exposures, the combination of DUNE and T2HK can achieve the sensitivities that are expected from these individual experiments using their full exposures. We also portray how the synergy between DUNE and T2HK can provide better constraints on (sin² θ₂₃–δ_CP) plane as compared to their individual reach.