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

A high-precision measurement of $Δm^2_{31}$ and $θ_{23}$ 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 $θ_{23}$, excluding the wrong-octant solution of $θ_{23}$, 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 $θ_{23}$ and exclude the wrong octant solution of $θ_{23}$ at around 7$σ$ C.L. with their nominal exposures. We find that DUNE + T2HK can improve the current relative 1$σ$ precision on $\sin^{2}θ_{23}~(Δm^{2}{31})$ 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 ($\sin^2θ{23}$ - $δ_{\mathrm{CP}}$) plane as compared to their individual reach.

💡 Research Summary

This paper presents a comprehensive analysis of the synergistic potential between the next-generation long-baseline neutrino oscillation experiments, DUNE and T2HK, in achieving unprecedented precision on the atmospheric oscillation parameters: the mixing angle θ₂₃ and the mass-squared difference Δm²₃₁.

The study begins by establishing the critical importance of precisely measuring these parameters for estimating Earth’s matter effects, resolving the neutrino mass ordering, and measuring the CP-violating phase δ_cp. It reviews the current status and limitations from existing experiments (T2K, NOvA, Super-K, IceCube DeepCore) and near-future projects (IceCube Upgrade, KM3NeT/ORCA, JUNO).

The core of the work lies in contrasting the complementary designs of DUNE and T2HK. DUNE features a long baseline (1285 km) with a wide-band on-axis beam, maximizing matter effects crucial for mass ordering determination. T2HK employs a shorter baseline (295 km) with a narrow-band off-axis beam, optimizing sensitivity to δ_cp and providing high-statistics, low-systematic data near the oscillation maximum. The analysis incorporates realistic details including detector specifications, systematic uncertainties, and the impact of wrong-sign contamination where detectors cannot fully distinguish neutrinos from antineutrinos.

The key findings, assuming Normal Mass Ordering and nominal exposures (DUNE: 5y ν + 5y ν̄; T2HK: 2.5y ν + 7.5y ν̄), demonstrate remarkable synergy:

1. Non-maximal θ₂₃ & Octant Resolution: The combined DUNE+T2HK setup can establish a deviation of θ₂₃ from maximal mixing (45°) and exclude the wrong octant solution at approximately 7σ confidence level.
2. Parameter Precision: The combined analysis can improve the current relative 1σ precision on sin²θ₂₃ and Δm²₃₁ by factors of 7 and 5, respectively.
3. Exposure Efficiency: Perhaps most strikingly, the combination of DUNE and T2HK can achieve the sensitivity expected from each experiment’s full nominal exposure using less than half of their individual planned exposures. This highlights the profound efficiency gained through collaboration.
4. Degeneracy Breaking: The study shows how the synergy provides far better constraints in the (sin²θ₂₃ – δ_cp) plane compared to individual experiments. DUNE’s strong matter effect alters the dependence on δ_cp, which, when combined with T2HK’s precision, effectively breaks parameter correlations.

In conclusion, the paper powerfully argues that the complementary strengths of DUNE and T2HK transcend simple data addition. Their combination offers a “stereoscopic view” of the oscillation parameter space, mitigating systematic uncertainties and lifting degeneracies. This synergy is presented as a cornerstone strategy for advancing neutrino physics into an era of precision measurement, ultimately leading to a more complete understanding of the three-neutrino paradigm.