DUNE-T2HK Synergy in Neutrino Research

• DUNE-T2HK synergy is a combined approach leveraging DUNE’s long baseline and T2HK’s high-statistics to resolve degeneracies in neutrino oscillation parameters.
• The collaboration enhances mass hierarchy discrimination, θ23 octant resolution, and CP violation sensitivity by exploiting complementary baseline and beam characteristics.
• This integrated analysis reduces exposure needs, mitigates systematic uncertainties, and tightens constraints on new physics such as nonstandard interactions and sterile neutrinos.

DUNE-T2HK synergy refers to the pronounced sensitivity gains and resolution of parameter degeneracies achieved by combining data from the Deep Underground Neutrino Experiment (DUNE) and the Tokai-to-Hyper-Kamiokande (T2HK) long-baseline neutrino oscillation experiments. Individually, these facilities bring complementary strengths: DUNE's long baseline and strong matter effects yield robust sensitivity to the neutrino mass hierarchy, while T2HK's extremely high statistics and narrow-band beam facilitate precise measurements of the CP-violating phase and the atmospheric mixing parameters. Their combined analysis enables breakthroughs in precision, the breaking of persistent parameter degeneracies, and the simultaneous constraining of both standard three-flavor oscillation parameters and exotic new-physics scenarios.

1. Underlying Principles of Complementarity

The DUNE-T2HK synergy exploits foundational differences:

• Baseline and matter effects: DUNE’s 1300 km baseline introduces strong matter effects (parameterized as $A = \sqrt{2}G_F N_e$), making it highly sensitive to the mass ordering via the oscillation frequency shift

$$\Delta \tilde{E}_{31} = \sqrt{(\Delta E_{31} \cos 2\theta_{13} - A)^2 + (\Delta E_{31} \sin 2\theta_{13})^2}$$

This distinguishes "normal" and "inverted" mass hierarchies unambiguously.

• Beam configuration and energy coverage: DUNE uses a wide-band beam, giving access to both first and second oscillation maxima, which enhances the extraction of subdominant CP-odd terms. T2HK, with a baseline of 295 km and a narrow-band, off-axis beam, minimizes matter effects, providing a "clean" environment for CP phase measurement and precise statistics.
• Parameter degeneracies: Different experimental designs yield different degeneracy structures. For T2HK, the “hierarchy–CP” and “octant–CP” degeneracies pose challenges—regions in parameter space where different values of δ_CP, the mass hierarchy, or θ_23 octant give similar oscillation probabilities. DUNE’s pronounced matter effect lifts the hierarchy degeneracy, while T2HK's statistics constrict remaining allowed regions.

This complementarity ensures that parameter regions ambiguous for one experiment become nondegenerate when dual datasets are analyzed, increasing overall robustness and statistical significance (Fukasawa et al., 2016, Ballett et al., 2016).

2. Resolution of the Mass Hierarchy and Octant

• Mass hierarchy discrimination: T2HK alone is limited in hierarchy sensitivity—especially for CP-values like δ_CP = +90° in normal ordering—because short baselines produce nearly overlapping oscillation probability “bands.” DUNE, in contrast, achieves $\gtrsim$8σ hierarchy sensitivity even in unfavorable δ_CP regions due to its pronounced matter-induced splitting of oscillation eigenfrequencies. By combining, T2HK+HK+DUNE achieves up to $\sim$15σ mass ordering significance, as the T2HK atmospheric data add complementary $L/E$ coverage (Fukasawa et al., 2016, Raut, 2017).
• θ_23 octant resolution: Both T2HK and DUNE face octant ambiguities due to the near-vacuum symmetry $$\sin^2 2\theta_{23} \to \sin^2 2(90^\circ-\theta_{23})$$ in the disappearance channel. However, the appearance channel's dependence $$P(\nu_\mu \to \nu_e) \propto \sin^2 \theta_{23} \sin^2(\Delta \tilde{E}_{31} L/2)$$ makes the amplitude octant-sensitive. The synergy further narrows allowed regions: the combined analysis, for both hierarchies, resolves the octant everywhere except in a narrow window $43.5^\circ < \theta_{23} < 48^\circ$ at 5σ C.L. (Fukasawa et al., 2016).

3. Enhancements in CP Violation and Parameter Precision

• CPV discovery fraction and precision: T2HK’s high event rate provides strong sensitivity to CP violation in “favorable” δ_CP regions (maximal CPV near ±90°), but limited hierarchy sensitivity causes a fall-off in coverage near unfavorable δ_CP. DUNE’s wide-band and matter effects generate a flatter, more uniform δ_CP sensitivity. In combination, the CP violation discovery fraction reaches ≥68% at $5σ$ (i.e., for ≥68% of δ_CP true values), with regions around δ_CP = ±90° achieving $10σ$ significance. Typical combined precisions improve to $\sim0.3\%$ for $\Delta m^2_{eff}$, $\sim2\%$ for $\sin^2\theta_{23}$, and $\sim20\%$ for δ_CP (Fukasawa et al., 2016, Ballett et al., 2016, Chakraborty et al., 2017).
• Synergy in the (sin²θ_23, δ_CP) and (Δm^2_31, sin²θ_23) planes: The complementary experiments shrink allowed confidence regions in two-parameter spaces, excluding degenerate (clone) solutions and facilitating high-precision global fits (Agarwalla et al., 22 Aug 2024).
• Exposure and runtime optimization: DUNE and T2HK, when combined, can achieve target sensitivities (e.g., >7σ octant exclusion, $3σ$ CPV for 75% of δ_CP) with only half the nominal exposure each, attributed to this synergy in lifting degeneracies and sharing parameter constraints (Agarwalla et al., 2022, Agarwalla et al., 22 Aug 2024).

4. Robustness and Systematic Uncertainties

• Systematics: Due to high statistics, T2HK is extremely sensitive to systematic uncertainties (signal/background normalization) while DUNE, with a lower event rate but stronger hierarchy sensitivity, is less systematics-limited. Combined analyses (especially with correlated systematics in T2HK/T2HKK) can achieve up to 25% increase in sensitivity, demonstrating the real-world necessity of multi-detector, multi-channel strategies (Ghosh et al., 2017, Raut, 2017).
• Earth density uncertainty: The long DUNE baseline makes δ_CP measurements more sensitive to uncertainties in matter density ($A \propto N_e$). The combined analysis mitigates this effect, allowing more robust extraction of subleading oscillation parameters even in the face of ±5–10% density uncertainties (Ghosh et al., 2022).

5. Probing New Physics: NSI, Long-Range Forces, Decoherence, and Sterile Neutrinos

• Nonstandard interactions (NSI): DUNE's long baseline and T2HK's statistics are highly complementary for probing extended matter effects from Z′-mediated NSI, especially for mediator masses 5–20 MeV where oscillation sensitivity is optimal. The combined reach substantially improves parameter bounds and enables exploration of complex flavor textures across a plethora of U(1)′ scenarios (Han et al., 2019, Agarwalla et al., 3 Apr 2024, Agarwalla et al., 23 Jan 2025).
• Long-range flavor-dependent forces: The combination is vital for constraining or discovering ultra-light mediator-induced potentials of the form

$$V_{\alpha\beta} \propto \frac{G'^{2}}{4\pi d} e^{-m_{Z'} d}$$

sourced by the large matter distributions at astronomical scales. Direct correlation and degeneracy breaking among V, δ_CP, and sin²θ_23 is only possible when using both DUNE (wide energy) and T2HK (precision statistics), as shown across dozens of operator/U(1)′ models (Singh et al., 2023, Singh et al., 21 Jan 2025, Agarwalla et al., 23 Jan 2025).

• Sterile neutrinos and exotic phases: In the 3+1 scenario, the sterile phase δ_24 contributes to the interference structure of ν_e appearance probabilities. Synergy between the narrow-band, high-statistics (T2HK) and matter-enhanced, wide-band (DUNE) experiments enables significantly tighter constraints on δ_24 and sterile mixing angles, reducing the degeneracy space for new physics searches (Choubey et al., 2017).
• Quantum decoherence: Sensitivity to Planck-scale or environmental decoherence is higher at DUNE due to longer baseline and damping terms (of Lindblad form) being proportional to L. However, only when data is combined with T2HK's less-affected sample can robust standard parameter extraction be guaranteed, revealing or excluding decoherence-induced distortions (Barenboim et al., 26 Feb 2024).
• Invisible neutrino decay: DUNE’s CC+NC capability, plus T2HK(T2HKK)'s multibaseline spectral coverage, tighten constraints on the decay lifetime τ₃/m₃ and mitigate octant ambiguities introduced by exponential damping (Dey et al., 20 Feb 2024).

6. Synergistic Probes of T and CP Violation

• L-odd (T-violating) component: A unique feature, accessed via DUNE and T2HK together, is that the L-odd component of the transition probability at fixed neutrino energy can be separated; DUNE's broad spectrum (especially 0.68–0.92 GeV, covering the 2nd maximum) enables sensitivity to genuine T violation up to 4σ, employing only neutrino data;

$$P_{odd} = 8\, \text{Im}[c_2^* c_3] \sin\phi_{21} \sin\phi_{31} \sin(\phi_{31} + \phi_{21})$$

This complements T2HK’s superior CPV sensitivity via direct comparison of neutrino to antineutrino appearance probabilities. The dual approach offers cross-validation for the leptonic Dirac phase, providing independent confirmation of complex-phase PMNS mixing (Chatterjee et al., 6 Aug 2025).

7. Future Impact and Broader Implications

By thoroughly combining DUNE and T2HK (and, in extended scenarios, data from atmospheric, Korean, or supernova neutrino channels), the global neutrino oscillation program:

• Achieves a $>15σ$ exclusion of the wrong mass hierarchy even for unfavorable parameter regions,
• Resolves θ_23 octant to $>7σ$ significance (excluding clones for all but a narrow parameter window),
• Measures the atmospheric mass splitting ($\Delta m^2_{31}$) to 0.3–0.5% and sin²θ_23 to 2%, improving upon standalone capabilities by a factor of 5–7,
• Enables robust, high-precision testing of new-physics scenarios, distinguishing among models by their characteristic signatures in oscillation spectra and parameter degeneracies,
• Sets a methodological paradigm for future oscillation facilities, emphasizing the necessity of strategically combined baselines, detector technologies, and comprehensive multi-channel analyses.

Such synergy is essential for breaking the remaining parameter degeneracies present in any single experiment, maximizing CPV discovery fraction, constraining flavor-violating new physics, and ensuring that the next generation of neutrino experiments realizes the full precision and discovery potential required for resolving outstanding open questions in the Standard Model and beyond.