Diagnosing Unmodeled Neutrino Physics via DUNE and T2HK Complementarity

Abstract

Unmodeled beyond Standard Model (BSM) physics in neutrino propagation can masquerade as parameter degeneracies in future precision measurements. Because the upcoming DUNE and T2HK experiments will operate at substantially different baselines, interpreting their data under the standard three-flavor framework provides a powerful diagnostic tool: any propagation BSM effect will inevitably manifest as an artificial tension between their extracted parameters. We demonstrate this principle using the complex non-standard interactions (NSI) currently favored to resolve the $\sim2σ$ tension between NO$ν$A and T2K. If these NSI solutions are realized, the NSI-induced interference term $\propto\sin(δ_{\rm CP}+φ)$ systematically distorts the DUNE appearance rates, leading to a correlated misidentification of the atmospheric mixing octant and the CP phase $δ_{\rm CP}$. Specifically, for $\varepsilon_{eμ}$ ($\varepsilon_{eτ}$) NSI, the DUNE fit shifts toward CP- conserving values (the opposite CP half-plane) along with a preference for the wrong octant. In contrast, the shorter-baseline T2HK experiment remains largely insensitive to this effect. The resulting $\sim3σ$ incompatibility between the DUNE and T2HK standard-fit results (after 6 years of data collection for each experiment) provides a robust experimental diagnostic for propagation NSI, illustrating how baseline complementarity is essential to uncover new physics in the precision era.

Figures (3)

• Figure 1: Bi-event plot for NO$\nu$A (upper) and T2K (lower). The ellipses are obtained by varying $\delta_{\rm CP}$ over $[-\pi,\pi]$ with all other oscillation parameters fixed at the combined best-fitEsteban:2024eli. The data points are represented by black squares and the predicted number of events are represented as stars. The circle, square, diamond and triangle shapes represents $\delta_{\rm CP}=0,\pi/2,\pi(-\pi)$ and $-\pi/2$ respectively.
• Figure 2: Bi-probability plot (upper) and bi-event (lower) plot for DUNE. For both cases, the ellipses are obtained by varying $\delta_{\rm CP}$ over $[-\pi,\pi]$ with all other oscillation parameters fixed at the combined best-fitEsteban:2024eli. The predicted probability and simulated data points use Benchmarks 1 and 2 from table as references and are represented by colored squares. The black ellipse represents the standard prediction for the upper octant (UO), while the gray dashed ellipse represents the standard prediction for the lower octant (LO). The blue and red ellipses represents the prediction for non-zero NSIs suggested by benchmarks 1 and 2, respectively. The circle, square, diamond and triangle shapes represents $\delta_{\rm CP}=0,\pi/2,\pi(-\pi)$ and $-\pi/2$ respectively.
• Figure 3: Allowed regions in the $\sin^2{\theta_{23}}$--$\delta_{\rm CP}$ plane from a standard three-flavor analysis of DUNE$\oplus$T2HK data. Left: The simulated data with true value driven by $\varepsilon_{e\mu}$ (benchmark 1); center: the simulated data with true value driven standard oscillations; right: the simulated data with true value driven by $\varepsilon_{e\tau}$ (benchmark 2). Contours at $1\sigma$, $2\sigma$ and $3\sigma$ are shown. The star marks the best-fit point after analyzing the simulated data using standard oscillation.