Diagnosing Unmodeled Neutrino Physics via DUNE and T2HK Complementarity
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 can 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)can provide a powerful experimental diagnostic for propagation NSI, illustrating how baseline complementarity can be a valuable tool to uncover new physics in the precision era.