Degeneracies in long-baseline neutrino experiments from nonstandard interactions

We study parameter degeneracies that can occur in long-baseline neutrino appearance experiments due to nonstandard interactions (NSI) in neutrino propagation. For a single off-diagonal NSI parameter, and neutrino and antineutrino measurements at a single $L/E$, there exists a continuous four-fold degeneracy (related to the mass hierarchy and θ₂₃ octant) that renders the mass hierarchy, octant, and $CP$ phase unknowable. Even with a combination of NOνA and T2K data, which in principle can resolve the degeneracy, both NSI and the $CP$ phase remain unconstrained because of experimental uncertainties. A wide-band beam experiment like DUNE will resolve this degeneracy if the nonzero off-diagonal NSI parameter is ε_eμ. If ε_eτ is nonzero, or the diagonal NSI parameter εₑₑ is O(1), a wrong determination of the mass hierarchy and of $CP$ violation can occur at DUNE. The octant degeneracy can be further complicated by ε_eτ, but is not affected by εₑₑ.