| Summary: | Non-standard interactions (NSI) between neutrinos and matter at long-baseline experiments could make determination of the CP-violating phase δ13 ambiguous due to interference with additional complex phases. Such degeneracies are often studied in the context of specific experiments and a few parameter choices, leaving it unclear how to extract a general understanding of when two sets of parameters may be degenerate or how different types of experiments in principle combine to lift such a degeneracy. This work complements detailed simulations of individual experiments by showing how underlying parameters relate to degeneracies as represented on a biprobability plot. We show how a range of energies near the oscillation maximum Δ31=π/2 separates some degenerate probabilities along the CP-conserving direction of biprobability space according to δ+≡δ13+δeτ, while near Δ31=3π/2 degenerate probabilities are separated along the CP-violating direction according to δeτ. We apply this to the experimental hints that suggest δ13∼−π/2 to see that this could also be consistent with δ13,δeτ=0 or π. The baseline and energy range characteristic of DUNE provides some resolution, but a further improvement comes from beams a few degrees off-axis at ≳1000 km baselines, including some proposed sites for T2HKK.
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