Degeneracy between $θ_{23}$ octant and neutrino non-standard interactions at DUNE

We expound in detail the degeneracy between the octant of $θ_{23}$ and flavor-changing neutral-current non-standard interactions (NSI's) in neutrino propagation, considering the Deep Underground Neutrino Experiment (DUNE) as a case study. In the presence of such NSI parameters involving the $e-μ$ ($\varepsilon_{eμ}$) and $e-τ$ ($\varepsilon_{eτ}$) flavors, the $ν_μ\to ν_e$ and $\barν_μ\to \barν_e$ appearance probabilities in long-baseline experiments acquire an additional interference term, which depends on one new dynamical CP-phase $ϕ_{eμ/eτ}$. This term sums up with the well-known interference term related to the standard CP-phase $δ$ creating a source of confusion in the determination of the octant of $θ_{23}$. We show that for values of the NSI coupling (taken one at-a-time) as small as $few\,\%$ (relative to the Fermi coupling constant $G_{\mathrm F}$), and for unfavorable combinations of the two CP-phases $δ$ and $ϕ_{eμ/eτ}$, the discovery potential of the octant of $θ_{23}$ gets completely lost.

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