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

TL;DR

This paper investigates how non-standard neutrino interactions can cause degeneracies with the $ heta_{23}$ octant determination in DUNE, showing that even small NSI effects can obscure the octant measurement due to interference with CP phases.

Contribution

It demonstrates the degeneracy between $ heta_{23}$ octant and flavor-changing NSI effects in neutrino oscillation experiments, highlighting the impact of interference terms involving new CP phases.

Findings

NSI effects can mimic standard oscillation signals.

Small NSI couplings can eliminate octant sensitivity.

Interference with CP phases complicates octant determination.

Abstract

We expound in detail the degeneracy between the octant of $\theta_{23}$ 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 $e-\mu$ ($\varepsilon_{e\mu}$) and $e-\tau$ ($\varepsilon_{e\tau}$) flavors, the $\nu_\mu \to \nu_e$ and $\bar\nu_\mu \to \bar\nu_e$ appearance probabilities in long-baseline experiments acquire an additional interference term, which depends on one new dynamical CP-phase $\phi_{e\mu/e\tau}$. This term sums up with the well-known interference term related to the standard CP-phase $\delta$ creating a source of confusion in the determination of the octant of $\theta_{23}$. 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 $\delta$ and $\phi_{e\mu/e\tau}$, the discovery potential of the octant of $\theta_{23}$ gets completely lost.