# Can we measure $\theta_{23}$ octant in 3+1 scheme?

## TL;DR

This paper investigates how the presence of a light sterile neutrino in the 3+1 scheme could hinder the ability of DUNE to determine the octant of the mixing angle θ23, due to new interference effects.

## Contribution

It analyzes the impact of a light sterile neutrino on θ23 octant measurement prospects in DUNE, revealing potential complete loss of octant sensitivity.

## Key findings

- Sterile neutrino introduces a new interference term in oscillation probability.
- This interference can completely obscure θ23 octant determination in DUNE.
- The effect challenges the standard three-neutrino paradigm for octant measurement.

## Abstract

Current 3$\nu$ global fits predict two degenerate solutions for $\theta_{23}$: one lies in lower octant ($\theta_{23} <\pi/4$), and the other belongs to higher octant ($\theta_{23} >\pi/4$). Here, we study how the measurement of $\theta_{23}$ octant would be affected in the upcoming Deep Underground Neutrino Experiment (DUNE) if there exist a light eV-scale sterile neutrino. We show that in 3+1 scheme, a new interference term in $\nu_\mu \to \nu_e$ oscillation probability can spoil the chances of measuring $\theta_{23}$ octant completely.