Octant of $\theta_{23}$ in danger with a light sterile neutrino

TL;DR

The presence of a light sterile neutrino can mimic the octant of b8_{23} in long-baseline experiments, potentially obscuring the determination of whether b8_{23} is less than or greater than c/4, thus complicating neutrino physics.

Contribution

This study demonstrates for the first time how a light sterile neutrino can interfere with b8_{23} octant measurements in DUNE, revealing a new degeneracy in neutrino oscillation analysis.

Findings

Sterile neutrino introduces a new interference term in b8_{23} measurement.

Octant sensitivity can be completely lost due to sterile neutrino effects.

Implications for future neutrino experiments and theoretical models are significant.

Abstract

Present global fits of world neutrino data hint towards non-maximal $\theta_{23}$ with two nearly degenerate solutions, one in the lower octant ($\theta_{23} <\pi/4$), and the other in the higher octant ($\theta_{23} >\pi/4$). This octant ambiguity of $\theta_{23}$ is one of the fundamental issues in the neutrino sector, and its resolution is a crucial goal of next-generation long-baseline (LBL) experiments. In this letter, we address for the first time, the impact of a light eV-scale sterile neutrino towards such a measurement, taking the Deep Underground Neutrino Experiment (DUNE) as a case study. In the so-called 3+1 scheme involving three active and one sterile neutrino, the $\nu_\mu \to \nu_e$ transition probability probed in the LBL experiments acquires a new interference term via active-sterile oscillations. We find that this novel interference term can mimic a swap of the $\theta_{23}$ octant, even if one uses the information from both neutrino and antineutrino channels. As a consequence, the sensitivity to the octant of $\theta_{23}$ can be completely lost and this may have serious implications in our understanding of neutrinos from both the experimental and theoretical perspectives.