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

Understand

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.