Octant of θ_{23} in Danger with a Light Sterile Neutrino.

Present global fits of world neutrino data hint towards nonmaximal θ_{23} with two nearly degenerate solutions, one in the lower octant (θ_{23}<π/4), and the other in the higher octant (θ_{23}>π/4). This octant ambiguity of θ_{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 as a case study. In the so-called 3+1 scheme involving three active and one sterile neutrinos, the ν_{μ}→ν_{e} transition probability probed in the LBL experiments acquires a new interference term via active-sterile oscillations. We find that this interference term can mimic a swap of the θ_{23} octant, even if one uses the information from both neutrino and antineutrino channels. As a consequence, the sensitivity to the octant of θ_{23} can be completely lost, and this may have serious implications for our understanding of neutrinos from both the experimental and theoretical perspectives.