Very Light Sterile Neutrinos at NOvA and T2K

Over the last several years, our understanding of neutrino oscillations has developed significantly due to the long-baseline measurements of muon-neutrino disappearance and muon-to-electron-neutrino appearance at the T2K and NOvA experiments. However, when interpreted under the standard-three-massive-neutrinos paradigm, a tension has emerged between the two experiments’ data. Here, we examine whether this tension can be alleviated when a fourth, very light neutrino is added to the picture. Specifically, we focus on the scenario in which this new neutrino has a mass similar to, or even lighter than, the three mostly active neutrinos that have been identified to date. We find that, for some regions of parameter space, the four-neutrino framework is favored over the three-neutrino one with moderate (<math display="inline"><mo>≲</mo><mn>2</mn><mi>σ</mi></math>) significance. Interpreting these results, we provide a future outlook for near-term and long-term experiments if this four-neutrino framework is indeed true.