A Review of the Tension between the T2K and NOνA Appearance Data and Hints to New Physics

In this article, we review the status of the tension between the long-baseline accelerator neutrino experiments T2K and NOνA. The tension arises mostly due to the mismatch in the appearance data of the two experiments. We explain how this tension arises based on ν_µ → ν_e and ¯ν_µ → ¯ν_e oscillation probabilities. We define the reference point of vacuum oscillation, maximal θ₂₃, and δ_CP = 0 and compute the ν_e / ¯ν_e appearance events for each experiment. We then study the effects of deviating the unknown parameters from the reference point and the compatibility of any given set of values of unknown parameters with the data from T2K and NOνA. T2K observes a large excess in the ν_e appearance event sample compared to the expected ν_e events at the reference point, whereas NOνA observes a moderate excess. The large excess in T2K dictates that δ_CP be anchored at −90^◦ and that θ₂₃ > π/4 with a preference for normal hierarchy. The moderate excess at NOνA leads to two degenerate solutions: (a) NH, 0 < δ_CP < 180^◦, and θ₂₃ > π/4; (b) IH, −180^◦ < δ_CP < 0, and θ₂₃ > π/4. This is the main cause of tension between the two experiments. We review the status of three beyond standard model (BSM) physics scenarios, (a) non-unitary mixing, (b) Lorentz invariance violation, and (c) non-standard neutrino interactions, to resolve the tension.