eV-scale sterile neutrino: A window open to non-unitarity?

TL;DR

This work tackles the appearance–disappearance tension in eV-scale sterile-neutrino interpretations by proposing a low-scale non-unitary framework. By embedding a visible sterile state into a non-unitary (3+1) model using the α parametrization, the authors show qualitatively how a positive real part of $\widetilde{α}_{μe}$ can enhance νμ→νe appearance while suppressing νμ disappearance, thereby easing the tension. An illustrative leading-order analysis finds tension-easing solutions that are consistent with reactor+Ga data for $\sin^2 2\theta_{14}$ near 0.3 and remain stable under parameter variations, suggesting a possible bridge between LSND–MiniBooNE and BEST signals and predicting a sizable neutrino–antineutrino asymmetry. The work also develops the Okubo-based interpretation of α, discusses bounds on $α_{ee}$, $α_{μμ}$ and $|α_{μe}|$, and outlines falsifiability tests via near-detector searches and CP-violating signatures, offering a testable path to validate or refute this non-unitary solution.

Abstract

An excess observed in the accelerator neutrino experiments in the $ν_μ \rightarrow ν_{e}$ channel at high confidence level (CL) has been interpreted as due to eV-scale sterile neutrino(s). But, it has been suffered from the problem of ``appearance-disappearance tension'' at the similarly high CL because the measurements of the $ν_μ \rightarrow ν_μ$ channel do not observe the expected event number depletion corresponding to the sterile contribution in the appearance channel. We suggest non-unitarity as a simple and natural way of resolving the tension, which leads us to construct the non-unitary $(3+1)$ model. With reasonable estimation of the $α$ parameters governing non-unitarity, which we argue growing when more sterile states added, we perform an illustrative analysis to illuminate if the tension can be resolved in this model. We have found the unique solution with $\sin^2 2θ_{14} \approx 0.3$, which is consistent with the (reactors + Ga) data and is stable against variation of the appearance signature. This solution bridges between the two high CL signatures, BEST and LSND-MiniBooNE, and implies a large neutrino-antineutrino asymmetry, given the much less anomaly indicated in the antineutrino sector.