Is neutrino decay really ruled out as a solution to the atmospheric neutrino problem from Super-Kamiokande data?

Abstract

In this paper, we perform a detailed χ²-analysis of the 848 days of Super-Kamiokande (SK) atmospheric neutrino data under the assumptions of ν_μ–ν_τ oscillation and neutrino decay. For the latter, we take the most general case of neutrinos with non-zero mixing and consider the possibilities of the unstable component in ν_μ decaying to a state with which it mixes (scenario (a)) and to a sterile state with which it does not mix (scenario (b)). In the first case, Δm² (mass squared difference between the two mass states which mix) has to be >0.1 eV² from constraints on K decays, whereas for the second case, Δm² can be unconstrained. For case (a), Δm² does not enter the χ²-analysis, while in case (b), it enters the χ²-analysis as an independent parameter. In scenario (a), there is Δm² averaged oscillation in addition to decay, and this gets ruled out at 100.0% CL by the latest SK data. Scenario (b) on the other hand, gives a reasonably good fit to the data for Δm² ∼0.001 eV².