Signatures of quasi-Dirac neutrinos in diffuse high-energy astrophysical neutrino data

Although the sources of astrophysical neutrinos are still unknown, they are believed to be produced by a population of sources in the distant Universe. Measurements of the diffuse, all-sky astrophysical flux can thus be sensitive to flavor- and energy-dependent propagation effects, such as very long–baseline oscillations. These oscillations are present in certain neutrino mass models, such as when neutrinos are quasi-Dirac. Assuming generic models for the source flux, we find that these oscillations can still be resolved even when integrated over wide distributions in source redshift. We use two sets of IceCube all-sky flux measurements, made with muon and all-flavor neutrino samples, to set constraints at the 3 σ level on quasi-Dirac mass splittings between ( 5 × 10 − 19 , 8 × 10 − 19 ) eV 2 . Additionally, we find that when the mass-square splittings are different between generations, the quasi-Dirac scenario can explain tensions between the all-flavor and muon-flavor measurements. We consider systematic uncertainties on the source population and find that our results are robust under alternate spectral hypotheses or physical redshift distributions. Our analysis shows that spectral features in the all-sky neutrino measurements provide strong constraints on massive neutrino scenarios and are sensitive to uncharted parameter space.