Flavor Composition of the High-Energy Neutrino Events in IceCube

The IceCube experiment has recently reported the observation of 28 high-energy ($>30\mathrm{TeV}$) neutrino events, separated into 21 showers and 7 muon tracks, consistent with an extraterrestrial origin. In this Letter, we compute the compatibility of such an observation with possible combinations of neutrino flavors with relative proportion $({\alpha }_e:{\alpha }_{\mu }:{\alpha }_{\tau }{)}_{\oplus }$. Although the $7:21$ track-to-shower ratio is naively favored for the canonical $(1:1:1{)}_{\oplus }$ at Earth, this is not true once the atmospheric muon and neutrino backgrounds are properly accounted for. We find that, for an astrophysical neutrino $E^{-2}$ energy spectrum, $(1:1:1{)}_{\oplus }$ at Earth is disfavored at 81% C.L. If this proportion does not change, 6 more years of data would be needed to exclude $(1:1:1{)}_{\oplus }$ at Earth at $3\sigma$ C.L. Indeed, with the recently released 3-yr data, that flavor composition is excluded at 92% C.L. The best fit is obtained for $(1:0:0{)}_{\oplus }$ at Earth, which cannot be achieved from any flavor ratio at sources with averaged oscillations during propagation. If confirmed, this result would suggest either a misunderstanding of the expected background events or a misidentification of tracks as showers, or even more compellingly, some exotic physics which deviates from the standard scenario.

Figure 1

Ternary plot of the exclusion C.L. for all possible flavor combinations (${\alpha }_{e,\oplus }:{\alpha }_{\mu ,\oplus }:{\alpha }_{\tau ,\oplus }$) as seen at Earth, given the 7 tracks and 21 showers observed at IceCube. The lower right corner corresponds to 100% electron neutrinos, the upper corner to 100% muon neutrinos, and the lower left corner to 100% tau neutrinos. The central sliver outlined in blue corresponds to the possible flavor combinations for astrophysical neutrinos, after oscillations have been averaged during propagation. The best fit is the darkest point, $(1:0:0{)}_{\oplus }$. The white star corresponds to $(1:1:1{)}_{\oplus }$, which is expected from a $(1:2:0{)}_S$ combination at the source. The color scale indicates the exclusion C.L. given an $E^{-2}$ spectrum of incoming neutrinos. Solid (dashed) lines show 68% C.L. (95% C.L.) contours, cyan for $E^{-1}$, thick black for $E^{-2}$, and pink for $E^{-3}$ spectra.

Figure 2

Same as Fig. 1, but for $\{{\alpha }_{i,S}\}$ at the source and assuming the signal neutrinos are astrophysical and oscillation probabilities are in the averaged regime. That is, the parameter space is restricted to the blue sliver shown in Fig. 1. The best fit is the darkest point, $(1:0:0{)}_S$. The white star corresponds to the $(1:2:0{)}_S$ flavor combination. Standard flavor compositions lie within a narrow band along the right side of the triangle. Note that all combinations are allowed at 95% C.L. for the three spectra, and even at 68% C.L. for $E^{-3}$.