Measuring flavor ratios of high-energy astrophysical neutrinos

We discuss the prospects for next generation neutrino telescopes, such as IceCube, to measure the flavor ratios of high-energy astrophysical neutrinos. The expected flavor ratios at the sources are φ_νₑ:φ_ν_μ:φ_ν_τ=1:2:0, and neutrino oscillations quickly transform these to $1:1:1.$ The flavor ratios can be deduced from the relative rates of showers (νₑ charged current, most ν_τ charged current, and all flavors neutral current), muon tracks (ν_μ charged current only), and tau lepton lollipops and double bangs (ν_τ charged current only). The peak sensitivities for these interactions are at different neutrino energies, but the flavor ratios can be reliably connected by a reasonable measurement of the spectrum shape. Measurement of the astrophysical neutrino flavor ratios tests the assumed production mechanism and also provides a very long baseline test of a number of exotic scenarios, including neutrino decay, CPT violation, and small-δm² oscillations to sterile neutrinos.