The fraction of muon tracks in cosmic neutrinos

The study of the distintive signatures of the ultra high energy events recently seen by IceCube can allow to single the neutrino origin out. The detection of tau neutrinos would be a clear way to prove that they come from cosmic distances, but at the highest energies currently seen, about 1 PeV, an experimental characterization of tau events is difficult. The study of the fraction of the muon tracks seems more promising. In fact, for any initial composition, because of the occurrence of flavor oscillations and despite their uncertainties, the fraction of muon tracks in the cosmic neutrinos is smaller than the one of atmospheric neutrinos, even hypothesizing an arbitrarily large contribution from charmed mesons. A good understanding of the detection efficiencies and the optimization of the analysis cuts, along with a reasonable increase in the statistics, should provide us with a significant test of the cosmic origin of these events.