Characterization of the Three-Flavor Composition of Cosmic Neutrinos with IceCube

Neutrinos oscillate over cosmic distances. Using 11.4 years of IceCube data, the flavor composition of the all-sky neutrino flux from 5,TeV–10,PeV is studied. We report the first measurement down to the $\mathcal{O}$(TeV) scale using events classified into three flavor-dependent morphologies. The best fit flavor ratio is $f_e:f_{\mu }:f_{\tau }=0.30:0.37:0.33$, consistent with the standard three-flavor neutrino oscillation model. Each fraction is constrained to be $>0$ at $>$ 90% confidence level, assuming a broken power law for cosmic neutrinos. We infer the flavor composition of cosmic neutrinos at their sources, and find production via neutron decay lies outside the 99% confidence interval.