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

The flavor composition of cosmic neutrinos probes both the physical properties of their sources and oscillations over energies and distances unreachable by terrestrial experiments. Using 11.4 yr of IceCube data, we analyze the flavor composition of the all-sky neutrino flux from 5 TeV–10 PeV, significantly lowering the analysis threshold from a previous minimum of 35 TeV. The best-fit flavor fractions are 𝑓$_𝑒$,𝑓$_𝜇$,𝑓$_𝜏$ =0.3⁢0$\frac{+0.20}{−0.15}$, 0.37$\frac{+0.14}{−0.16}$, 0.33$\frac{+0.24}{−0.25}$. For the first time, we report that neutrinos of all three flavors are present in the observed cosmic neutrino flux at >90% confidence level. We infer the flavor composition at their sources, finding good agreement with pion decay while pure neutron decay is disfavored, with posterior probability 2.7 ×10$^{−5}$ ( ∼4⁢𝜎).