Measuring neutrino mixing above 1 TeV with astrophysical neutrinos

We assess the potential for measuring neutrino mixing parameters at energies above 1~TeV, for the first time, using the flavor composition of TeV–PeV astrophysical neutrinos, i.e., the proportion of ${\nu }_e$, ${\nu }_{\mu }$, and ${\nu }_{\tau }$. Today, flavor measurements inferred from the 11.4-year IceCube Medium Energy Starting Events sample are insufficient to constrain the mixing parameters due to limited statistics, challenges in flavor identification, and uncertainty in neutrino production. Yet, upcoming multi-neutrino-telescope observations – even using only existing telescopes – may achieve sensitivity to ${\theta }_{23}$ and ${\theta }_{13}$ when combined with traditional oscillation experiments. We establish the current status and future prospects for testing the three-flavor mixing framework in the previously unexplored TeV–PeV regime and quantify the minimum detectable size of flavor-modifying beyond-Standard-Model effects, providing a roadmap for high-energy neutrino mixing measurements.