Measuring neutrino mixing above 1 TeV with astrophysical neutrinos

Abstract

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 $ν_e$, $ν_μ$, and $ν_τ$. 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 $θ_{23}$ and $θ_{13}$ 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.

Figures (9)

• Figure 1: Evolution of measurements of neutrino mixing parameters above 1 TeV. Present results are derived from IceCube observations of the flavor composition of high-energy astrophysical neutrinos (11.4-yr MESE sample). Projections (2040/2050) combine HESE and through-going muon samples across multiple detectors under two neutrino production scenarios. Each parameter is constrained individually, profiling over remaining parameters within current global sub-TeV ranges. Future neutrino telescopes will constrain $\theta_{23}$ and $\theta_{13}$ (and possibly $\delta_{\rm CP}$) above 1 TeV for the first time.
• Figure 2: High-energy neutrino flavor composition at Earth. Theory-allowed regions vary $f_{e, {\rm S}} \in [0, 1]$ and mixing parameters within experimental uncertainties ("theoretically palatable" Bustamante:2015waaSong:2020nfh) or unitarity constraints ("unitarity-allowed" Ahlers:2018yom). No $\nu_\tau$ production is assumed. Present IceCube MESE measurements Abbasi:2025fjc have large uncertainty. They are consistent with standard mixing expectations, but cannot constrain mixing parameters. Projected multi-telescope combinations, assuming neutrino production via full pion decay and muon-damped decay, reveal a path to improved precision.
• Figure 3: Constraints on neutrino mixing parameters with high-energy astrophysical neutrinos. Present constraints from the 11.4-yr IceCube MESE sample Abbasi:2025fjc are consistent with three-flavor mixing but cannot constrain individual parameters. Projected measurements from multi-telescope observations show sensitivity to $\theta_{23}$ and $\theta_{13}$ (and $\delta_{\rm CP}$ under muon-damped production). For comparison, we show current Esteban:2024eli and projected Song:2020nfh sub-TeV global-fit ranges. These results represent the first rigorous assessment of mixing parameter sensitivity at TeV--PeV energies.
• Figure A1: Present experimental likelihood of flavor-composition measurements. This likelihood, $\mathcal{L}$, is a parametric form of the allowed contours reported Ref. Abbasi:2025fjc, inferred from the 11.4-yr IceCube MESE sample IceCube:2025tgp.
• Figure A2: Forecasts of experimental likelihood of flavor-composition measurements. Same as Fig. \ref{['fig:flavor_likelihood_mese']}, but for forecasts using HESE plus through-going muons and assuming (left column) neutrino production via pion decay, i.e., $(\frac{1}{3}, \frac{2}{3}, 0)_{\rm S}$, and (right column) via muon-damped pion decay, i.e., $(0, 1, 0)_{\rm S}$Top: Using combined observations by multiple existing neutrino telescopes with km$^3$ size. Center: Combination of existing and future neutrino telescopes, including telescopes of 8 km$^3$. Bottom: Combination including one telescope with tens of km$^3$.