Identifying Energy-Dependent Flavor Transitions in High-Energy Astrophysical Neutrino Measurements

The flavor composition of TeV--PeV astrophysical neutrinos, i.e., the proportion of neutrinos of different flavors in their flux, is a versatile probe of high-energy astrophysics and fundamental physics. Because flavor identification is challenging and the number of detected high-energy astrophysical neutrinos is limited, so far measurements of the flavor composition have represented an average over the range of observed neutrino energies. Yet, this washes out the potential existence of changes in the flavor composition with energy and weakens our sensitivity to the many models that posit them. For the first time, we measure the energy dependence of the flavor composition, looking for a transition from low to high energies. Our present-day measurements, based on the 7.5-year public sample of IceCube High-Energy Starting Events (HESE), find no evidence of a flavor transition. The observation of HESE and through-going muons jointly by next-generation neutrino telescopes Baikal-GVD, IceCube-Gen2, KM3NeT, P-ONE, TAMBO, and TRIDENT may identify a flavor transition around 200TeV by 2030. By 2040, we could infer the flavor composition with which neutrinos are produced with enough precision to establish the transition from neutrino production via the full pion decay chain at low energies to muon-damped pion decay at high energies.