Mauricio Bustamante (Ohio State University)

What to Expect for the Flavor Composition of High-Energy Astrophysical Neutrinos

After decades of searching, high-energy astrophysical neutrinos (10's TeV - 2 PeV) were recently detected by IceCube. Among the wealth of information gathered, the flavor composition --the proportion of electron-, muon-, and tau-neutrinos in the flux-- stands out as one of the richest. Due to neutrino oscillations, it does not equal the flavor composition that leaves the sources. A precise measurement of the flavor composition at Earth can potentially reveal the identity of the sources and of the neutrino production process, whether there is new physics during the propagation, and of what kind. It is therefore useful and timely to explore what we should expect from theory. I will show that, despite the flavor composition at the astrophysical sources being unknown and the neutrino mixing parameters being varied within their experimental uncertainties, the region of allowed composition at Earth is surprisingly small, only ~10% of the total flavor space. It remains small --only ~25% of the total-- even if a broad class of new physics, which includes neutrino decay, is present during propagation. To access the remaining region of flavor space, a broader class of new physics models, which includes CPT violation, would be required. Future improvements in both neutrino telescopes and oscillation experiments will further cement these observations, providing key information to solve the puzzle of the origin of the high-energy astrophysical neutrinos.