Electron anti-neutrinos at the Glashow resonance (GR, at Eν¯e∼6.3 PeV) have an enhanced probability to be detected. With three neutrinos detected by IceCube in the (1–2) PeV energy range at present, one would expect that about 1 to 4 GR ν¯e should have been detected. The high-energy ∼8.7 PeV muon neutrino detected by IceCube may not be a GR event. If so, we expect to detect 50 to 70 GR ν¯e, then one would have a “missing Glashow-resonance problem”. This would suggest (1) that pγ interaction rather than pp interaction is the dominant channel to produce the observed IceCube high-energy neutrinos; (2) that multi-pion pγ interactions are suppressed; and (3) that the magnetic field and photon energy density in the pγ emission region is such that significant μ+ cooling occurs before decaying, yet π+'s essentially do not cool before decaying.
Sahu, S., & Zhang, B. (2018). On the non-detection of Glashow resonance in IceCube. Journal of High Energy Astrophysics, 18, 1–4. https://doi.org/10.1016/j.jheap.2018.01.003