When high‐energy cosmic ray protons and nuclei hit the top of the atmosphere, they generate showers of mesons that quickly decay to muons, electrons, positrons and neutrinos. A few kilometers below the surface of the Earth, nothing remains but the most energetic muons and almost all of the neutrinos. Irrespective of the complicated details, unimpeachable particle theory dictates that these hadronic showers produce, on average, twice as many muon‐flavored neutrinos ( and ) as electron‐flavored neutrinos ( and ). The details leave wiggle room for only a few percent departure from this predicted 2:1 ratio.
Bertram Schwarzschild; Anomalous Cosmic‐Ray Data Suggest Oscillation Between Neutrino Flavors. Physics Today 1 October 1994; 47 (10): 22–24. https://doi.org/10.1063/1.2808665
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