Atmospheric neutrino

An atmospheric neutrino is produced in a cosmic-ray air shower, principally through decays of secondary mesons and muons. For example, a charged pion can produce a muon and a muon-flavor neutrino; the later muon decay produces additional electron- and muon-flavor neutrinos or antineutrinos.

The direction determines a broad range of propagation distances: downward events originate relatively nearby, whereas upward events can arrive through Earth from the opposite atmosphere. Comparing event distributions by direction and energy therefore tests flavor conversion over different baselines. The 1998 Super-Kamiokande analysis found a direction-dependent muon-neutrino deficit consistent with oscillations, using a large water detector to observe charged interaction products.^[ref] The incident spectrum and interaction probabilities remain part of the inference.