High energy cosmic rays hitting the atmosphere of the earth produce a cascade of secondary particles. Among them pions are particularly numerous. A negative pion decays to a muon and a muon antineutrino, and the resulting muon decays to an electron, an electron antineutrino, and a muon neutrino. A positively charged pion produces the charge conjugates of the respective leptons.
The resulting total neutrino flux can be estimated if the original cosmic ray flux is known. Unfortunately this involves lots of uncertainties, both theoretical and experimental. However, the ratio of muon neutrinos to electron neutrinos can be predicted to be 2, with a good accuracy. On the other hand, muon and electron neutrinos have different spectra that are known a little less accurately.
In water Cerenkov detectors the muon and electron signals can be distinguished by the shape of the ring. In matter electrons lose energy by bremstrahlung (which is proportional to mass to the fourth) which causes minimal changes in their trajectories. Consequently, the Cerenkov cone is varied, so that the rings are less sharp.
Experiments measuring atmospheric neutrino flux
L/E plot of SuperKamiokande:
Explanation of the data
The ratio refers to the ratio of electron-like (non-showering) events to
muon-like (showering) events.
Theoretical explanationsThe data is consistent with neutrino oscillation from muon neutrinos to tau neutrinos. Oscillation to electron neutrinos do not fit the Superkamiokande data. Oscillation to sterile neutrinos is disfavored, at least in 2 sigma level.
The favored values (SK) for the oscillation parameters are
m2 ~ 1.5 10-3 ... 5 10-3
sin2 > 0.88
PlotsA two flavor fit to SuperKamiokande data, presented by Sobel at Neutrino 2000
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