Observation of atmospheric neutrinos

Atmospheric neutrinos are produced as decay products in hadronic showers resulting from collisions of cosmic rays with nuclei in the atmosphere. Electron neutrinos and muon neutrinos are produced mainly by the decay chain of charged pions to muons to electrons. Atmospheric neutrinos have been observed by large underground detectors. Depending on their energy, these neutrinos are observed as fully contained events, partially contained events, or upward going muon events. The energy range covered by these events is from a few hundred MeV to above 100 GeV. It has been known for about ten years that some data suggested the existence of neutrino oscillations. With the recent increase in event data, especially from Super-Kamiokande, it is concluded that the atmospheric neutrino data give evidence for neutrino oscillations. Two-flavor ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations, with ${\sin }^{2}2\theta >0.88\mathrm{}$ and $\Delta m^2$ in the region of $2\times {10}^{-3}$ to $5\times {10}^{-3}{\mathrm{eV}}^2,$ explain all these data.