Observation of atmospheric neutrinos

Atmospheric neutrinos are produced as decay products in hadronic showers re sulting from collisions of cosmic rays with nuclei in the atmosphere. Elect ron neutrinos and muon neutrinos are produced mainly by the decay chain of charged pions to muons to electrons. Atmospheric neutrinos have been observ ed by large underground detectors. Depending on their energy, these neutrin os are observed as fully contained events. partially contained events, or u pward 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 th at some data suggested the existence of neutrino oscillations. With the rec ent increase in event data, especially from Super-Kamiokande, it is conclud ed that the atmospheric neutrino data give evidence for neutrino oscillatio ns. Two-flavor nu (mu)-->nu (tau) oscillations, with sin(2) 2 theta >0.88 a nd Deltam(2) in the region of 2x10(-3) to 5x10(-3) eV(2), explain all these data.