Atmospheric neutrinos

Neutrino oscillation was discovered through the study 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 and electrons. Depending on the energy of the neutrinos, atmospheric neutrinos are observed as fully contained events, partially contained events and upward-going muon events. The energy range covered by these events is from a few hundred MeV to >1 TeV. Data from various experiments showed zenith angle- and energy-dependent deficit of ν_μ events, while ν_e events did not show any such effect. It was also shown that the ν_μ survival probability obeys the sinusoidal function as predicted by neutrino oscillations. Two-flavour ν_μ ↔ ν_τ oscillations, with sin² 2θ > 0.90 and Δm² in the region of 1.9 × 10⁻³ to 3.0 × 10⁻³ eV², explain all these data. Various detailed studies using high statistics atmospheric neutrino data excluded the alternative hypotheses that were proposed to explain the ν_μ deficit.