atmosphericneutrino

Atmospheric neutrinos are neutrinos produced when cosmic rays interact with nuclei in Earth's atmosphere, creating cascades that generate charged pions and kaons. These mesons decay to muons and muon neutrinos, and the muons further decay to electrons and additional neutrinos, resulting in a flux of electron and muon neutrinos arriving at the surface.

The production chain begins with primary cosmic rays, mostly protons, colliding with atmospheric nuclei. Charged pions and kaons decay to muons and muon neutrinos; the muons then decay to electrons (or positrons) and further neutrinos. At very high energies, decays of charmed hadrons contribute a prompt neutrino component with a harder spectrum. The resulting atmospheric neutrino flux covers roughly 100 MeV to well above TeV energies. The conventional component from pion/kaon decays dominates at GeV–TeV energies, and muon neutrinos generally outnumber electron neutrinos, with electron neutrinos forming a smaller but non-negligible fraction. The flux also depends on zenith angle, reflecting the varying path length and atmospheric density encountered by cosmic-ray cascades.

Atmospheric neutrinos have played a pivotal role in neutrino physics. They provided clear evidence for neutrino oscillations, notably the disappearance of muon neutrinos and the observation of muon-to-tau oscillations, through detectors such as Super-Kamiokande. The up-down flux asymmetry and energy- and angle-dependent flavor composition constrain oscillation parameters. In addition, atmospheric neutrinos serve as a calibration and background source for underground and undersea neutrino telescopes studying astrophysical neutrinos, and they test hadronic interaction models and charm production in the atmosphere.