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Atmospheric neutrinos are the result of cosmic rays (typically protons) striking a nucleus in the upper atmosphere. The shower of particles starts with pions, which decay via the weak interaction to muons and muon neutrinos. The muons subsequently decay to electrons, an electron neutrino and another muon neutrino. The Super-Kamiokande experiment did not measure the expected two-to-one ratio of muon to electron neutrinos, and this provided an important element of their evidence for neutrino oscillation in 1998.
Primary cosmic rays arrive at the near vicinity of the earth isotropically, having been randomized by interstellar magnetic fields. The Super-K group relied on the up-down symmetry of high energy neutrinos (with energy greater than 1 GeV) from high energy cosmic rays as another element of their evidence for neutrino oscillation. However, low energy cosmic rays are deflected by the earths magentic field and are therefore more complicated to study.In particular, low energy cosmic rays from the east are suppressed compared to those from the west, because the presence of the earth effectively shadows certain trajectories, which are therefore forbidden. In the 1930's this east-west effect was detected in charged secondary cosmic rays and was used to infer that the sign of the primary cosmic rays charge must be positive. Super-K has for the first time detected this asymmetry in the flux of atmospheric neutrinos.
Reported by: the Super Kamiokande collaboration in the 21 June issue of Physical Review Letters