A typical particle shower from a cosmic ray proton.

A Muon decays into a muon neutrion, an electron,
and an electron antineutrino.

A tau can decay into a tau neutrino, a muon, and a
muon antineutrino.

The Plot Thickens

  • The discovery of different neutrino types came from study of cosmic rays.
  • Cosmic rays are constantly bombarding our atmosphere. These rays are predominantly fast moving protons from deep space.
  • The cosmic rays collide with atoms in the air and create a variety of exotic particles which continue on to the surface of the Earth.
  • High energy cosmic rays come from massive stars which undergo supernova explosions, but some cosmic rays with an unknown source have even higher energies.
  • The muons in the diagram are similar to the electron, though they are heavier and susceptible to spontaneous decay into lighter particles.
  • One of these lighter particles is a neutrino, but it was found to have a different signature than the neutrinos made in nuclear reactions which Cowan and Reines discovered.
  • This new particle was named 'muon neutrino', while the previously discovered neutrino was renamed 'electron neutrino'.
  •  In 1977, an even heavier electron-like particle was discovered: the tau lepton. It too had a neutrino associated with it, and it was named 'tau neutrino'.