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'.