Muon decay. Computer illustration showing muons decaying. Muons are unstable elementary particles that are heavier than electrons and neutrinos but lighter than all other matter particles. The dominant muon decay mode (sometimes called the Michel decay after Louis Michel) is the simplest possible: the muon decays to an electron, an electron-antineutrino, and a muon-neutrino. Muons are produced in large quantities when cosmic rays bombard the Earth's atmosphere. These collisions result in the production of a shower of pions and kaons which quickly decay into muons.