MUON

subatomic particle similar to the electron but 207 times heavier; it has two forms, the negative muon and its positive antiparticle. The muon was discovered in 1936 by the American physicist Carl D. Anderson. Because of its mass, it was at first thought to be the particle predicted by the Japanese physicist Yukawa Hideki (1935) to explain the strong interaction that binds protons and neutrons together in atomic nuclei. It has since been discovered, however, that a muon never reacts with nuclei or other particles through the strong interaction; rather, it decays by the weak interaction into an electron and two kinds of neutrinos. Because muons are charged, before decaying they lose energy by knocking electrons out of atoms (ionization). At high particle velocities close to the speed of light, ionization dissipates energy in relatively small amounts, so muons in cosmic radiation are extremely penetrating and can travel thousands of metres below the Earth's surface.