muon (myōˈŏn) [key], elementary particle heavier than an electron but lighter than other particles having nonzero rest mass. The name muon is derived from mu meson, the former name of the particle. The muon was first observed in cosmic rays by Carl D. Anderson and Seth Neddermeyer in 1936, the year after the existence of a particle of about the same mass had been predicted by Hideki Yukawa. However, the muon's behavior did not conform to that of Yukawa's meson theory (which actually describes the pion, discovered more than 10 years later), and the muon is now classed as a lepton rather than a meson. The muon resembles the electron in every way except mass, the muon having 207 times the mass of the electron. Each particle is negatively charged and has a positively charged antiparticle; each has half-integer spin and participates in the weak nuclear force but not in the strong force; and each has an associated neutrino and antineutrino. Muons are produced by the weak decay of pions into a muon and a muon antineutrino. The muon differs from the electron in that it is unstable, decaying with an average lifetime of 2.2 × 10 - 6 sec (2.2 microseconds) into an electron or positron and a pair of neutrinos, but this difference is related to the difference in mass; the electron is stable because there is no lighter particle into which it can decay. Muons can be substituted for electrons in orbit around the nucleus of an atom; the resulting atom is long-lived enough to exhibit behavior that further supports the close resemblance between the muon and the electron. Recent studies of muons have included the production of "muonic atoms" (ordinary atoms to which an orbiting muon is added) and muonium, which consists of an electron in orbit around a positive muon.