A muon ( M(Y)OO-on; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of 1/2 ħ, but with a much greater mass. It is classified as a lepton. As with other leptons, the muon is not thought to be composed of any constituent particles. The muon is an unstable subatomic particle with a mean lifetime of 2.2 μs. Muon decay is slower than many other unstable particles because the decay is mediated by the weak interaction and because the mass difference between the muon and the set of its decay products is small, providing few kinetic degrees of freedom for decay. Muon decay always produces an electron (or positron) and two types of neutrinos. Like all elementary particles, the muon has a corresponding antiparticle of opposite charge (+1 e) but equal mass and spin: the antimuon (also called a positive muon). Muons are denoted by μ− and antimuons by μ+. Formerly, muons were called mu mesons, but are not classified as mesons by modern particle physicists (see § History of discovery), and that name is no longer used by the physics community. Muons have a mass of 105.66 MeV/c2, which is 206.7682827(46) times that of the electron, me. There is also a third lepton, the tau, approximately 17 times heavier than the muon. Due to their greater mass, muons accelerate more slowly than electrons in electromagnetic fields, and emit less bremsstrahlung (deceleration radiation). This allows muons of a given energy to penetrate far deeper into matter because the deceleration of electrons and muons is primarily due to energy loss by the bremsstrahlung mechanism. For example, so-called secondary muons, created by cosmic rays hitting the atmosphere, can penetrate the atmosphere and reach Earth's land surface and even into deep mines. Because muons have a greater mass and energy than the decay energy of radioactivity, they are not produced by radioactive decay. Nonetheless, they are produced in great amounts in high-energy interactions in normal matter, in certain particle accelerator experiments with hadrons, and in cosmic ray interactions with matter. These interactions usually produce pi mesons initially, which almost always decay to muons. As with the other charged leptons, the muon has an associated muon neutrino, denoted by νμ, which differs from the electron neutrino and participates in different nuclear reactions.
History of discovery Muons were discovered by Carl D. Anderson and Seth Neddermeyer at Caltech in 1936 while studying cosmic radiation. Anderson noticed particles that curved differently from electrons and other known particles when passed through a magnetic field. They were negatively charged but curved less sharply than electrons, but more sharply than protons, for particles of the same velocity. It was assumed that the magnitude of their negative electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that their mass was greater than an electron's but smaller than a proton's. Thus Anderson initially called the new particle a mesotron, adopting the prefix meso- from the Greek word for "mid-". The existence of the muon was confirmed in 1937 by J. C. Street and E. C. Stevenson's cloud chamber experiment. A particle with a mass in the meson range had been predicted before the discovery of any mesons, by theorist Hideki Yukawa:
It seems natural to modify the theory of Heisenberg and Fermi in the following way. The transition of a heavy particle from neutron state to proton state is not always accompanied by the emission of light particles. The transition is sometimes taken up by another heavy particle.
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