Muon

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...