Muon

A muon is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of 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. 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 and two types of neutrinos.
Like all elementary particles, the muon has a corresponding antiparticle of opposite charge but equal mass and spin: the antimuon. Muons are denoted by and antimuons by. Formerly, muons were called mu mesons, but are not classified as mesons by modern particle physicists, and that name is no longer used by the physics community.
Muons have a mass of, which is approximately times that of the electron, m. 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. 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, 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.

Because of its mass, the mu meson was initially thought to be Yukawa's particle and some scientists, including Niels Bohr, originally named it the yukon.
The fact that the mesotron was not Yukawa's particle was established in 1946 by an experiment conducted by Marcello Conversi, Oreste Piccioni, and Ettore Pancini in Rome. In this experiment, which Luis Walter Alvarez called the "start of modern particle physics" in his 1968 Nobel lecture, they showed that the muons from cosmic rays were decaying without being captured by atomic nuclei, contrary to what was expected of the mediator of the nuclear force postulated by Yukawa. Yukawa's predicted particle, the pi meson, was finally identified in 1947.
With two particles now known with the intermediate mass, the more general term meson was adopted to refer to any such particle within the correct mass range between electrons and nucleons. Further, in order to differentiate between the two different types of mesons after the second meson was discovered, the initial mesotron particle was renamed the mu meson, and the new 1947 meson was named the pi meson.
As more types of mesons were discovered in accelerator experiments later, it was eventually found that the mu meson significantly differed not only from the pi meson, but also from all other types of mesons. The difference, in part, was that mu mesons did not interact with the nuclear force, as pi mesons did. Newer mesons also showed evidence of behaving like the pi meson in nuclear interactions, but not like the mu meson. Also, the mu meson's decay products included both a neutrino and an antineutrino, rather than just one or the other, as was observed in the decay of other charged mesons.
In the eventual Standard Model of particle physics codified in the 1970s, all mesons other than the mu meson were understood to be hadrons – that is, particles made of quarks – and thus subject to the nuclear force. In the quark model, a meson was no longer defined by mass, but instead were particles composed of exactly two quarks, unlike the baryons, which are defined as particles composed of three quarks. Mu mesons, however, had shown themselves to be fundamental particles like electrons, with no quark structure. Thus, mu "mesons" were not mesons at all, in the new sense and use of the term meson used with the quark model of particle structure.
With this change in definition, the term mu meson was abandoned, and replaced whenever possible with the modern term muon, making the term "mu meson" only a historical footnote. In the new quark model, other types of mesons sometimes continued to be referred to in shorter terminology, but in the case of the muon, it retained the shorter name and was never again properly referred to by older "mu meson" terminology.
The eventual recognition of the muon as a simple "heavy electron", with no role at all in the nuclear interaction, seemed so incongruous and surprising at the time, that Nobel laureate I. I. Rabi famously quipped, "Who ordered that?".
In the Rossi–Hall experiment, muons were used to observe the time dilation predicted by special relativity, for the first time.

Muon sources

Muons arriving on the Earth's surface are created indirectly as decay products of collisions of cosmic rays with particles of the Earth's atmosphere.
When a cosmic ray proton impacts atomic nuclei in the upper atmosphere, pions are created. These decay within a relatively short distance into muons, and muon neutrinos. The muons from these high-energy cosmic rays generally continue in about the same direction as the original proton, at a velocity near the speed of light. Although their lifetime without relativistic effects would allow a half-survival distance of only about 456 m at most, the time dilation effect of special relativity allows cosmic ray secondary muons to survive the flight to the Earth's surface, since in the Earth frame the muons have a longer half-life due to their velocity. From the viewpoint of the muon, on the other hand, it is the length contraction effect of special relativity that allows this penetration, since in the muon frame its lifetime is unaffected, but the length contraction causes distances through the atmosphere and Earth to be far shorter than these distances in the Earth rest-frame. Both effects are equally valid ways of explaining the fast muon's unusual survival over distances.
Since muons are unusually penetrative of ordinary matter, like neutrinos, they are also detectable deep underground and underwater, where they form a major part of the natural background ionizing radiation. Like cosmic rays, as noted, this secondary muon radiation is also directional.
The same nuclear reaction described above is used by particle physicists to produce muon beams, such as the beam used for the muon g−2 experiment.

Muon decay

Muons are unstable elementary particles and are more massive than electrons and neutrinos. Since muons are less massive than all hadrons, no hadrons appear in their decays. Muons decay via the weak interaction. Because leptonic family numbers are conserved in the absence of an extremely unlikely immediate neutrino oscillation, one of the product neutrinos of muon decay must be a muon-type neutrino and the other an electron-type antineutrino.
Because charge must be conserved, one of the products of muon decay is always an electron of the same charge as the muon. Thus all muons decay to at least an electron, and two neutrinos. Sometimes, besides these necessary products, additional other particles that have no net charge and spin of zero, are produced.
The mean lifetime,, of the muon is. The equality of the muon and antimuon lifetimes has been established to better than one part in 10⁴.

Decay modes

The dominant muon decay mode is the simplest possible: the muon decays to an electron, an electron antineutrino, and a muon neutrino. Antimuons, in mirror fashion, most often decay to the corresponding antiparticles: a positron, an electron neutrino, and a muon antineutrino. In formulaic terms, these two decays are:
In addition, five body decays occur with a branching ratio of approximately.

Radiative decays also occur with a branching ratio of approximately.