In the Standard Model of particle physics, a muon (Greek μείον = 'minus') is a semistable fundamental particle with negative electric charge and a spin of 1/2. Together with the electron, the tauon and the neutrinos, it is classified as part of the lepton family of fermions. Like all fundamental particles, the muon has an antimatter partner of opposite charge but equal mass and spin: the antimuon.
For historical reasons, muons are sometimes referred to as mu mesons, even though they are not classified as mesons by modern particle physicists (see History). Muons have a mass that is 207 times greater than the electron (105.6 MeV). Because of this, a muon can be thought of as an extremely heavy electron. Muons are denoted by μ- and antimuons by μ+.
On earth, muons are created when a charged pion decays. The pions are created in an upper atmosphere by cosmic radiation and have a very short decay time--a few nanoseconds. The muons created when the pion decays are also short-lived: their decay time is 2.2 microseconds. However, muons in the atmosphere are moving at very high velocities, so that the time dilation effect of special relativity make them easily detectable at the earth's surface.
As with the case of the other charged leptons, there is a muon-neutrino which is associated with the muon. Muon-neutrinos are denoted by νμ. Muons naturally decay into an electron, an electron-antineutrino, and a muon-neutrino.
The muon was the first elementary particle discovered that does not appear in ordinary atoms. Muons can, however, form muonic atoms by replacing the electrons in ordinary atoms. Muonic atoms are much smaller than typical atoms because, in order to conserve angular momentum, the more massive muon must be closer to the atomic nucleus than its less massive electron counterpart.
Muons were discovered by Carl D. Anderson in 1936 while he studied cosmic radiation. He had noticed particles that curved in a manner distinct from that of electrons and other known particles when passed through a magnetic field. In particular, these new particles curved to a smaller degree than electrons, but more sharply than protons. It was assumed that their electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that these particles were of intermediate mass (lying somewhere between that of an electron and that of a proton).
For this reason, Anderson initially called the new particle a mesotron, adopting the prefix meso- from the Greek word for "intermediate". Shortly thereafter, additional particles of intermediate mass were discovered, and the more general term meson was adopted to refer to any such particle. Faced with the need to differentiate between different types of mesons, the mesotron was renamed the mu meson (with the Greek letter mu used to approximate the sound of the English letter m).
However, it was soon found that the mu meson significantly differed from other mesons (e.g. its breakdown products included a neutrino and an antineutrino, rather than one or the other as was observed in other mesons). Thus mu mesons were not mesons at all, and so the term mu meson was abandoned and replaced with the modern term muon.
In the mid 1970s, experimental physicists working at the European Center for Nuclear Research fired neutrinos at a proton target. According to what was then known about the weak interaction, they expected the collision to turn the neutrino into a muon, and the proton into debris. They were surprised to discover that two muons, one negatively and one positively charged, result from such collision.
This generated a good deal of theoretical discussion, until a consensus emerged on how that positive muon comes about. The neutrino/proton collision produces not only proton debris and a negative muon, but a charmed quark, and the quark soon decays into a strange quark, a muon neutrino, and a positive muon.
- Serway & Faughn, College Physics, Fourth Edition (Fort Worth TX: Saunders, 1995) page 841
- Emanuel Derman, My Life As A Quant (Hoboken, NJ: Wiley, 2004) pp. 58-62.