Muon
A muon (M(Y)OO-on; from the Greek letter mu (μ) used to represent it) is an elementary particle classified as a lepton. It is similar to the electron in that it possesses an electric charge of −1 e and a spin of 1/2 ħ, but it is characterized by a significantly greater mass. Like 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. Its decay process is slower than many other unstable particles because it is mediated by the weak interaction and because the mass difference between the muon and its set of decay products is small, providing limited kinetic degrees of freedom for decay. The decay of a muon always produces an electron (or positron) and two types of neutrinos.
As with 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+. Although they were formerly referred to as “mu mesons,” this term is no longer used by the modern physics community.
The physical properties of the muon include:
- Mass: The muon has a mass of 105.66 MeV/c², which is approximately 206.77 times that of an electron.
- Comparison: There is a third lepton, the tau, which is 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 characteristic allows muons of a given energy to penetrate much deeper into matter because the deceleration of electrons and muons is primarily caused by energy loss through the bremsstrahlung mechanism. For example, secondary muons—created by cosmic rays hitting the atmosphere—can penetrate the atmosphere to reach the Earth’s surface and even penetrate into deep mines.
Because muons possess greater mass and energy than the decay energy of radioactivity, they are not produced by radioactive decay. However, they are produced in large quantities in high-energy interactions in normal matter, in specific particle accelerator experiments with hadrons, and in cosmic ray interactions with matter. These interactions typically produce pi mesons initially, which almost always decay into muons.
Additionally, the muon has an associated muon neutrino (denoted by νμ), which differs from the electron neutrino and participates in different nuclear reactions.
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