Muon

According to the Standard Model of particle physics, a muon (also known as a mu meson) is a collective name for two semistable fundamental particles with positive and negative charge. Muons have a mass that is 207 times greater than the electron (105.6 MeV) and a spin of 1/2. Both electrons and muons belong to to the same family of fermions (i.e., fundamental particles) called the leptons. Because of this, a negatively-charged muon can be thought of as an extremely heavy electron. Muons are denoted by μ^- and μ⁺ depending on their charge.

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 mouns created when the pion decays are also short-lived: their decay time is 2.2 microseconds. However, the muons have high energies, so the time dilation[?] effects of special relativity make them easily detectable at the earth's surface.

As with the case of electrons there is a muon neutrino which is associated with the muon. Muon neutrinos are denoted by ν_μ.

Positive muons can form a particle called muonium, or μ⁺e^–. Due to the mass difference between the muon and the electron, muonium is more similar to atomic hydrogen than positronium.

Reference: Serway & Faughn, College Physics, Fourth Edition (Fort Worth TX: Saunders, 1995) page 841