Muons were discovered by Carl D. Anderson and Seth Neddermeyer at Caltech in 1936, while studying cosmic radiation. Muons make up much of the cosmic radiation reaching the earth's surface.

The muon (from the Greek letter mu (μ) which used to represent it) is an elementary particle similar to the electron, with an electric charge, but with a much greater mass (approx. 200 times). It is classified as a lepton. A lepton is an elementary particle that does not undergo strong interactions. As is the case with other leptons, the muon is not believed to have any sub-structure, that is, it is not thought to be composed of any simpler particles.

The muon is an unstable subatomic particle. In the physical sciences, subatomic particles are particles much smaller than atoms. It has a mean lifetime (the decaying quantity) of 2.2 µs (microsecond), much longer than many other subatomic particles.

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