Definition of "muon"

The μ-meson of Powell (called here muon) is instead a disintegration product of the pion, only weakly linked to the nucleons and therefore of little importance in the explanation of nuclear forces.

1951, Enrico Fermi, Elementary Particles

The spectrum of electrons arising from the decay of the negative mu meson has been determined. The muons are arrested in the gas of a high pressure hydrogen filled diffusion cloud chamber.

1955 March, CP Sargent, “Diffusion Cloud-Chamber Study of Very Slow Mesons”, in Physical Review

The data comes from experiments at the Fermilab US particle accelerator facility, which explored how subatomic particles called muons – similar to electrons but about 200 times heavier – move in a magnetic field.

2023 August 11, Nicola Davis, “Scientists may be on brink of discovering fifth force of nature”, in The Guardian‎