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A muon (/ ˈ m (j) uː ɑː n / M(Y)OO -on; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of − 1 e and spin-1/2, but with a much greater mass. It is classified as a lepton. As with other leptons, the muon is not thought to be composed of any simpler particles.

The muon is an unstable subatomic particle with a mean lifetime of 2.2 μs, much longer than many other subatomic particles. As with the decay of the free neutron (with a lifetime around 15 minutes), muon decay is slow (by subatomic standards) because the decay is mediated only by the weak interaction (rather than the more powerful strong interaction or electromagnetic interaction), and because the mass difference between the muon and the set of its decay products is small, providing few kinetic degrees of freedom for decay. Muon decay almost always produces at least three particles, which must include an electron of the same charge as the muon and two types of neutrinos.

Like 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. Formerly, muons were called mu mesons, but are not classified as mesons by modern particle physicists, and that name is no longer used by the physics community.

Muons have a mass of 105.66 MeV/ c, which is approximately 206.768 2827 (46) times that of the electron, m. There is also a third lepton, the tau, approximately 17 times heavier than the muon.

Due to their greater mass, muons accelerate slower than electrons in electromagnetic fields, and emit less bremsstrahlung (deceleration radiation). This allows muons of a given energy to penetrate far deeper into matter because the deceleration of electrons and muons is primarily due to energy loss by the bremsstrahlung mechanism. For example, so-called secondary muons, created by cosmic rays hitting the atmosphere, can penetrate the atmosphere and reach Earth's land surface and even into deep mines.

Because muons have a greater mass and energy than the decay energy of radioactivity, they are not produced by radioactive decay. Nonetheless, they are produced in great amounts in high-energy interactions in normal matter, in certain particle accelerator experiments with hadrons, and in cosmic ray interactions with matter. These interactions usually produce pi mesons initially, which almost always decay to muons.

As with the other charged leptons, the muon has an associated muon neutrino, denoted by ν, which differs from the electron neutrino and participates in different nuclear reactions.