Muons (from the Greek letter mu (μ) used to represent it)  are elementary particles similar to electrons, with an electric charge of −1 e and a spin of  1/2 , but with a much greater mass.(105.6583715(35) MeV/c2  vs  0.5109989461(31) MeV/c2 for the electron)

They are classified as a leptons. As is the case with other leptons, Muons are not believed to have any sub-structure—that is, it is not thought to be composed of any simpler particles.

Muons are unstable subatomic particles with a mean lifetime of 2.2 µs. Among all known unstable subatomic particles, only neutrons (lasting around 15 minutes) has a longer decay lifetime; others decay significantly faster. 

Muons decay by the weak interaction exclusively, as is neutron decay. Muons decay to always produce at least three particles, which must include an electron of the same charge as the muon and two neutrinos of different types.

The most common decay of the muon produces neutrinos and an electron

Where do Muons come from?

Muons arriving on the Earth's surface are created indirectly as decay products of collisions of cosmic rays with particles of the Earth's atmosphere. 

About 10,000 muons reach every square meter of the earth's surface a minute; these charged particles form as by-products of cosmic rays colliding with molecules in the upper atmosphere. Travelling at relativistic speeds, muons can penetrate tens of meters into rocks and other matter before attenuating as a result of absorption or deflection by other atoms. 

When a cosmic ray proton impacts atomic nuclei in the upper atmosphere, pions(pi mesons, quark pairs) are created. These decay within a relatively short distance (metres) into muons (their preferred decay product), and muon neutrinos. The muons from these high energy cosmic rays generally continue in about the same direction as the original proton, at a velocity near the speed of light. 

Although their lifetime without relativistic effects would allow a half-survival distance of only about 456 m (2,197 µs×ln(2) × 0,9997×c) at most (as seen from Earth) the time dilation effect of special relativity (from the viewpoint of the Earth) allows cosmic ray secondary muons to survive the flight to the Earth's surface, since in the Earth frame, the muons have a longer half life due to their velocity. From the viewpoint (inertial frame) of the muon, on the other hand, it is the length contraction effect of special relativity which allows this penetration, since in the muon frame, its lifetime is unaffected, but the length contraction causes distances through the atmosphere and Earth to be far shorter than these distances in the Earth rest-frame. Both effects are equally valid ways of explaining the fast muon's unusual survival over distances.

Since muons unusually go through ordinary matter, like neutrinos, they are also detectable deep underground (700 meters at the Soudan 2 detector) and underwater, where they form a major part of the natural background ionizing radiation. Like cosmic rays, as noted, this secondary muon radiation is also directional.

Adapted from:

Muon. (2016, September 3). In Wikipedia, The Free Encyclopedia. Retrieved 00:36, September 12, 2016, from

See also

 Street, J.; Stevenson, E. (1937). "New Evidence for the Existence of a Particle of Mass Intermediate Between the Proton and Electron". Physical Review. 52 (9): 1003. doi:10.1103/PhysRev.52.1003