A muon is a fundamental constituent of matter. Its properties are similar to those of the more-familiar electron, but it is roughly 206 times more massive. Muons belong to a category of elementary particles known as leptons.
Muons were discovered by Carl D. Anderson, n American physicist who also first observed the positron, in 1936.
Links with relativity
Muons can be formed by interactions between cosmic rays and the upper atmosphere. The resulting muons have extremely high energies, and therefore have speeds close to that of light. When the flux of muons reaching the ground is compared to that in the upper atmosphere, we observe many more muon reach the ground than expected (fewer have decayed than expected). This can be explained using time dilation and length contraction. Not only does a muon experience less time than an on the ground, but the distance from the top of the atmosphere and the surface of the earth is greatly reduced in the muon's frame of reference. Hence there will be a greater flux at sea level.
Uses in nuclear fusion
Hypothetically, a steady stream of muons could be used to jumpstart nuclear fusion, by replacing the electrons. Because Muons are 217 times heavier than electrons, an atom with muons orbiting it would be more attracted to other atoms. However, the difficulty in acquiring muons and preventing them from decaying presents a hurdle to nuclear fusion with muons.