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, an 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. An observer on the ground measures time flow more slowly for the muon, meaning fewer will have decay by the time the muon reaches the ground. In the muon's frame of reference, it observes the distance between where it started and the ground contracted and so the time it takes to reach the ground is less than in an observer's frame of reference. Hence many more reach the ground than Newtonian mechanics would suggest.
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.