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.
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, a 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.