Muon is a 1/2 spin fermion and it's one of the fundamental physics particles known as leptons. Muons have similar characterstics to electron except that their mass is about 200 times greater. Muons that are found on Earth are mostly product of a reaction between cosmic rays and matter in Earth's athmosphere. Even though muons have a very short lifetime (2.2 microseconds) we still see a relatively large flux of muons on Earth's surface, since muons travel at high velocities and thust time dialation effect allows them to travel far.

Muons are one of the biggest sources of background radiation on Earth's surface. This is precisely why many experiments are moved deep into Earth in order to eliminate the background noise of muons.