The muon can be considered the big sibling of the electron: they share the same properties, but the muon is about 200 times heavier than the electron.
Muons are produced naturally in our atmosphere when cosmic rays, protons and other particles from the Universe, hit the atmosphere, thus creating a shower of new, secondary particles. They are also created in particle accelerators or as decay products of heavier particles.
The muon was discovered by C. Anderson in his studies on cosmic rays in 1936, but only in 1947 it was correctly identified as a "heavy electron". Muons can be used to confirm Einstein’s theory of special relativity: given their lifetime, they would cross only a small fraction of the atmosphere before decaying into an electron and two neutrinos. However, as they are travelling at almost the speed of light, we still measure about 100 muons per square metre and per second at sea level, 1 million times more than otherwise expected! This is a demonstration that time passes more slowly for very fast particles. This process is known as time dilation.
In interactions involving muons, there is a conserved quantity that says that the total number of muons and muon neutrinos should remain constant. Hints of New Physics are searched in hypothetical decays, which violate this quantity and thus hint towards new physics.