Muon

A muon is a semistable fundamental particle with negative electric charge and a spin of 1/2. Together with the electron, the tau lepton and the neutrinos, it is classified as part of the lepton family of fermions. Like all fundamental particles, the muon has an antimatter partner of opposite charge but equal mass and spin: the antimuon.

Since their interactions are very similar to that of the electron, a muon can often be thought of as an extremely heavy electron. Muons are denoted by μ− and antimuons by μ+.

In planetary conditions, muons are created when a charged pion decays. The pions are created in the upper atmosphere by cosmic radiation and have a very short decay time — a few nanoseconds. The muons created when the pion decays are also short-lived: their decay time is 2.2 microseconds. However, muons in the atmosphere are moving at very high velocities, so that the time dilation effect of special relativity makes them easily detectable.

As with the case of the other charged leptons, there is a muon-neutrino which has the same flavor as the muon. Muon-neutrinos are denoted by νμ. Muons naturally decay into an electron, an electron-antineutrino, and a muon-neutrino.

Muonic atoms

Muons do not appear in ordinary atoms. Negative muons can, however, form muonic atoms by replacing an electron in ordinary atoms. Muonic atoms are much smaller than typical atoms because, in order to conserve angular momentum, the more massive muon must be closer to the atomic nucleus than its less massive electron counterpart.

A positive muon, when stopped in ordinary matter, can also bind an electron and form the muonium (Mu) atom, in which the muon acts as the nucleus. The reduced mass of muonium is very close to that of hydrogen, hence this short lived atom behaves chemically − in first approximation − like its heavier isotopes, hydrogen, deuterium and tritium.