The muon (; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with unitary negative electric charge of −1 and a spin of , but with a much greater mass (). It is classified as a lepton, together with the electron (mass ), the tau (mass ), and the three neutrinos. As is the case with other leptons, the muon is not believed to have any sub-structure—that is, it is not thought to be composed of any simpler particles. The muon is an unstable subatomic particle with a mean lifetime of . Among all known unstable subatomic particles, only the neutron (~15 minutes) and some atomic nuclei have a longer decay lifetime; others decay significantly faster. The decay of the muon (as well as of the neutron, the longest-lived unstable baryon), is mediated by the weak interaction exclusively. Muon decay always produces at least three particles, which must include an electron of the same charge as the muon and two neutrinos of different types. Like all elementary particles, the muon has a corresponding antiparticle of opposite charge (+1) but equal mass and spin: the antimuon (also called a positive muon). Muons are denoted by and antimuons by . Muons were previously called mu mesons, but are not classified as mesons by modern particle physicists (see History), and that name is no longer used by the physics community. Muons have a mass of , which is about 200 times that of the electron. Due to their greater mass, muons are not as sharply accelerated when they encounter electromagnetic fields, and do not emit as much bremsstrahlung (deceleration radiation). This allows muons of a given energy to penetrate far more deeply into matter than electrons, since the deceleration of electrons and muons is primarily due to energy loss by the bremsstrahlung mechanism. As an example, so-called “secondary muons”, generated by cosmic rays hitting the atmosphere, can penetrate to the Earth’s surface, and even into deep mines. Because muons have a very large mass and energy compared with the decay energy of radioactivity, they are never produced by radioactive decay. They are, however, produced in copious amounts in high-energy interactions in normal matter, in certain particle accelerator experiments with hadrons, or naturally in cosmic ray interactions with matter. These interactions usually produce pi mesons initially, which most often decay to muons. As with the case of the other charged leptons, the muon has an associated muon neutrino, denoted by , which is not the same particle as the electron neutrino, and does not participate in the same nuclear reactions.