Muon

A muon is an <a href="/facts/Elementary_particle/CTFDM97U">elementary particle</a> similar to the <a href="/facts/Electron/L0KBo5cW">electron</a>, with an <a href="/facts/Electric_charge/xxsBfmlB">electric charge</a> of −1 <a href="/facts/Elementary_charge/lPcj6CoQ">e</a> and a <a href="/facts/Spin-1%2f2/3I929M4X">spin</a> of ⁠1/2⁠ ħ, but with a much greater mass. It is classified as a <a href="/facts/Lepton/X4a6MGop">lepton</a>. As with other leptons, the muon is not thought to be composed of any simpler particles.
The muon is an unstable <a href="/facts/Subatomic_particle/tMEzcTWQ">subatomic particle</a> with a <a href="/facts/Mean_lifetime/wLB1zYDP">mean lifetime</a> of 2.2 <a href="/facts/Microsecond/zdW1DEb2">μs</a>, much longer than many other subatomic particles. As with the decay of the free <a href="/facts/Neutron/oRCM1geb">neutron</a> (with a lifetime around 15 minutes), muon decay is slow (by subatomic standards) because the decay is mediated only by the <a href="/facts/Weak_interaction/f6upmKMs">weak interaction</a> (rather than the more powerful <a href="/facts/Strong_interaction/JjRsKcLg">strong interaction</a> or <a href="/facts/Electromagnetic_interaction/oAkmALHT">electromagnetic interaction</a>), and because the mass difference between the muon and the set of its decay products is small, providing few kinetic <a href="/facts/Degrees_of_freedom/D9RKXgml">degrees of freedom</a> for decay. Muon decay almost always produces at least three particles, which must include an electron of the same charge as the muon and two types of <a href="/facts/Neutrino/yGWVfSg2">neutrinos</a>.
Like all elementary particles, the muon has a corresponding <a href="/facts/Antiparticle/7Ic43ThI">antiparticle</a> of opposite charge (+1 e) but equal <a href="/facts/Mass/HHdc8XmF">mass</a> and spin: the antimuon (also called a positive muon). Muons are denoted by μ− and antimuons by μ+. Formerly, muons were called mu mesons, but are not classified as <a href="/facts/Meson/9R0kIc1q">mesons</a> by modern particle physicists (see § History of discovery), and that name is no longer used by the physics community.
Muons have a <a href="/facts/Mass/HHdc8XmF">mass</a> of 105.66 <a href="/facts/Electronvolt/YMT1fqX9">MeV/c2</a>, which is approximately 206.7682827(46)‍ times that of the electron, me. There is also a third lepton, the <a href="/facts/Tau_(particle)/6DW8j6PC">tau</a>, approximately 17 times heavier than the muon.
Due to their greater mass, muons accelerate more slowly than electrons in electromagnetic fields, and emit less <a href="/facts/Bremsstrahlung/XafQtbx6">bremsstrahlung</a> (deceleration radiation). This allows muons of a given energy to <a href="/facts/Stopping_power_(particle_radiation)/JRCnYIpH">penetrate far deeper</a> into matter because the deceleration of electrons and muons is primarily due to energy loss by the bremsstrahlung mechanism. For example, so-called secondary muons, created by <a href="/facts/Cosmic_rays/uxCVdTKd">cosmic rays</a> hitting the atmosphere, can penetrate the atmosphere and reach Earth's land surface and even into deep mines.
Because muons have a greater mass and energy than the <a href="/facts/Decay_energy/g4wDGC8r">decay energy</a> of radioactivity, they are not produced by <a href="/facts/Radioactive_decay/Ya6FVjt4">radioactive decay</a>. Nonetheless, they are produced in great amounts in high-energy interactions in normal matter, in certain <a href="/facts/Particle_accelerator/Pkuc08xC">particle accelerator</a> experiments with <a href="/facts/Hadron/C0sd1zty">hadrons</a>, and in cosmic ray interactions with matter. These interactions usually produce <a href="/facts/Pi_meson/x5HamzG3">pi mesons</a> initially, which almost always decay to muons.
As with the other charged leptons, the muon has an associated <a href="/facts/Muon_neutrino/A8kPHDKd">muon neutrino</a>, denoted by νμ, which differs from the <a href="/facts/Electron_neutrino/e5hQPWIQ">electron neutrino</a> and participates in different nuclear reactions.

Muon open-in-new

Muon