Editing Muon (section)

{{short description|Subatomic particle}}
{{Use dmy dates|date=March 2020}}
{{Infobox Particle
 |bgcolour =
 |name = Muon
 |image = Moon's shadow in muons.gif
 |image_size = 
 |caption = The Moon's [[cosmic ray]] shadow, as seen in secondary muons generated by cosmic rays in the atmosphere, and detected 700&nbsp;m below ground, at the [[Soudan 2]] detector
 |num_types =
 |composition = [[Elementary particle]]
 |statistics = [[Fermionic]]
 |group = [[Lepton]]
 |generation = Second
 |interaction = [[Gravity]], [[Electromagnetic interaction|electromagnetic]], <br/>[[Weak interaction|weak]]
 |antiparticle = Antimuon ({{SubatomicParticle|Antimuon}})
 |theorized =
 |discovered = [[Carl D. Anderson]], [[Seth Neddermeyer]] (1936)
 |symbol = {{SubatomicParticle|muon-}}
 |mass = {{physconst|mmu}}<br />{{physconst|mmu_Da}}<br />{{physconst|mmuc2_MeV|unit={{val|ul=MeV/c2}}}}
 |mean_lifetime = {{val|2.1969811|(22)|e=-6|ul=s}}<ref name="PDG2012">
  {{cite web
   |first1=J. |last1=Beringer
   |display-authors=etal
   |collaboration=[[Particle Data Group]]
   |url=http://pdg.lbl.gov/2012/tables/rpp2012-sum-leptons.pdf
   |series=PDGLive Particle Summary
   |title=Leptons (e, mu, tau, ... neutrinos ...)
   |publisher=[[Particle Data Group]]
   |year=2012
   |access-date=2013-01-12
   |df=dmy-all
  }}</ref><ref name="PDG2016">
  {{cite journal
   |title=Review of Particle Physics
   |first1=C. |last1=Patrignani
   |display-authors=etal
   |collaboration=Particle Data Group
   |journal=Chinese Physics C |year=2016 |volume=40 |issue=10 |page=100001
   |doi=10.1088/1674-1137/40/10/100001 |hdl=1983/989104d6-b9b4-412b-bed9-75d962c2e000
   |bibcode=2016ChPhC..40j0001P
   |s2cid=125766528
   |url=http://bib-pubdb1.desy.de/record/311549/files/cpc_40_10_100001.pdf
  }}</ref>
 |decay_particle = [[Electron|{{Subatomic particle|electron-}}]], [[Electron antineutrino|{{Subatomic particle|electron antineutrino}}]], [[Muon neutrino|{{Subatomic particle|Muon neutrino}}]]<ref name="PDG2016" /> (most common)
 |electric_charge = &minus;1&nbsp;[[Elementary charge|''e'']]
 |magnetic_moment = {{physconst|mumu}}<br/>{{physconst|mumu/muB|unit=[[Bohr magneton|''μ''<sub>B</sub>]]}}
 |color_charge = None
 |spin = {{sfrac|1|2}}&nbsp;[[reduced Planck constant|''ħ'']]
 |num_spin_states =
 |weak_isospin = {{nowrap|[[Chirality (physics)|LH]]: &minus;{{sfrac|1|2}}, [[Chirality (physics)|RH]]: 0}}
 |weak_hypercharge= {{nowrap|[[Chirality (physics)|LH]]: &minus;1, [[Chirality (physics)|RH]]: &minus;2}}
}}
A '''muon''' ({{IPAc-en|ˈ|m|(|j|)|uː|.|ɒ|n}} {{Respell|M(Y)OO|on}}; from the [[Greek alphabet|Greek]] letter [[mu (letter)|mu]] (μ) used to represent it) is an [[elementary particle]] similar to the [[electron]], with an [[electric charge]] of &minus;1&nbsp;''[[elementary charge|e]]'' and a [[spin-1/2|spin]] of {{sfrac|1|2}}&nbsp;''ħ'', but with a much greater mass. It is classified as a [[lepton]]. As with other leptons, the muon is not thought to be composed of any simpler particles.

The muon is an unstable [[subatomic particle]] with a [[mean lifetime]] of {{val|2.2|ul=μs}}, much longer than many other subatomic particles. As with the decay of the free [[neutron]] (with a lifetime around 15 minutes), muon decay is slow (by subatomic standards) because the decay is mediated only by the [[weak interaction]] (rather than the more powerful [[strong interaction]] or [[electromagnetic interaction]]), and because the mass difference between the muon and the set of its decay products is small, providing few kinetic [[degrees of freedom]] 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 [[neutrino]]s.

Like all elementary particles, the muon has a corresponding [[antiparticle]] of opposite charge (+1&nbsp;''e'') but equal [[mass]] and spin: the '''antimuon''' (also called a ''positive muon''). Muons are denoted by {{SubatomicParticle|Muon-}} and antimuons by {{SubatomicParticle|Muon+}}. Formerly, muons were called ''mu mesons'', but are not classified as [[meson]]s by modern particle physicists (see {{crossreference|text={{Section link||History of discovery}}|printworthy=1}}), and that name is no longer used by the physics community.

Muons have a [[mass]] of {{physconst|mmuc2_MeV|round=2|unit={{val|ul=MeV/c2}}|ref=no}}, which is approximately {{physconst|mmu/me}} times that of the electron, ''m''{{sub|e}}. There is also a third lepton, the [[tau (particle)|tau]], approximately 17 times heavier than the muon.

Due to their greater mass, muons accelerate more slowly than electrons in electromagnetic fields, and emit less [[bremsstrahlung]] (deceleration radiation). This allows muons of a given energy to [[Stopping power (particle radiation)|penetrate far deeper]] 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 [[cosmic rays]] 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 [[decay energy]] of radioactivity, they are not produced by [[radioactive decay]]. Nonetheless, they are produced in great amounts in high-energy interactions in normal matter, in certain [[particle accelerator]] experiments with [[hadron]]s, and in cosmic ray interactions with matter. These interactions usually produce [[pi meson]]s initially, which almost always decay to muons.

As with the other charged leptons, the muon has an associated [[muon neutrino]], denoted by {{SubatomicParticle|Muon neutrino}}, which differs from the [[electron neutrino]] and participates in different nuclear reactions.