Editing Pion (section)

{{Short description|Subatomic particle; lightest meson}}
{{Other uses}}
{{Infobox Particle
| name        = Pion
| image       = Image:Quark structure pion.svg
| caption     = The quark structure of the positively charged pion.
| num_types   = 3
| composition =
 {{plainlist|
* {{math|{{SubatomicParticle|Pion+}}}} : {{SubatomicParticle|Up quark}}{{SubatomicParticle|Down antiquark}}
* {{math|{{SubatomicParticle|Pion0}}}} : <math>\rm \tfrac{u\overline{u} - d\overline{d}}{\sqrt{2}}</math>
* {{math|{{SubatomicParticle|Pion-}}}} : {{SubatomicParticle|Down quark}}{{SubatomicParticle|Up antiquark}}
 }}
| statistics  = [[Boson]]ic
| group       = [[Meson]]s
| interaction = [[Strong interaction|Strong]], [[Weak interaction|weak]], [[Electromagnetic interaction|electromagnetic]], and [[gravity]]
| antiparticle=
 {{plainlist|
* {{math|{{SubatomicParticle|Pion+}}}} :  {{math|{{SubatomicParticle|Pion-}}}}
* {{math|{{SubatomicParticle|Pion0}}}} :  self
 }}
| theorized   = [[Hideki Yukawa]] (1935)
| discovered  =
 {{plainlist|
* {{math|{{SubatomicParticle|Pion+-}}}} : [[César Lattes]], [[Giuseppe Occhialini]], [[Cecil Powell]] (1947)
* {{math|{{SubatomicParticle|Pion0}}}} : 1950
 }} 
| symbol      = {{math|{{SubatomicParticle|Pion+}}}}, {{math|{{SubatomicParticle|Pion0}}}}, and {{math|{{SubatomicParticle|Pion-}}}}
| mass        =
 {{plainlist|
* {{math|{{SubatomicParticle|Pion+-}}}} : {{val|139.57039|(18)|ul=MeV/c2}}{{px2}}<ref name=pdg>{{cite journal | title=Review of Particle Physics | last1=Zyla | first1=P.A. | last2=Barnett | first2=R.M. | display-authors=1 | collaboration = Particle Data Group | journal=Progress of Theoretical and Experimental Physics | volume = 2020 | page = 083C01 | year = 2020 | issue=8 | doi=10.1093/ptep/ptaa104| doi-access=free | hdl=11585/772320 | hdl-access=free }}</ref>
* {{math|{{SubatomicParticle|Pion0}}}} : {{val|134.9768|(5)|u=MeV/c2}}{{px2}}<ref name=pdg/>
 }}
| mean_lifetime =
 {{plainlist|
* {{math|{{SubatomicParticle|Pion+-}}}} : {{val|2.6|e=-8|ul=s}}
* {{math|{{SubatomicParticle|Pion0}}}} : {{val|8.5|e=-17|u=s}}
 }}
| decay_particle = 
| electric_charge =
 {{plainlist|
* {{math|{{SubatomicParticle|Pion+-}}}} : {{+-}}1 [[Elementary charge|''e'']] 
* {{math|{{SubatomicParticle|Pion0}}}} : 0&nbsp;''e''
 }}
| charge_radius = {{math|{{SubatomicParticle|Pion+-}}}} : {{+-}}{{val|0.659|(4)|u=[[Femtometre|fm]]}}{{px2}}<ref name=pdg/>
| color_charge=  0
| spin        =  0&nbsp;[[reduced Planck constant|''ħ'']]
| strangeness = 
| charm = 
| bottomness = 
| topness = 
| isospin =
 {{plainlist|
* {{math|{{SubatomicParticle|Pion+-}}}} : ±1
* {{math|{{SubatomicParticle|Pion0}}}} : 0
 }}
| hypercharge = 0
| parity = −1
| c_parity = +1
}}

In [[particle physics]], a '''pion''' ({{IPAc-en|'|p|ai|.|Q|n}}, {{respell|PIE|on}}) or '''pi meson''', denoted with the [[Greek alphabet|Greek]] letter [[pi (letter)|pi]] ({{math|{{SubatomicParticle|Pion}}}}), is any of three [[subatomic particle]]s: {{math|{{SubatomicParticle|Pion0}}}}, {{math|{{SubatomicParticle|Pion+}}}}, and {{math|{{SubatomicParticle|Pion-}}}}. Each pion consists of a [[quark]] and an [[antiquark]] and is therefore a [[meson]]. Pions are the lightest mesons and, more generally, the lightest [[hadron]]s. They are unstable, with the charged pions {{math|{{SubatomicParticle|Pion+}}}} and {{math|{{SubatomicParticle|Pion-}}}} decaying after a [[mean lifetime]] of 26.033&nbsp;[[nanosecond]]s ({{val|2.6033|e=-8}}&nbsp;seconds), and the neutral pion {{math|{{SubatomicParticle|Pion0}}}} decaying after a much shorter lifetime of 85&nbsp;[[attosecond]]s ({{val|8.5|e=-17}}&nbsp;seconds).<ref name=pdg/> Charged pions most often [[particle decay|decay]] into [[muon]]s and [[muon neutrino]]s, while neutral pions generally decay into [[gamma ray]]s.

The exchange of [[virtual particle|virtual]] pions, along with [[vector meson|vector]], [[rho meson|rho]] and [[omega meson]]s, provides an explanation for the [[nuclear force|residual strong force]] between [[nucleon]]s. Pions are not produced in [[radioactive decay]], but commonly are in high-energy collisions between [[hadron]]s. Pions also result from some matter–antimatter [[Annihilation#Proton–antiproton annihilation|annihilation]] events. All types of pions are also produced in natural processes when high-energy [[cosmic-ray]] protons and other hadronic cosmic-ray components interact with [[matter]] in Earth's [[atmosphere]]. In 2013, the detection of characteristic gamma rays originating from the decay of neutral pions in two [[supernova remnants]] has shown that pions are produced copiously after supernovas, most probably in conjunction with production of high-energy protons that are detected on Earth as cosmic rays.<ref name=ackermann-2013>
{{cite journal
 | last1=Ackermann | first1 = M. 
 | display-authors=etal
 | year=2013
 | title=Detection of the characteristic pion-decay signature in supernova remnants
 | journal=[[Science (journal)|Science]]
 | volume=339 | issue=6424 | pages=807–811
 | arxiv = 1302.3307
 | bibcode = 2013Sci...339..807A
 | doi =10.1126/science.1231160
 | pmid=23413352| s2cid=29815601
}}</ref>

The pion also plays a crucial role in cosmology, by imposing an upper limit on the energies of cosmic rays surviving collisions with the [[cosmic microwave background]], through the [[Greisen–Zatsepin–Kuzmin limit]].<ref>{{cite journal |last=Greisen |first=K. |year=1966 |title=End to the Cosmic-Ray Spectrum? |journal=Physical Review Letters |volume=16 |issue=17 |pages=748–750 |doi=10.1103/PhysRevLett.16.748}}</ref>