Editing Pion (section)

=== Neutral pion decays ===
The {{math|{{SubatomicParticle|Pion0}}}} meson has a mass of {{val|135.0|u=MeV/c2}} and a mean lifetime of {{val|8.5|e=-17|u=s}}.<ref name=pdg/> It decays via the [[electromagnetism|electromagnetic force]], which explains why its mean lifetime is much smaller than that of the charged pion (which can only decay via the [[weak force]]). 
[[Image:Anomalous-pion-decay.png |right|thumb| [[Chiral anomaly#Calculation|Anomaly]]-induced  neutral pion decay.]]
The dominant {{math|{{SubatomicParticle|Pion0}}}} decay mode, with a [[branching fraction|branching ratio]] of {{nowrap|1=BR{{sub|γγ}} = 0.98823}}, is into two [[photon]]s:
<math display=block>\pi^0 \longrightarrow 2\ \gamma</math>

The decay {{nobr|{{math| {{SubatomicParticle|Pion0}} → 3 {{SubatomicParticle|link=yes|Gamma}} }}}} (as well as decays into any odd number of photons) is forbidden by the [[C-symmetry]] of the electromagnetic interaction: The intrinsic C-parity of the {{math|{{SubatomicParticle|Pion0}}}} is +1, while the C-parity of a system of {{mvar|n}} photons is {{math|(&minus;1){{sup|''n''}}}}.

The second largest {{math|{{SubatomicParticle|Pion0}}}} decay mode ({{nobr|BR{{sub|''γ''e{{overbar|e}}}} {{=}} 0.01174}}) is the Dalitz decay (named after [[Richard Dalitz]]), which is a two-photon decay with an internal photon conversion resulting in a photon and an [[electron]]-[[positron]] pair in the final state:
<math display=block>\pi^0 \longrightarrow \gamma + \rm e^- + e^+</math>

The third largest established decay mode ({{nobr|BR{{sub|2e2{{overbar|e}}}} {{=}} 3.34{{x10^|-5}}}}) is the double-Dalitz decay, with both photons undergoing internal conversion which leads to further suppression of the rate:
<math display=block>\pi^0 \longrightarrow \rm 2 \ e^- + 2\ e^+</math>

The fourth largest established decay mode is the [[Feynman diagram|loop-induced]] and therefore suppressed (and additionally [[Helicity (particle physics)|helicity]]-suppressed) leptonic decay mode ({{nowrap|1=BR{{sub|e{{overbar|e}}}} = 6.46{{x10^|-8}}}}):
<math display=block>\pi^0 \longrightarrow \rm e^- + e^+</math>

The neutral pion has also been observed to decay into [[positronium]] with a branching fraction on the order of {{10^|-9}}. No other decay modes have been established experimentally. The branching fractions above are the [[Particle Data Group|PDG]] central values, and their uncertainties are omitted, but available in the cited publication.<ref name=pdg/>

{| class="wikitable sortable" style="text-align: center;"
|+ Pions
|-
! class=unsortable | Particle <br/>name
! Particle <br/>symbol
! Antiparticle <br/>symbol
! class=unsortable | Quark <br/>content<ref name=PDGQuarkmodel>{{cite web |first1=C. |last1=Amsler |display-authors=etal |collaboration=[[Particle Data Group]] |year=2008 |url=http://pdg.lbl.gov/2008/reviews/quarkmodrpp.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://pdg.lbl.gov/2008/reviews/quarkmodrpp.pdf |archive-date=2022-10-09 |url-status=live |title=Quark Model |publisher=[[Lawrence Berkeley Laboratory]]}}</ref>
! [[Rest mass]] {{bracket|[[electron volt|MeV]]/[[speed of light|''c'']]<sup>2</sup>}}
! width="50" | [[Isospin|''I'']]<sup>[[G parity|G]]</sup>
! width="50" | [[Total angular momentum|''J'']]<sup>[[Parity (physics)|P]][[C parity|C]]</sup>
! width="50" | [[strangeness|''S'']]
! width="50" | [[charm (quantum number)|''C'']]
! width="50" | [[bottomness|''B''′]]
! [[Mean lifetime]] {{bracket|[[second|s]]}}
! class=unsortable|Commonly decays to <br/>(>&nbsp;5% of decays)
|-
| Pion<ref name=pdg/>
| {{math|{{SubatomicParticle|Pion+}}}}
| {{math|{{SubatomicParticle|Pion-}}}}
| {{SubatomicParticle|link=yes|Up quark}}{{SubatomicParticle|link=yes|Down antiquark}}
| {{nobr|{{gaps|139.570|39}} ± {{gaps|0.000|18}}}}
| 1<sup>&minus;</sup>
| 0<sup>&minus;</sup>
| 0
| 0
| 0
| {{val|2.6033|0.0005|e=-8}}
| {{SubatomicParticle|link=yes|antimuon}} + {{SubatomicParticle|link=yes|muon neutrino}}
|-
| Pion<ref name=pdg/>
| {{math|{{SubatomicParticle|Pion0}}}}
| Self
| <math>\tfrac{\mathrm{u\bar{u}} - \mathrm{d\bar{d}}}{\sqrt 2}</math><sup>{{ref|quarkcontent|[a]}}</sup>
| {{val|134.9768|0.0005}}
| 1<sup>&minus;</sup>
| 0<sup>&minus;+</sup>
| 0
| 0
| 0
| {{val|8.5|0.2|e=-17}}
| {{math|{{SubatomicParticle|link=yes|Photon}} + {{SubatomicParticle|link=yes|Photon}}}}
|}
<sup>[a]</sup> {{note|quarkcontent}} The quark composition of the {{math|{{SubatomicParticle|Pion0}}}} is not exactly divided between up and down quarks, due to complications from non-zero quark masses.<ref>{{cite book |last=Griffiths |first=D.J. |author-link=David J. Griffiths |year=1987 |title=Introduction to Elementary Particles |publisher=[[John Wiley & Sons]] |isbn=0-471-60386-4 }}</ref>