Editing Pion (section)

=== Charged pion decays ===
[[Image:PiPlus muon decay.svg|right|thumb|[[Feynman diagram]] of the dominant leptonic pion decay.]]
[[File:K meson decay.jpg|thumb|[[Kaon]] decay in a [[nuclear emulsion]]. The positively-charged kaon enters at the top of the image and decays into a {{SubatomicParticle|Pion-}} meson (''a'') and two {{math|{{SubatomicParticle|Pion+}}}} mesons (''b'' and ''c''). The {{math|{{SubatomicParticle|Pion-}}}} meson interacts with a [[Atomic nucleus|nucleus]] in the emulsion at ''B''.]]
The {{math|{{SubatomicParticle|Pion+-}}}} mesons have a [[mass]] of {{val|139.6|ul=MeV/c2}} and a [[mean life]]time of {{val|2.6033|e=-8|ul=s}}. They decay due to the [[weak force|weak interaction]]. The primary decay mode of a pion, with a [[branching fraction]] of 0.999877, is a [[lepton]]ic decay into a [[muon]] and a [[muon neutrino]]:
<math display=block>\begin{align}
  \pi^+ &\longrightarrow \mu^+ + \nu_\mu \\[2pt]
  \pi^- &\longrightarrow \mu^- + \overline\nu_\mu
\end{align}</math>

The second most common decay mode of a pion, with a branching fraction of 0.000123, is also a leptonic decay into an [[electron]] and the corresponding [[electron antineutrino]]. This "electronic mode" was discovered at [[CERN]] in 1958:<ref>{{cite journal | last1 = Fazzini | first1 = T. | last2 = Fidecaro | first2 = G. | last3 = Merrison | first3 = A. | last4 = Paul | first4 = H. | last5 = Tollestrup | first5 = A. | year = 1958 | title = Electron Decay of the Pion | journal = Physical Review Letters | volume = 1 | issue = 7 | pages = 247–249 | bibcode=1958PhRvL...1..247F | doi = 10.1103/PhysRevLett.1.247 | url = https://cds.cern.ch/record/342714}}</ref>
<math display=block>\begin{align}
  \pi^+ &\longrightarrow {\rm e}^+ + \nu_e \\[2pt]
  \pi^- &\longrightarrow {\rm e}^- + \overline\nu_e
\end{align}</math>

The suppression of the electronic decay mode with respect to the muonic one is given approximately (up to a few percent effect of the radiative corrections) by the ratio of the half-widths of the pion–electron and the pion–muon decay reactions,
<math display="block"> R_\pi = \left(\frac{m_e}{m_\mu}\right)^2 \left(\frac{m_\pi^2 - m_e^2}{m_\pi^2 - m_\mu^2}\right)^2 = 1.283 \times 10^{-4}</math>
and is a [[Spin (physics)|spin]] effect known as [[helicity (particle physics)|helicity]] suppression.

Its mechanism is as follows: The negative pion has spin zero; therefore the lepton and the antineutrino must be emitted with opposite spins (and opposite linear momenta) to preserve net zero spin (and conserve linear momentum). However, because the weak interaction is sensitive only to the left [[Chirality (physics)|chirality]] component of fields, the antineutrino has always left chirality, which means it is right-handed, since for massless anti-particles the helicity is opposite to the chirality. This implies that the lepton must be emitted with spin in the direction of its linear momentum (i.e., also right-handed). If, however, leptons were massless, they would only interact with the pion in the left-handed form (because for massless particles helicity is the same as chirality) and this decay mode would be prohibited. Therefore, suppression of the electron decay channel comes from the fact that the electron's mass is much smaller than the muon's. The electron is relatively massless compared with the muon, and thus the electronic mode is greatly suppressed relative to the muonic one, virtually prohibited.<ref>{{cite web |url=http://hyperphysics.phy-astr.gsu.edu/hbase/particles/hadron.html |title=Mesons |website=Hyperphysics |publisher=Georgia State U.}}</ref>

Although this explanation suggests that parity violation is causing the helicity suppression, the fundamental reason lies in the vector-nature of the interaction which dictates a different handedness for the neutrino and the charged lepton. Thus, even a parity conserving interaction would yield the same suppression.

Measurements of the above ratio have been considered for decades to be a test of [[lepton universality]]. Experimentally, this ratio is {{val|1.233|(2)|e=-4}}.<ref name=pdg/>

Beyond the purely leptonic decays of pions, some structure-dependent radiative leptonic decays (that is, decay to the usual leptons plus a gamma ray) have also been observed.

Also observed, for charged pions only, is the very rare "pion [[beta decay]]" (with branching fraction of about {{10^|−8}}) into a neutral pion, an electron and an electron antineutrino (or for positive pions, a neutral pion, a positron, and electron neutrino).
<math display=block>\begin{align}
  \pi^+ &\longrightarrow \pi^0 + {\rm e}^+ + \nu_e \\[2pt]
  \pi^- &\longrightarrow \pi^0 + {\rm e}^- + \overline\nu_e
\end{align}</math>

The rate at which pions decay is a prominent quantity in many sub-fields of particle physics, such as [[chiral perturbation theory]]. This rate is parametrized by the [[pion decay constant]] ({{math|''f''{{sub|&pi;}}}}), related to the [[wave function]] overlap of the quark and antiquark, which is about {{val|130|u=MeV}}.<ref>{{cite report |first1=J.L. |last1=Rosner |first2=S. |last2=Stone |collaboration=[[Particle Data Group]] |date=18 December 2013 |title=Leptonic decays of charged pseudo- scalar mesons |website=pdg.lbl.gov |place=Lawrence, CA |publisher=[[Lawrence Berkeley Lab]] |url=http://pdg.lbl.gov/2014/reviews/rpp2014-rev-pseudoscalar-meson-decay-cons.pdf}}</ref>