Pion (source code)
= Pion
{title2=$\pi$}
{wiki}
A pion is a light spin-zero pseudoscalar <meson>, with charge states $\pi^+$, $\pi^0$ and $\pi^-$. They form an approximate <isospin> triplet. In the light-quark description they are pseudo-Goldstone modes associated with <chiral symmetry breaking>, with nonzero masses due to explicit symmetry breaking.
The valence assignments $u\bar d$, $(u\bar u-d\bar d)/\sqrt2$ and $d\bar u$ organize the triplet without describing the full interacting QCD state as just two free particles. Charged pions commonly decay through the <weak charged current>, whereas the <neutral pion> decays predominantly to two photons. This contrast makes pion identity and charge important when reconstructing particle showers and decay chains. Their light mass also makes pions central degrees of freedom in low-energy descriptions of hadron interactions.
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