Pion

A pion is the lightest meson, a quark–antiquark particle that mediates much of the residual nuclear force between protons and neutrons.

Yukawa interaction: A quantum-field interaction in which particles exchange mesons, producing a force between matter particles. Yukawa’s meson-exchange model first explained how pions can bind nucleons.
Meson: A hadron composed of a quark and an antiquark. Pions are the lightest members of this particle family.
Charged pion decay: The decay of a charged pion, predominantly into a muon and a muon neutrino or antineutrino. This dominant decay reveals the pion’s coupling to the weak interaction.
Hideki Yukawa: A Japanese theoretical physicist who proposed a meson-mediated nuclear force in 1935. His prediction gave pions their original role as carriers of nuclear attraction.
Kaon: A meson containing a strange quark or antiquark, heavier than a pion. Kaons share the meson structure but contain strange quarks and have greater mass.
Nuclear force: The interaction that binds protons and neutrons inside atomic nuclei. Pion exchange accounts for the long-range part of this force.
Up quark: A first-generation quark with electric charge +2/3 of the elementary charge. Up quarks and antiquarks appear in the pion’s charged and neutral states.
Neutral pion decay: The decay of a neutral pion, predominantly into two photons. Its rapid two-photon decay distinguishes it sharply from charged pions.
Cecil Powell: A British physicist who led the experimental discovery of the pion in cosmic-ray tracks. His group identified the particle predicted by Yukawa.
Eta meson: A neutral meson made chiefly from up, down, and strange quark–antiquark components. The eta is another light meson, but it is substantially heavier than the pion.

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