In particle physics, a meson is a strongly interactingboson, that is, it is a hadron with integral spin. In the Standard Model, mesons are composite (non-elementary) particles composed of an even number of quarks and antiquarks. Until the discovery of the tetraquark, all known mesons were believed to consist of a quark-antiquark pair - the so-called valence quarks - plus a "sea" of virtual quark-antiquark pairs and virtual gluons. The valence quarks may exist in a superposition of flavor states; for example, the neutral pion is neither an up-antiup pair nor a down-antidown pair, but an equal superposition of both. Pseudoscalar mesons (spin 0) have the lowest rest energy, where the quark and antiquark have opposite spin, and then the vector mesons (spin 1), where the quark and antiquark have parallel spin. Both come in higher energy versions where the spin is augmented by orbital angular momentum. Most of a meson's mass comes from binding energy, rather than the sum of the mass of its components. All mesons are unstable.
Mesons were originally predicted as carriers of the force that bind protons and neutrons together. When first discovered, the muon was identified with this family from its similar mass and was named "mu meson", however it did not show a strong attraction to nuclear matter and is actually a lepton. The pion was the first true meson to be discovered. (The current picture of intranuclear forces is quite complicated; see quantum hadrodynamics for a discussion of modern theories in which nucleon-nucleon interactions are mediated by meson exchange.)
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