From HEP at Tennessee
In particle physics, a meson is a strongly interacting boson, 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. Most known mesons are believed to consist of a quark-antiquark pair — the so-called valence quarks — plus a "sea" of virtual quark-antiquark pairs and virtual gluons. Searches for exotic mesons that have different constituents are one of the main foci of meson spectoscopy today. The valence quarks may also 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. All mesons are unstable.
Mesons were originally predicted as carriers of the force that binds 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.
In 1949 Hideki Yukawa was awarded the Physics Nobel Prize for predicting the existence of the meson. He originally named it 'mesotron', but was corrected by Werner Heisenberg (whose father was a professor in Greek at University of Munich) that there is no 'tr' in the Greek word 'mesos'.
Mesonic Nomenclature
The name of a meson is devised so that its main properties can be inferred. Conversely, given a meson's properties, its name is clearly determined. The naming conventions fall in two categories based on flavor: flavorless mesons and flavored mesons. See the PDG for more details.
Spectroscopy
Relatively stable mesons like pions and kaons are directly detectable by particle detectors, but more short lived particles have to be detected as resonances in the invariant masses of longer-lived particles. The meson spectroscopist's is to discover and characterize these states, in terms of charge, parity, spin, etc. Most states so far discovered can be fit into the simple quark-antiquark model, but there remain some which do not. These Exotic Mesons like X(3872), X(4260) and others are currently being studied by experiments like BaBar to elucidate their properties.