Mesons
Mesons are hadrons composed of a quark–antiquark pair. They are bosons (integral spin – i.e. an even multiple of ½ – as they have an even number of quarks). They have baryon number B = 0. Examples of mesons commonly produced in particle physics experiments include pions and kaons. Pions also play a role in holding atomic nuclei together via the residual strong force.
In principle, mesons with more than one quark–antiquark pair may exist; a hypothetical meson with two pairs is called a tetraquark. Several tetraquark candidates were found in the 2000s, but their status is under debate. Several other hypothetical "exotic" mesons lie outside the quark model of classification. These include glueballs and hybrid mesons (mesons bound by excited gluons).
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