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Mesons

Mesons are hadrons containing an even number of valence quarks (at least 2). Most well known mesons are composed of a quark-antiquark pair, but possible tetraquarks (4 quarks) and hexaquarks (6 quarks, comprising either a dibaryon or three quark-antiquark pairs) may have been discovered and are being investigated to confirm their nature. Several other hypothetical types of exotic meson may exist which do not fall within the quark model of classification. These include glueballs and hybrid mesons (mesons bound by excited gluons).

Because mesons have an even number of quarks, they are also all bosons, with integer spin, i.e., 0, +1, or −1. They have baryon number B = 1 / 3 − 1 / 3 = 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.