History
When the quark model was first postulated by Murray Gell-Mann and others in the 1960s it was to organize the states then known to be in existence in a meaningful way. As Quantum Chromodynamics (QCD) developed over the next decade; however, it became apparent that there was no fundamental reason why only 3-quark and quark-antiquark combinations should exist. In addition it seemed that gluons, the force carrying particles of the strong interaction, should also form bound states by themselves (glueballs) and with quarks (hybrid hadrons). Nevertheless, several decades have passed without conclusive evidence of an exotic hadron.
Read more about this topic: Exotic Hadron
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