States Outside The Quark Model
While the quark model is derivable from the theory of quantum chromodynamics, the structure of hadrons is more complicated than this model reveals. The full quantum mechanical wave function of any hadron must include virtual quark pairs as well as virtual gluons. Also, there may be hadrons which lie outside the quark model. Among these are the glueballs (which contain only valence gluons), hybrids (which contain valence quarks as well as gluons) and "exotic hadrons" (such as tetraquarks or pentaquarks).
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