摘要
The constituent quark model describes the observed meson spectrum as bound qq states grouped into SU（N） flavor multiplets （see our review on the ＇Quark Model＇ in this issue of the Review）. However, the self-coupling of gluons in QCD suggests that additional mesons made of bound gluons （glueballs）, or qq-pairs with an excited gluon （hybrids）, may exist.
The constituent quark model describes the observed meson spectrum as bound qq states grouped into SU（N） flavor multiplets （see our review on the ＇Quark Model＇ in this issue of the Review）. However, the self-coupling of gluons in QCD suggests that additional mesons made of bound gluons （glueballs）, or qq-pairs with an excited gluon （hybrids）, may exist.
