Non-
quark model mesons include
- exotic mesons, which have quantum numbers not possible for mesons in the quark model;
- glueballs or gluonium, which have no valence quarks at all;
- tetraquarks, which have two valence quark-antiquark pairs; and
- hybrid mesons, which contain a valence quark-antiquark pair and one or more gluons.
All of these can be classed as
mesons, because they are
hadrons and carry zero
baryon number. Of these, glueballs must be flavor singlets; that is, have zero
isospin,
strangeness,
charm,
bottomness, and
topness. Like all particle states, they are specified by the quantum numbers which label representations of the
Poincaré symmetry, q.e.,
JPC (where
J is the
angular momentum,
P is the
intrinsic parity, and
C is the
charge conjugation parity) and by the
mass. One also specifies the
isospin I of the meson. Typically, every
quark model meson comes in
SU(3) flavor nonet: an octet and a flavor singlet. A glueball shows up as an extra (
supernumerary) particle outside the nonet.