What is "exotic meson"

n. (context physics English) A meson existing outside of the quark model, such as a hybrid meson or a glueball.

Non- quark model mesons include

1. exotic mesons, which have quantum numbers not possible for mesons in the quark model;
2. glueballs or gluonium, which have no valence quarks at all;
3. tetraquarks, which have two valence quark-antiquark pairs; and
4. 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., J (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.

In spite of such seemingly simple counting, the assignment of any given state as a glueball, tetraquark, or hybrid remains tentative even today. Even when there is agreement that one of several states is one of these non- quark model mesons, the degree of mixing, and the precise assignment is fraught with uncertainties. There is also the considerable experimental labor of assigning quantum numbers to each state and crosschecking them in other experiments. As a result, all assignments outside the quark model are tentative. The remainder of this article outlines the situation as it stood at the end of 2004.