In particle physics, an exotic meson is a meson (a strongly interacting boson) that does not contain exactly one valence quark-antiquark pair. Exotic mesons can be:
- tetraquarks, which can be interpreted as two quark-antiquark pairs or a meson-antimeson pair (e.g. a
pair);
- mesons with no valence quarks but two or more real gluons
- gluonium states, made up of two bound gluons;
- glueballs, with three gluons or more;
- hybrids, composed of one or more valence quark-antiquark pairs plus one or more real gluons.
Currently, there is experimental evidence for only one type of exotic meson, the tetraquark. There are some gluonium candidates, but they are still unconfirmed. Some mesons are believed to be mixes of purequarkonium states with pure gluonium states.
The composition of exotic mesons is constrained by the fact that the total number of quarks and antiquarks must be even (if it were odd, the particle would be a fermion, not a boson, and hence by definition a baryon), and that all mesons must be "colorless", which can only be arranged if the quarks either come in a triplet, with one quark of each color (i.e. a red quark, a green quark, and a blue quark are all present), or in a quark-antiquark pair (e.g. a red quark and an anti-red antiquark).
The possible existence of exotic mesons has been contemplated by physicists since the early 1970s, as it is an immediate consequence of quantum chromodynamics (the quantum field theory which describes hadrons).