Exotic meson - Definition and Overview

In particle physics, an exotic meson is a meson (a strongly interacting boson) that does not contain exactly one valence quark-antiquark pair. Currently, there is experimental evidence for only one type of exotic meson, the tetraquark. It consists of two valence quark-antiquark pairs. 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). Other hypothetical exotic mesons include the glueball, with no valence quarks but two or more real gluons, and hybrids, composed of one or more valence quark-antiquark pairs plus one or more real gluons.