Exotic meson

Like all particle states, exotic mesons are specified by the quantum numbers which label representations of the Poincaré symmetry, q.e., by the mass (enclosed in parentheses), and by JPC, where J is the angular momentum, P is the intrinsic parity, and C is the charge conjugation parity; One also often specifies the isospin I of the meson.

In spite of such seemingly simple counting, the assignment of any given state as a glueball, tetraquark, or hybrid remains tentative even today, hence the preference for the more generic term exotic meson.

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.

The best lattice computations to date are made in the quenched approximation, which neglects virtual quarks loops.

[1][2][3] A recent coupled-channel analysis has shown these states, which were initially considered separate, are consistent with a single pole.

Lattice QCD calculations show the lightest π1 with 1−+ quantum numbers has strong overlap with operators featuring gluonic construction.

If this identification is correct, then it is a remarkable agreement with lattice computations, which place several hybrids in this range of masses.