Exotic meson

Lattice QCD predictions for glueballs are now fairly settled, at least when virtual quarks are neglected. The two lowest states are ::0++ with mass of and ::2++ with mass of The 0−+ and exotic glueballs such as 0−− are all expected to lie above . Glueballs are necessarily isoscalar (both for strong isospin, and trivially, weak isospin), with The ground state hybrid mesons 0−+, 1−+, 1−−, and 2−+ all lie a little below . The hybrid with exotic quantum numbers 1−+ is at . The best lattice computations to date are made in the quenched approximation, which neglects virtual quarks loops. As a result, these computations miss mixing with meson states. ==0++ states==

The data show five isoscalar resonances: (500), (980), (1370), (1500), and (1710). Of these the (500) is usually identified with the of chiral models. The decays and production of (1710) give strong evidence that it is also a meson. Glueball candidate The (1370) and (1500) cannot both be a quark model meson, because one is supernumerary. The production of the higher mass state in two photon reactions such as or reactions is highly suppressed. The decays also give some evidence that one of these could be a glueball. Tetraquark candidate The (980) has been identified by some authors as a tetraquark meson, along with the  = 1 states (980) and (800). Two long-lived (narrow in the jargon of particle spectroscopy) states: the scalar (0) state (2317) and the vector (1) meson (2460), observed at CLEO and BaBar, have also been tentatively identified as tetraquark states. However, for these, other explanations are possible. ==2 states==

Two isoscalar states are definitely identified: (1270) and the ′(1525). No other states have been consistently identified by all experiments. Hence it is difficult to say more about these states. ==1 and other states==

The two isovector exotics 1(1400) and 1(1600) seem to be well established experimentally. A recent coupled-channel analysis has shown these states, which were initially considered separate, are consistent with a single pole. A second exotic state is disfavored. The assignment of these states as hybrids is favored. Lattice QCD calculations show the lightest with 1 quantum numbers has strong overlap with operators featuring gluonic construction. The (1800) 0, (1900) 1 and the (1870) 2 are fairly well identified states, which have been tentatively identified as hybrids by some authors. If this identification is correct, then it is a remarkable agreement with lattice computations, which place several hybrids in this range of masses. ==See also==