Exotic meson

File:Exotic mesons.svg

Identities and classification of possible tetraquark mesons, where Template:Mvar denotes isospin. Template:Legend inline; Template:Legend inline Template:Legend inline The vertical axis is the mass.

In particle physics, exotic mesons are mesons that have quantum numbers not possible in the quark model; some proposals for non-standard quark model mesons could be:

glueballs or gluonium

Glueballs have no valence quarks at all.

tetraquarks

Tetraquarks have two valence quark–antiquark pairs.

hybrid mesons

Hybrid mesons contain a valence quark–antiquark pair and one or more gluons.

All exotic mesons are classed as mesons because they are hadrons and carry zero baryon number. Of these, glueballs must be flavor singlets – that is, must have zero isospin, strangeness, charm, bottomness, and topness. 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 Template:Math, where Template:Mvar is the angular momentum, Template:Mvar is the intrinsic parity, and Template:Mvar is the charge conjugation parity; One also often specifies the isospin Template:Mvar of the meson. Typically, every quark model meson comes in SU(3) flavor nonet: an octet and an associated 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, 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. 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.

Lattice predictions

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 Template:Val and

2⁺⁺ with mass of Template:Val

The 0⁻⁺ and exotic glueballs such as 0⁻⁻ are all expected to lie above Template:Val. Glueballs are necessarily isoscalar (both for strong isospin, and trivially, weak isospin), with Template:Nobr

The ground state hybrid mesons 0⁻⁺, 1⁻⁺, 1⁻⁻, and 2⁻⁺ all lie a little below Template:Val. The hybrid with exotic quantum numbers 1⁻⁺ is at Template:Val. 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: Template:Math₀(500), Template:Math₀(980), Template:Math₀(1370), Template:Math₀(1500), and Template:Math₀(1710). Of these the Template:Math₀(500) is usually identified with the Template:Mvar of chiral models. The decays and production of Template:Math₀(1710) give strong evidence that it is also a meson.

Glueball candidate

The Template:Math₀(1370) and Template:Math₀(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 Template:Nowrap or Template:Nowrap reactions is highly suppressed. The decays also give some evidence that one of these could be a glueball.

Tetraquark candidate

The Template:Math₀(980) has been identified by some authors as a tetraquark meson, along with the Template:Mvar = 1 states Template:Math₀(980) and Template:Math₀(800). Two long-lived (narrow in the jargon of particle spectroscopy) states: the scalar (0⁺⁺) state Template:PhysicsParticle(2317) and the vector (1⁺) meson Template:PhysicsParticle(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: Template:Math₂(1270) and the Template:Math₂′(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 Template:Math₁(1400) and Template:Math₁(1600) seem to be well established experimentally.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> The assignment of these states as hybrids is favored. Lattice QCD calculations show the lightest Template:Math₁ with 1⁻⁺ quantum numbers has strong overlap with operators featuring gluonic construction.<ref>Template:Cite journal</ref>

The Template:Math(1800) 0⁻⁺, Template:Math(1900) 1⁻⁻ and the Template:Math²(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

• Quark model, mesons, baryons, quarks, and gluons
• Exotic hadrons and exotic baryons
• Quantum chromodynamics, flavor, and the QCD vacuum
• GlueX, an experiment which will explore the spectrum of glueballs and exotic mesons

Further reading

• Template:Cite journal