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In
particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...
, exotic mesons are
meson In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...
s that have
quantum number In quantum physics and chemistry, quantum numbers are quantities that characterize the possible states of the system. To fully specify the state of the electron in a hydrogen atom, four quantum numbers are needed. The traditional set of quantu ...
s not possible in the
quark model In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks that give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the Eig ...
; some proposals for non-standard quark model mesons could be: ;glueballs or gluonium: Glueballs have no valence
quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...
s at all. ;tetraquarks: Tetraquarks have two valence quark–antiquark pairs. ;hybrid mesons: Hybrid mesons contain a valence quark–antiquark pair and one or more
gluon A gluon ( ) is a type of Massless particle, massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a Spin (physi ...
s. All exotic mesons are classed as mesons because they are
hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...
s and carry zero baryon number. Of these, glueballs must be flavor singlets – that is, must have zero
isospin In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. Isospin is also known as isobaric spin or isotopic spin. Isospin symmetry is a subset of the flavour symmetr ...
, 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 Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
(enclosed in parentheses), and by , where is the
angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
, is the
intrinsic parity In quantum mechanics, the intrinsic parity is a phase factor that arises as an eigenvalue of the parity operation x_i \rightarrow x_i' = -x_i (a reflection about the origin). To see that the parity's eigenvalues are phase factors, we assume an ...
, and is the
charge conjugation In physics, charge conjugation is a transformation that switches all particles with their corresponding antiparticles, thus changing the sign of all charges: not only electric charge but also the charges relevant to other forces. The term C- ...
parity; One also often specifies the
isospin In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. Isospin is also known as isobaric spin or isotopic spin. Isospin symmetry is a subset of the flavour symmetr ...
of the meson. Typically, every
quark model In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks that give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the Eig ...
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 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 A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
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

*
Quark model In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks that give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the Eig ...
,
meson In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...
s,
baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle that contains an odd number of valence quarks, conventionally three. proton, Protons and neutron, neutrons are examples of baryons; because baryons are ...
s,
quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...
s, and
gluon A gluon ( ) is a type of Massless particle, massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a Spin (physi ...
s * Exotic hadrons and
exotic baryon In particle physics, exotic baryons are a type of hadron (bound states of quarks and gluons) with half-integer spin, but with a quark content different from the three quarks (''qqq'') present in conventional baryons. An example would be pentaq ...
s *
Quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the study of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of ...
, flavor, and the QCD vacuum * GlueX, an experiment which will explore the spectrum of glueballs and exotic mesons


References


Further reading

* {{particles Mesons Hypothetical composite particles