Exotic meson
Encyclopedia
Non-quark model
mesons include
All of these can be classed as meson
s, because they are hadron
s and carry zero baryon number. Of these, glueballs must be flavor singlets; that is, have zero isospin
, strangeness
, charm
, bottomness
, and topness
. Like all particle states, they are specified by the quantum numbers which label representations of the Poincaré symmetry
, q.e., JPC (where J is the angular momentum
, P is the intrinsic parity
, and C is the charge conjugation parity) and by the mass
. One also specifies the isospin
I of the meson.
Typically, every quark model
meson comes in SU(3) flavor nonet: an octet and a 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. 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.
predictions for glueballs are now fairly stable, at least when virtual
quarks are neglected. The two lowest states are
The 0−+ and exotic glueballs such as 0−− are all expected to lie above . Glueballs are necessarily isoscalar, with isospin
I = 0.
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.
s. The decays and production of f0(1710) give strong evidence that it is also a meson.
reactions such as or reactions is highly suppressed. The decays also give some evidence that one of these could be a glueball.
and BaBar
, have also been tentatively identified as tetraquark states. However, for these, other explanations are possible.
The π(1800) (0−+), ρ(1900) (1−−) and the η2(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.
Quark model
In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons....
mesons include
- exotic mesons, which have quantum numberQuantum numberQuantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities. This is distinguished from classical mechanics where the values can range continuously...
s not possible for mesons in the quark modelQuark modelIn physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons....
; - glueballGlueballIn particle physics, a glueball is a hypothetical composite particle. It consists solely of gluon particles, without valence quarks. Such a state is possible because gluons carry color charge and experience the strong interaction...
s or gluonium, which have no valenceQuark modelIn physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons....
quarkQuarkA quark is an 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 nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
s at all; - tetraquarkTetraquarkIn particle physics a tetraquark is a hypothetical meson composed of four valence quarks. In principle, a tetraquark state may be allowed in quantum chromodynamics, the modern theory of strong interactions. However, there has been no confirmed report of a tetraquark state to date...
s, which have two valence quark-antiquark pairs; and - hybrid mesons, which contain a valence quark-antiquark pair and one or more gluonGluonGluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....
s.
All of these can be classed as meson
Meson
In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...
s, because they are hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s and carry zero baryon number. Of these, glueballs must be flavor singlets; that is, have zero isospin
Isospin
In physics, and specifically, particle physics, isospin is a quantum number related to the strong interaction. This term was derived from isotopic spin, but the term is confusing as two isotopes of a nucleus have different numbers of nucleons; in contrast, rotations of isospin maintain the number...
, strangeness
Strangeness
In particle physics, strangeness S is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic reactions, which occur in a short period of time...
, charm
Charm quark
The charm quark or c quark is the third most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks...
, bottomness
Bottom quark
The bottom quark, also known as the beauty quark, is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large bare mass , combined with low values of the CKM matrix elements Vub and Vcb, gives it a...
, and topness
Top quark
The top quark, also known as the t quark or truth quark, is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and...
. Like all particle states, they are specified by the quantum numbers which label representations of the Poincaré symmetry
Poincaré group
In physics and mathematics, the Poincaré group, named after Henri Poincaré, is the group of isometries of Minkowski spacetime.-Simple explanation:...
, q.e., JPC (where J is the angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
, P is the intrinsic parity
Intrinsic parity
In quantum mechanics, the intrinsic parity is a phase factor that arises as an eigenvalue of the parity operation...
, and C is the charge conjugation parity) and by the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
. One also specifies the isospin
Isospin
In physics, and specifically, particle physics, isospin is a quantum number related to the strong interaction. This term was derived from isotopic spin, but the term is confusing as two isotopes of a nucleus have different numbers of nucleons; in contrast, rotations of isospin maintain the number...
I of the meson.
Typically, every quark model
Quark model
In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons....
meson comes in SU(3) flavor nonet: an octet and a 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. Even when there is agreement that one of several states is one of these non-quark model
Quark model
In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons....
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
Quark model
In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons....
are tentative. The remainder of this article outlines the situation as it stood at the end of 2004.
Lattice predictions
Lattice QCDLattice QCD
Lattice QCD is a well-established non-perturbative approach to solving the quantum chromodynamics theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time....
predictions for glueballs are now fairly stable, at least when virtual
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...
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, with isospin
Isospin
In physics, and specifically, particle physics, isospin is a quantum number related to the strong interaction. This term was derived from isotopic spin, but the term is confusing as two isotopes of a nucleus have different numbers of nucleons; in contrast, rotations of isospin maintain the number...
I = 0.
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
Quenched approximation
In particle physics, the quenched approximation is an approximation often used in lattice gauge theory in which the quantum loops of fermions in Feynman diagrams are neglected. Equivalently, the corresponding one-loop determinants are set to one...
, which neglects virtual
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...
quarks loops. As a result, these computations miss mixing with meson states.
The 0++ states
The data show five isoscalar resonances: f0(600), f0(980), f0(1370), f0(1500), and f0(1710). Of these the f0(600) is usually identified with the σ of chiral modelChiral model
In nuclear physics, the chiral model is a phenomenological model describing mesons in the chiral limit where the masses of the quarks go to zero . It's a nonlinear sigma model with the principal homogeneous space of the Lie group SU as its target manifold where N is the number of quark flavors...
s. The decays and production of f0(1710) give strong evidence that it is also a meson.
Glueball candidate
The f0(1370) and f0(1500) cannot both be a quark model meson, because one is supernumerary. The production of the higher mass state in two photonPhoton
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
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 f0(980) has been identified by some authors as a tetraquark meson, along with the I = 1 states a0(980) and κ0(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 CLEOCLEO (particle detector)
CLEO was a general purpose particle detector at the Cornell Electron Storage Ring , and the name of the collaboration of physicists who operated the detector. The name CLEO is not an acronym; it is short for Cleopatra and was chosen to go with CESR . CESR was a particle accelerator designed to...
and BaBar
BaBar experiment
The BaBar experiment, or simply BaBar, is an international collaboration of more than 500 physicists and engineers studying the subatomic world at energies of approximately ten times the rest mass of a proton . Its design was motivated by the investigation of CP violation...
, have also been tentatively identified as tetraquark states. However, for these, other explanations are possible.
The 2++ states
Two isoscalar states are definitely identified—f2(1270) and the f′2(1525). No other states have been consistently identified by all experiments. Hence it is difficult to say more about these states.The 1−+ exotics and other states
The two isovector exotics π1(1400) and π1(1600) seem to be well established experimentally. They are clearly not glueballs, but could be either a tetraquark or a hybrid. The evidence for such assignments is weak.The π(1800) (0−+), ρ(1900) (1−−) and the η2(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 modelQuark modelIn physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons....
, mesonMesonIn particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...
s, baryonBaryonA baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
s, quarkQuarkA quark is an 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 nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
s, and gluonGluonGluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....
s - Exotic hadronExotic hadronExotic hadrons are subatomic particles made of quarks , but which do not fit into the usual scheme of hadrons. While bound by the strong interaction they are not predicted by the simple quark model...
s and exotic baryonExotic baryonExotic baryons are hypothetical composite particles which are bound states of 3 quarks and additional elementary particles. This is to be contrasted with ordinary baryons, which are bound states of just 3 quarks. The additional particles may include quarks, antiquarks or gluons.One such exotic...
s - Quantum chromodynamicsQuantum chromodynamicsIn theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
, flavor, and the QCD vacuumQCD vacuumThe QCD vacuum is the vacuum state of quantum chromodynamics . It is an example of a non-perturbative vacuum state, characterized by many non-vanishing condensates such as the gluon condensate or the quark condensate... - GlueXGlueXGlueX will be a particle physics experiment located at the Thomas Jefferson National Accelerator Facility accelerator. Its primary purpose is to understand the nature of confinement in quantum chromodynamics by mapping the spectrum of exotic mesons generated by the excitation of the gluonic field...
, an experiment which will explore the spectrum of glueballs and exotic mesons