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Identities and classification of possible tetraquark mesons. Green denotes I = 0 states, purple, I = 1/2 and red, I = 1. The vertical axis is the mass.
Identities and classification of possible tetraquark mesons. Green denotes I = 0 states, purple, I = 1/2 and red, I = 1. The vertical axis is the mass.

Non-quark model mesons include

  1. exotic mesons, which have quantum numbers not possible for mesons in the quark model;
  2. glueballs or gluonium, which have no valence quarks at all;
  3. tetraquarks, which have two valence quark-antiquark pairs; and
  4. hybrid mesons, which contain a valence quark-antiquark pair and one or more gluons. In Physics, the quark model is a classification scheme for Hadrons in terms of their valence quarks, i Quantum numbers describe values of conserved numbers in the dynamics of the Quantum system. In Physics, the quark model is a classification scheme for Hadrons in terms of their valence quarks, i In Particle physics, a glueball is a strongly interacting particle containing no valence Quarks It is composed entirely of Gluons Such a state In Physics, the quark model is a classification scheme for Hadrons in terms of their valence quarks, i In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. In Particle physics a tetraquark is a hypothetical Meson composed of four valence Quarks. Gluons ( Glue and the suffix -on) are Elementary particles that cause Quarks to interact and are indirectly responsible for the

All of these can be classed as mesons, because they are hadrons and carry zero baryon number. In Particle physics, a meson is a strongly interacting Boson &mdashthat is a Hadron with integer spin. In Particle physics, a hadron ( from the ἁδρός hadrós, " stout, thick " ( In Particle physics, the baryon number is an approximate conserved Quantum number of a system Of these, glueballs must be flavor singlets; that is, have zero isospin, strangeness, charm, bottomness and topness. In Physics, and specifically Particle physics, isospin ( isotopic spin, isobaric spin) is a Quantum number related to the The charm Quark is a second-generation quark with an electric charge of +(2/3 e. The bottom quark is a third-generation Quark with a charge of − e. The top quark is the third- generation up-type Quark with a charge of +(2/3 e. Like all particle states, they are specified by the quantum numbers which label representations of the Poincaré symmetry, ie, JPC (where J is the angular momentum, P is the intrinsic parity and C is the charge conjugation parity) and by the mass. In Physics and Mathematics, the Poincaré group, named after Henri Poincaré, is the group of isometries of Minkowski spacetime In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position In Physics, a parity transformation (also called parity inversion) is the flip in the sign of one Spatial Coordinate. In Physics, C-symmetry means the symmetry of physical laws under a charge -conjugation transformation. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object 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. In Physics, the quark model is a classification scheme for Hadrons in terms of their valence quarks, i In Particle physics, flavour or flavor (see spelling differences) is a Quantum number of Elementary particles related to their 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. In Physics, the quark model is a classification scheme for Hadrons in terms of their valence quarks, i There is also the considerable experimental labour of assigning quantum numbers to each state and crosschecking them in other experiments. As a result, all assignments outside the quark model are tentative. In Physics, the quark model is a classification scheme for Hadrons in terms of their valence quarks, i The remainder of this article outlines the situation as it stood at the end of 2004.

Contents

Lattice predictions

Lattice QCD predictions for glueballs are now fairly stable, at least when virtual quarks are neglected. In Physics, lattice quantum chromodynamics (lattice QCD is a theory of Quarks and Gluons formulated on a space-time lattice. In Physics, a virtual particle is a particle that exists for a limited time and space introducing uncertainty in their energy and momentum due to the Heisenberg Uncertainty The two lowest states are

0+ + with mass of 1611±163 MeV and
2+ + with mass of 2232±310 MeV.

The 0− + and exotic glueballs such as 0− − are all expected to lie above 2 GeV. Glueballs are necessarily isoscalar, with isospin I=0. In Physics, and specifically Particle physics, isospin ( isotopic spin, isobaric spin) is a Quantum number related to the

The ground state hybrid mesons 0− +, 1− +, 1− − and 2− + all lie a little below 2 GeV. The hybrid with exotic quantum numbers 1− + is at 1. 9±0. 2 GeV. The best lattice computations to date are made in the quenched approximation, which neglects virtual quarks loops. In Physics, lattice quantum chromodynamics (lattice QCD is a theory of Quarks and Gluons formulated on a space-time lattice. 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 models. In Nuclear physics, the chiral model is a phenomenological model describing Mesons in the Chiral limit where the masses of the Quarks 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 photon reactions such as 2γ → 2π or 2γ → 2K reactions is highly suppressed. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena 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 longlived (narrow in the jargon of particle spectroscopy) states: the scalar (0++) state DsJ(2317)*± and the vector (1+) meson DsJ(2460)*±, observed at CLEO and BaBar, have also been tentatively identified as tetraquark states. CLEO was a general purpose Particle detector at the Cornell Electron Storage Ring (CESR and the name of the collaboration of physicists who operated the detector The BaBar experiment is an international collaboration of more than 550 physicists and engineers studying the subatomic world at energy of approximately ten times the rest mass of a proton 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 0− + π(1800), 1− − ρ(1900) and the 2− + η2<(1870) 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

References and external links

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