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In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle. The name "quarkonium" is analogous to positronium, the bound state of electron and anti-electron. The particles are short-lived due to matter-antimatter annihilation.
Light quarks (up, down, and strange) are much less massive than the heavier quarks, and so the physical states actually seen in experiments (η, η′, and π0 mesons) are quantum mechanical mixtures of the light quark states. The much larger mass differences between the charm and bottom quarks and the lighter quarks results in states that are well defined in terms of a quark–antiquark pair of a given flavor.
Examples of quarkonia are the J/ψ meson (the ground state of charmonium,
c
c
) and the
ϒ
meson (bottomonium,
b
b
). Because of the high mass of the top quark, toponium (θ meson) does not exist, since the top quark decays through the electroweak interaction before a bound state can form (a rare example of a weak process proceeding more quickly than a strong process). Usually, the word "quarkonium" refers only to charmonium and bottomonium, and not to any of the lighter quark–antiquark states.
In the following table, the same particle can be named with the spectroscopic notation or with its mass. In some cases excitation series are used: ψ′ is the first excitation of ψ (which, for historical reasons, is called
J/ψ
particle); ψ″ is a second excitation, and so on. That is, names in the same cell are synonymous.
Some of the states are predicted, but have not been identified; others are unconfirmed. The quantum numbers of the X(3872) particle have been measured recently by the LHCb experiment at CERN. [1] This measurement shed some light on its identity, excluding the third option among the three envisioned, which are:
In 2005, the BaBar experiment announced the discovery of a new state: Y(4260). [2] [3] CLEO and Belle have since corroborated these observations. At first, Y(4260) was thought to be a charmonium state, but the evidence suggests more exotic explanations, such as a D "molecule", a 4-quark construct, or a hybrid meson.
| Term symbol n2S+1LJ | I G ( J P C ) | Particle | mass (MeV/c2) [4] |
|---|---|---|---|
| 11S0 | 0+(0−+) | ηc(1S) | 2983.4±0.5 |
| 13S1 | 0−(1−−) | J/ψ(1S) | 3096.900±0.006 |
| 11P1 | 0−(1+−) | hc(1P) | 3525.38±0.11 |
| 13P0 | 0+(0++) | χc0(1P) | 3414.75±0.31 |
| 13P1 | 0+(1++) | χc1(1P) | 3510.66±0.07 |
| 13P2 | 0+(2++) | χc2(1P) | 3556.20±0.09 |
| 21S0 | 0+(0−+) | ηc(2S), or η′ c | 3639.2±1.2 |
| 23S1 | 0−(1−−) | ψ(2S) or ψ(3686) | 3686.097±0.025 |
| 11D2 | 0+(2−+) | ηc2(1D) | |
| 13D1 | 0−(1−−) | ψ(3770) | 3773.13±0.35 |
| 13D2 | 0−(2−−) | ψ2(1D) | |
| 13D3 | 0−(3−−) | ψ3(1D) [‡] | |
| 21P1 | 0−(1+−) | hc(2P) [‡] | |
| 23P0 | 0+(0++) | χc0(2P) [‡] | |
| 23P1 | 0+(1++) | χc1(2P) [‡] | |
| 23P2 | 0+(2++) | χc2(2P) [‡] | |
| ???? | 0+(1++)[*] | X(3872) | 3871.69±0.17 |
| ???? | ??(1−−) [†] | Y(4260) | 4263+8 −9 |
Notes:
In the following table, the same particle can be named with the spectroscopic notation or with its mass. Some of the states are predicted, but have not been identified; others are unconfirmed.
| Term symbol n2S+1LJ | I G ( J P C ) | Particle | mass (MeV/c2) [5] |
|---|---|---|---|
| 11S0 | 0+(0−+) | η b (1S) | 9390.9±2.8 |
| 13S1 | 0−(1−−) | ϒ (1S) | 9460.30±0.26 |
| 11P1 | 0−(1+−) | h b(1P) | 9899.3±0.8 |
| 13P0 | 0+(0++) | χ b0 (1P) | 9859.44±0.52 |
| 13P1 | 0+(1++) | χ b1 (1P) | 9892.76±0.40 |
| 13P2 | 0+(2++) | χ b2 (1P) | 9912.21±0.40 |
| 21S0 | 0+(0−+) | η b (2S) | |
| 23S1 | 0−(1−−) | ϒ (2S) | 10023.26±0.31 |
| 11D2 | 0+(2−+) | η b 2(1D) | |
| 13D1 | 0−(1−−) | ϒ (1D) | |
| 13D2 | 0−(2−−) | ϒ 2(1D) | 10161.1±1.7 |
| 13D3 | 0−(3−−) | ϒ 3(1D) | |
| 21P1 | 0−(1+−) | h b(2P) | 10259.8±1.2 |
| 23P0 | 0+(0++) | χ b0 (2P) | 10232.5±0.6 |
| 23P1 | 0+(1++) | χ b1 (2P) | 10255.46±0.55 |
| 23P2 | 0+(2++) | χ b2 (2P) | 10268.65±0.55 |
| 33S1 | 0−(1−−) | ϒ (3S) | 10355.2±0.5 |
| 33P1 | 0+(1++) | χ b1 (3P) | 10513.42±0.41 (stat.) ± 0.53 (syst.) [6] |
| 33P2 | 0+(2++) | χ b2 (3P) | 10524.02±0.57 (stat.) ± 0.53 (syst.) [6] |
| 43S1 | 0−(1−−) | ϒ (4S) or ϒ (10580) | 10579.4±1.2 |
| 53S1 | 0−(1−−) | ϒ (5S) or ϒ (10860) | 10865±8 |
| 63S1 | 0−(1−−) | ϒ (11020) | 11019±8 |
Notes:
The
ϒ
(1S) state was discovered by the E288 experiment team, headed by Leon Lederman, at Fermilab in 1977, and was the first particle containing a bottom quark to be discovered. On 21 December 2011, the
χ
b2 (3P) state was the first particle discovered in the Large Hadron Collider; the discovery article was first posted on arXiv. [7] [8] In April 2012, Tevatron's DØ experiment confirmed the result in a paper published in Physical Review D . [9] [10] The J = 1 and J = 2 states were first resolved by the CMS experiment in 2018. [6]
The theta meson hasn't been and isn't expected to be observed in nature, as top quarks decay too fast to form mesons in nature (and be detected).
 | This section needs expansion. You can help by adding to it. (April 2017) |
The computation of the properties of mesons in quantum chromodynamics (QCD) is a fully non-perturbative one. As a result, the only general method available is a direct computation using lattice QCD (LQCD) techniques.[ citation needed ] However, for heavy quarkonium, other techniques are also effective.
The light quarks in a meson move at relativistic speeds, since the mass of the bound state is much larger than the mass of the quark. However, the speed of the charm and the bottom quarks in their respective quarkonia is sufficiently small for relativistic effects in these states to be much reduced. It is estimated that the velocity, , is roughly 0.3 times the speed of light for charmonia and roughly 0.1 times the speed of light for bottomonia. The computation can then be approximated by an expansion in powers of and . This technique is called non-relativistic QCD (NRQCD).
NRQCD has also been quantized as a lattice gauge theory, which provides another technique for LQCD calculations to use. Good agreement with the bottomonium masses has been found, and this provides one of the best non-perturbative tests of LQCD. For charmonium masses the agreement is not as good, but the LQCD community is actively working on improving their techniques. Work is also being done on calculations of such properties as widths of quarkonia states and transition rates between the states.
An early, but still effective, technique uses models of the effective potential to calculate masses of quarkonium states. In this technique, one uses the fact that the motion of the quarks that comprise the quarkonium state is non-relativistic to assume that they move in a static potential, much like non-relativistic models of the hydrogen atom. One of the most popular potential models is the so-called Cornell (or funnel) potential: [11]
where is the effective radius of the quarkonium state, and are parameters.
This potential has two parts. The first part, , corresponds to the potential induced by one-gluon exchange between the quark and its anti-quark, and is known as the Coulombic part of the potential, since its form is identical to the well-known Coulombic potential induced by the electromagnetic force.
The second part, , is known as the confinement part of the potential, and parameterizes the poorly understood non-perturbative effects of QCD. Generally, when using this approach, a convenient form for the wave function of the quarks is taken, and then and are determined by fitting the results of the calculations to the masses of well-measured quarkonium states. Relativistic and other effects can be incorporated into this approach by adding extra terms to the potential, much as is done for the model hydrogen atom in non-relativistic quantum mechanics.
This form was derived from QCD up to by Sumino (2003). [12] It is popular because it allows for accurate predictions of quarkonium parameters without a lengthy lattice computation, and provides a separation between the short-distance Coulombic effects and the long-distance confinement effects that can be useful in understanding the quark / anti-quark force generated by QCD.
Quarkonia have been suggested as a diagnostic tool of the formation of the quark–gluon plasma: Both disappearance and enhancement of their formation depending on the yield of heavy quarks in plasma can occur.
In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the electric force. Most of the mass of ordinary matter comes from two hadrons: the proton and the neutron, while most of the mass of the protons and neutrons is in turn due to the binding energy of their constituent quarks, due to the strong force.
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 nuclei. All commonly observable matter is composed of up quarks, down quarks and electrons. Owing to a phenomenon known as color confinement, quarks are never found in isolation; they can be found only within hadrons, which include baryons and mesons, or in quark–gluon plasmas. For this reason, much of what is known about quarks has been drawn from observations of hadrons.
In theoretical physics, quantum chromodynamics (QCD) is the theory 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 quantum field theory called a non-abelian gauge theory, with symmetry group SU(3). The QCD analog of electric charge is a property called color. Gluons are the force carriers of the theory, just as photons are for the electromagnetic force in quantum electrodynamics. The theory is an important part of the Standard Model of particle physics. A large body of experimental evidence for QCD has been gathered over the years.
The Standard Model of particle physics is the theory describing three of the four known fundamental forces in the universe and classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists worldwide, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, proof of the top quark (1995), the tau neutrino (2000), and the Higgs boson (2012) have added further credence to the Standard Model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.
The charm quark, charmed quark, or c quark is an elementary particle of the second generation. It is the third-most massive quark, with a mass of 1.27±0.02 GeV/c2 and a charge of +2/3e. It carries charm, a quantum number. Charm quarks are found in various hadrons, such as the J/psi meson and the charmed baryons. There are also several bosons, including the W and Z bosons and the Higgs boson, that can decay into charm quarks.
A tetraquark, in particle physics, is an exotic meson composed of four valence quarks. A tetraquark state has long been suspected to be allowed by quantum chromodynamics, the modern theory of strong interactions. A tetraquark state is an example of an exotic hadron which lies outside the conventional quark model classification. A number of different types of tetraquark have been observed.
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:
In 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 between themselves. Glueballs are extremely difficult to identify in particle accelerators, because they mix with ordinary meson states. In pure gauge theory, glueballs are the only states of the spectrum and some of them are stable.
The
J/ψ
(J/psi) meson is a subatomic particle, a flavor-neutral meson consisting of a charm quark and a charm antiquark. Mesons formed by a bound state of a charm quark and a charm anti-quark are generally known as "charmonium" or psions. The
J/ψ
is the most common form of charmonium, due to its spin of 1 and its low rest mass. The
J/ψ
has a rest mass of 3.0969 GeV/c2, just above that of the
η
c, and a mean lifetime of 7.2×10−21 s. This lifetime was about a thousand times longer than expected.
Exotic hadrons are subatomic particles composed of quarks and gluons, but which – unlike "well-known" hadrons such as protons, neutrons and mesons – consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic. In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral.
The Upsilon meson is a quarkonium state formed from a bottom quark and its antiparticle. It was discovered by the E288 experiment team, headed by Leon Lederman, at Fermilab in 1977, and was the first particle containing a bottom quark to be discovered because it is the lightest that can be produced without additional massive particles. It has a lifetime of 1.21×10−20 s and a mass about 9.46 GeV/c2 in the ground state.
An onium is a bound state of a particle and its antiparticle. These states are usually named by adding the suffix -onium to the name of one of the constituent particles, with one exception for "muonium"; a muon–antimuon bound pair is called "true muonium" to avoid confusion with old nomenclature.
The OZI rule is a consequence of quantum chromodynamics (QCD) that explains why certain decay modes appear less frequently than otherwise might be expected. It was independently proposed by Susumu Okubo, George Zweig and Jugoro Iizuka in the 1960s. It states that any strongly occurring process will be suppressed if, through only the removal of internal gluon lines, its Feynman diagram can be separated into two disconnected diagrams: one containing all of the initial-state particles and one containing all of the final-state particles.
In particle physics, the phi meson or
ϕ
meson is a vector meson formed of a strange quark and a strange antiquark. It was the
ϕ
meson's unusual propensity to decay into
K0
and
K0
that led to the discovery of the OZI rule. It has a mass of 1019.461±0.020 MeV/c2 and a mean lifetime of 1.55±0.01 × 10−22s.
The bottom eta meson or eta-b meson is a flavourless meson formed from a bottom quark and its antiparticle. It was first observed by the BaBar experiment at SLAC in 2008, and is the lightest particle containing a bottom and anti-bottom quark.
In particle physics, XYZ particles are recently-discovered heavy mesons whose properties do not appear to fit the standard picture of charmonium and bottomonium states. They are therefore types of exotic mesons. The term arises from the names given to some of the first such particles discovered: X(3872), Y(4260) and Zc(3900), although the symbols X and Y have since been deprecated by the Particle Data Group.
SooKyung Choi is a South Korean particle physicist at Gyeongsang National University. She is part of the Belle experiment and was the first to observe the X(3872) meson in 2003. She won the 2017 Ho-Am Prize in Science.
The Cornell Potential is an effective method to account for the confinement of quarks. It was developed in the 1970s to explain the masses of quarkonium states and account for the relation between the mass and angular momentum of the hadron. The potential has the form:
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