HOME

TheInfoList



OR:

In
particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
, quarkonium (from
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 nuclei. All commonly o ...
and -
onium An onium (plural: onia) 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 (replacing an ''-on'' suffix when present), with one except ...
, pl. quarkonia) is a flavorless
meson In particle physics, a meson ( or ) 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 ...
whose constituents are a heavy
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 nuclei. All commonly o ...
and its own
antiquark 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 o ...
, making it both a
neutral particle In physics, a neutral particle is a particle with no electric charge, such as a neutron. The term ''neutral particles'' should not be confused with ''truly neutral particles'', the subclass of neutral particles that are also identical to their ow ...
and its own antiparticle.


Light quarks

Light quarks ( up, down, and
strange Strange may refer to: Fiction * Strange (comic book), a comic book limited series by Marvel Comics * Strange (Marvel Comics), one of a pair of Marvel Comics characters known as The Strangers * Adam Strange, a DC Comics superhero * The title char ...
) 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 Charm may refer to: Social science * Charisma, a person or thing's pronounced ability to attract others * Superficial charm, flattery, telling people what they want to hear Science and technology * Charm quark, a type of elementary particle * Ch ...
and
bottom Bottom may refer to: Anatomy and sex * Bottom (BDSM), the partner in a BDSM who takes the passive, receiving, or obedient role, to that of the top or dominant * Bottom (sex), a term used by gay couples and BDSM * Buttocks or bottom, part of th ...
quarks and the lighter quarks results in states that are well defined in terms of a quark–antiquark pair of a given flavor.


Heavy quarks

Examples of quarkonia are the J/ψ meson (the ground state of charmonium, ) and the meson (bottomonium, ). Because of the high mass of the
top quark The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs Boson. This coupling y_ is very close to unity; in the Standard ...
, toponium ( θ meson) does not exist, since the top quark decays through the
electroweak interaction In particle physics, the electroweak interaction or electroweak force is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very differe ...
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.


Charmonium

In the following table, the same particle can be named with the
spectroscopic notation Spectroscopic notation provides a way to specify atomic ionization states, atomic orbitals, and molecular orbitals. Ionization states Spectroscopists customarily refer to the spectrum arising from a given ionization state of a given element by ...
or with its mass. In some cases excitation series are used: ψ′ is the first excitation of ψ (which, for historical reasons, is called 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) The X(3872) is an exotic meson candidate with a mass of 3871.68 MeV/c2 which does not fit into the quark model because of its quantum numbers. It was first discovered in 2003 by the Belle experiment in Japan and later confirmed by several other exp ...
particle have been measured recently by the LHCb experiment at CERN. This measurement shed some light on its identity, excluding the third option among the three envisioned, which are: * a charmonium hybrid state * a molecule * a candidate for the 11D2 state In 2005, the
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 (~10 GeV). Its design was motivat ...
announced the discovery of a new state:
Y(4260) The Y(4260) is an anomalous particle with an energy of 4260 MeV which does not appear to fit into the quark model. It was discovered by the BaBar experiment at Stanford University for the Department of Energy in California and later confirmed by sev ...
. 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 In particle physics, a meson ( or ) 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 ...
. Notes: :[] Needs confirmation. :[†] Interpretation as a 1−− charmonium state not favored. :[‡] Predicted, but not yet identified.


Bottomonium

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. Notes: : sub>*/span> Preliminary results. Confirmation needed. The (1S) state was discovered by the
E288 experiment E, or e, is the fifth letter and the second vowel letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''e'' (pronounced ); plur ...
team, headed by
Leon Lederman Leon, Léon (French) or León (Spanish) may refer to: Places Europe * León, Spain, capital city of the Province of León * Province of León, Spain * Kingdom of León, an independent state in the Iberian Peninsula from 910 to 1230 and again fro ...
, at
Fermilab Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy national laboratory specializing in high-energy particle physics. Since 2007, Fermilab has been operat ...
in 1977, and was the first particle containing a bottom quark to be discovered. On 21 December 2011, the (3P) state was the first particle discovered in the
Large Hadron Collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundred ...
; the discovery article was first posted on arXiv. In April 2012, Tevatron's DØ experiment confirmed the result in a paper published in ''
Physical Review D Physical may refer to: *Physical examination In a physical examination, medical examination, or clinical examination, a medical practitioner examines a patient for any possible medical signs or symptoms of a medical condition. It generally co ...
''. The ''J'' = 1 and ''J'' = 2 states were first resolved by the
CMS experiment CMS may refer to: Computing * Call management system * CMS-2 (programming language), used by the United States Navy * Code Morphing Software, a technology used by Transmeta * Collection management system for a museum collection * Color managem ...
in 2018.


Toponium

The
theta meson The theta meson () is a hypothetical form of quarkonium (i.e. a flavour (particle physics), flavourless meson) formed by a top quark () and top antiquark (). As a parity (physics), P-odd and charge parity, C-odd state, it is analogous to the ph ...
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).


QCD and quarkonium

The computation of the properties of
meson In particle physics, a meson ( or ) 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 ...
s in
quantum chromodynamics 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 ...
(QCD) is a fully non-perturbative one. As a result, the only general method available is a direct computation using
lattice QCD Lattice QCD is a well-established non-perturbative approach to solving the quantum chromodynamics (QCD) theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time. When the size of the ...
(LQCD) techniques. 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, \mathbf, is roughly 0.3 times the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
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 \mathbf/c and v^2/c^2. This technique is called non-relativistic QCD (NRQCD). NRQCD has also been quantized as a
lattice gauge theory In physics, lattice gauge theory is the study of gauge theories on a spacetime that has been discretized into a lattice. Gauge theories are important in particle physics, and include the prevailing theories of elementary particles: quantum elec ...
, 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'': : V(r) = -\frac + b\,r, where r is the effective radius of the quarkonium state, a and b are parameters. This potential has two parts. The first part, a/r, 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 1/r form is identical to the well-known Coulombic potential induced by the electromagnetic force. The second part, b\,r, 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 a and b 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 \mathcal(\Lambda^3_\text\,r^2) by Sumino (2003). 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 Quark–gluon plasma (QGP) or quark soup is an interacting localized assembly of quarks and gluons at thermal (local kinetic) and (close to) chemical (abundance) equilibrium. The word ''plasma'' signals that free color charges are allowed. In a 1 ...
: Both disappearance and enhancement of their formation depending on the yield of heavy quarks in plasma can occur.


See also

*
OZI rule 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 196 ...


References

{{Authority control Mesons Onia