In particle physics, a diquark, or diquark correlation/clustering, is a hypothetical state of two quarks grouped inside a baryon (that consists of three quarks) (Lichtenberg 1982). Corresponding models of baryons are referred to as quark–diquark models. The diquark is often treated as a single subatomic particle with which the third quark interacts via the strong interaction. The existence of diquarks inside the nucleons is a disputed issue, but it helps to explain some nucleon properties and to reproduce experimental data sensitive to the nucleon structure. Diquark–antidiquark pairs have also been advanced for anomalous particles such as the X(3872).


The forces between the two quarks in a diquark is attractive when both the colors and spins are antisymmetric. When both quarks are correlated in this way they tend to form a very low energy configuration. This low energy configuration has become known as a diquark.


Many scientists theorize that a diquark should not be considered a particle. Even though they may contain two quarks they are not colour neutral, and therefore cannot exist as isolated bound states. So instead they tend to float freely inside hadrons as composite entities; while free-floating they have a size of about fm. This also happens to be the same size as the hadron itself.


Diquarks are the conceptual building blocks, and as such give scientists an ordering principle for the most important states in the hadronic spectrum. There are many different pieces of evidence that prove diquarks are fundamental in the structure of hadrons. One of the most compelling pieces of evidence comes from a recent study of baryons. In this study the baryon had one heavy and two light quarks. Since the heavy quark is inert, the scientists were able to discern the properties of the different quark configurations in the hadronic spectrum.

Λ and Σ baryon experiment

An experiment was conducted using diquarks in an attempt to study the Λ and Σ baryons that are produced in the creation of hadrons created by fast-moving quarks. In the experiment the quarks ionized the vacuum area. This produced the quark–antiquark pairs, which then converted themselves into mesons. When generating a baryon by assembling quarks, it is helpful if the quarks first form a stable two-quark state. The Λ and the Σ are created as a result of up, down and strange quarks. Scientists found that the Λ contains the [ud] diquark, however the Σ does not. From this experiment scientists inferred that Λ baryons are more common than Σ baryons, and indeed they are more common by a factor of 10.


  • D. B. Lichtenberg; W. Namgung; E. Predazzi; J. G. Wills (1982). "Baryon Masses In A Relativistic Quark-Diquark Model". Physical Review Letters. 48 (24): 1653–1656. Bibcode:1982PhRvL..48.1653L. doi:10.1103/PhysRevLett.48.1653.
  • R. Rapp; T. Schëfer; E. Shuryak; M. Velkovsky (1998). "Diquark bose condensates in high density matter and instantons". Physical Review Letters. 81 (1): 53–56. arXiv:hep-ph/9711396. Bibcode:1998PhRvL..81...53R. doi:10.1103/PhysRevLett.81.53.
Charmed baryons

Charmed baryons are a category of composite particles comprising all baryons made of at least one charm quark. Since their first observation in the 1970s, a large number of distinct charmed baryon states have been identified. Observed charmed baryons have masses ranging between 2300 and 2700 MeV/c2. In 2002, the SELEX collaboration, based at Fermilab published evidence of a doubly charmed baryon (Ξcc), containing two charm quarks) with a mass of ~3520 MeV/c2, but has yet to be confirmed by other experiments. One triply charmed baryon (Ωccc) has been predicted but not yet observed.

Color superconductivity

Color superconductivity is a phenomenon predicted to occur in quark matter if the baryon density is sufficiently high (well above nuclear density) and the temperature is not too high (well below 1012 kelvin). Color superconducting phases are to be contrasted with the normal phase of quark matter, which is just a weakly interacting Fermi liquid of quarks.

In theoretical terms, a color superconducting phase is a state in which the quarks near the Fermi surface become correlated in Cooper pairs, which condense. In phenomenological terms, a color superconducting phase breaks some of the symmetries of the underlying theory, and has a very different spectrum of excitations and very different transport properties from the normal phase.

Exotic hadron

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.Consistent with ordinary hadrons, exotic hadrons are classified as being either fermions, like ordinary baryons, or bosons, like ordinary mesons. According to this classification scheme, pentaquarks, containing five valence quarks, are exotic baryons, while tetraquarks (four valence quarks) and hexaquarks (six quarks, consisting of either a dibaryon or three quark-antiquark pairs) would be considered exotic mesons. Tetraquark and pentaquark particles are believed to have been observed and are being investigated; Hexaquarks have not yet been confirmed as observed.

Exotic hadrons can be searched for by looking for S-matrix poles with quantum numbers forbidden to ordinary hadrons. Experimental signatures for such exotic hadrons have been seen by at least 2003 but remain a topic of controversy in particle physics.

Jaffe and Low suggested that the exotic hadrons manifest themselves as poles of the P matrix, and not of the S matrix. Experimental P-matrix poles are determined reliably in both the meson-meson channels and nucleon-nucleon channels.

Index of physics articles (D)

The index of physics articles is split into multiple pages due to its size.

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Pervez Hoodbhoy

Pervez Amirali Hoodbhoy (Urdu: پرویز ہودبھائی; born 11 July 1950) is a Pakistani nuclear physicist and activist who serves as Zohra and ZZ Ahmed Foundation distinguished professor at the Forman Christian College and previously taught physics at the Quaid-e-Azam University. Hoodbhoy is also a prominent activist in particular concerned with promotion of freedom of speech, secularism and education in Pakistan.Born and raised in Karachi, Hoodbhoy studied at the Massachusetts Institute of Technology for nine years, where he received degrees in electrical engineering, mathematics and solid-state physics, eventually leading to a PhD in nuclear physics. In 1981, Hoodbhoy went on to conduct post-doctoral research at the University of Washington, before leaving to serve as a visiting professor at the Carnegie Mellon University in 1985. While still a professor at the Quaid-e-Azam University, Hoodbhoy worked as a guest scientist at the International Centre for Theoretical Physics between 1986 until 1994. He remained with the Quaid-e-Azam University until 2010, throughout which he held visiting professorships at MIT, University of Maryland and Stanford Linear Collider.In 2011, Hoodbhoy joined LUMS while simultaneously working as a researcher with the Princeton University and a columnist with the Express Tribune. His contract with LUMS was terminated in 2013 which resulted in a controversy. He is a sponsor of the Bulletin of the Atomic Scientists, and a member of the monitoring panel on terrorism of the World Federation of Scientists. Hoodbhoy has won several awards including the Abdus Salam Prize for Mathematics (1984); the Kalinga Prize for the popularization of science (2003); the Burton Award (2010) from the American Physical Society. In 2011, he was included in the list of 100 most influential global thinkers by Foreign Policy. In 2013, he was made a member of the UN Secretary General's Advisory Board on Disarmament.Hoodbhoy remains one of Pakistan's most prominent academics. He is the author of Islam and Science: Religious Orthodoxy and the Battle for Rationality He is the head of Mashal Books in Lahore, which claims to make "a major translation effort to produce books in Urdu that promote modern thought, human rights, and emancipation of women". Hoodbhoy has written for Project Syndicate, DAWN, The New York Times and The Express Tribune. Hoodbhoy is generally considered one of the most vocal, progressive and liberal member of the Pakistani intelligentsia.

Quark star

A quark star is a hypothetical type of compact exotic star, where extremely high temperature and pressure has forced nuclear particles to form a continuous state of matter that consists primarily of free quarks.

It is well known that massive stars can collapse to form neutron stars, under extreme temperatures and pressures. In simple terms, neutrons usually have space separating them due to degeneracy pressure keeping them apart. Under extreme conditions such as a neutron star, the pressure separating nucleons is overwhelmed by gravity, and the separation between them breaks down, causing them to be packed extremely densely and form an immensely hot and dense state known as neutron matter, where they are only held apart by the strong interaction. Because these neutrons are made of quarks, it is hypothesized that under even more extreme conditions, the degeneracy pressure keeping the quarks apart within the neutrons might break down in much the same way, creating an ultra-dense phase of degenerate matter based on densely packed quarks. This is seen as plausible, but is very hard to prove, as scientists cannot easily create the conditions needed to investigate the properties of quark matter, so it is unknown whether this actually occurs.

If quark stars can form, then the most likely place to find quark star matter would be inside neutron stars that exceed the internal pressure needed for quark degeneracy - the point at which neutrons (which are formed from quarks bound together) break down into a form of dense quark matter. They could also form if a massive star collapses at the end of its life, provided that it is possible for a star to be large enough to collapse beyond a neutron star but not large enough to form a black hole. However, as scientists are unable so far to explore most properties of quark matter, the exact conditions and nature of quark stars, and their existence, remain hypothetical and unproven. The question whether such stars exist and their exact structure and behavior is actively studied within astrophysics and particle physics.

If they exist, quark stars would resemble and be easily mistaken for neutron stars: they would form in the death of a massive star in a Type II supernova, they would be extremely dense and small, and possess a very high gravitational field. They would also lack some features of neutron stars, unless they also contained a shell of neutron matter, because free quarks are not expected to have properties matching degenerate neutron matter. For example, they might be radio-silent, or not have typical size, electromagnetic, or temperature measurements, compared to other neutron stars.

The hypothesis about quark stars was first proposed in 1965 by Soviet physicists D. D. Ivanenko and D. F. Kurdgelaidze. Their existence has not been confirmed. The equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 1012 K) quark-gluon plasma blobs the size of atomic nuclei, which decay immediately after formation. The conditions inside compact stars with extremely high densities and temperatures well below 1012 K can not be recreated artificially, as there are no known methods to produce, store or study "cold" quark matter directly as it would be found inside quark stars. The theory predicts quark matter to possess some peculiar characteristics under these conditions.

SU(2) color superconductivity

Several hundred metals, compounds, alloys and ceramics possess the property of superconductivity at low temperatures. The SU(2) color quark matter adjoins the list of superconducting systems. Although it is a mathematical abstraction, its properties are believed to be closely related to the SU(3)

color quark matter, which exists in nature when ordinary matter is compressed at supranuclear densities above ~ 0.5 1039 nucleon/cm3.


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.

Thomas Carlos Mehen

Thomas Carlos Mehen (born September 8, 1970) is an American physicist. His research has consisted of primarily Quantum chromodynamics (QCD) and the application of effective field theory to problems in hadronic physics. He has also worked on effective field theory for non-relativistic particles whose short range interactions are characterized by a large scattering length, as well as novel field theories which arise from unusual limits of string theory.

Mehen was born in Tegucigalpa, Honduras where he learned Spanish as his first language. In 1974 at the age of three he relocated with his family to McLean, Virginia, USA. He was educated at the University of Virginia (B.S., 1992), and Johns Hopkins University (M.A., Ph.D., 1998). He served as a research associate and John A. McCone Postdoctoral Scholar in the Division of Mathematics, Physics and Astronomy at the California Institute of Technology from 1997 to 2000. He served as a research associate and University Postdoctoral Fellow in the Department of Physics at the Ohio State University from 2000–2001. In 2002 he joined the Department of Physics at Duke University as assistant professor where he is currently a tenured faculty member.In 2005 Mehen received an Outstanding Junior Investigator Award in Nuclear Physics by the United States Department of Energy. He has contributed over 50 published works and is a lecturer in his field.

Three-body force

A three-body force is a force that does not exist in a system of two objects but appears in a three-body system. In general, if the behaviour of a system of more than two objects cannot be described by the two-body interactions between all possible pairs, as a first approximation, the deviation is mainly due to a three-body force.

The fundamental strong interaction does exhibit such behaviour, the most important example being the stability experimentally observed for the helium-3 isotope, which can be described as a 3-body quantum cluster entity of two protons and one neutron [PNP] in stable superposition. Direct evidence of a 3-body force in helium-3 is known: [1]. The existence of stable [PNP] cluster calls into question models of the atomic nucleus that restrict nucleon interactions within shells to 2-body phenomenon. The three-nucleon-interaction is fundamentally possible because gluons, the mediators of the strong interaction, can couple to themselves. In particle physics, the interactions between the three quarks that compose hadrons can be described in a diquark model which might be equivalent to the hypothesis of a three-body force. There is growing evidence in the field of nuclear physics that three-body forces exist among the nucleons inside atomic nuclei for many different isotopes (three-nucleon force).


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 experimental collaborations. Several theories have been proposed for its nature, such as a mesonic molecule or a diquark-antidiquark pair (tetraquark).

The quantum numbers of X(3872) have been determined by the LHCb Experiment at CERN in March 2013. The values for JPC is 1++.

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