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A quark () is a type of
elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, antiqu ...
and a fundamental constituent of
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...
. Quarks combine to form
composite particle This is a list of known and hypothesized particles. Elementary particles Elementary particles are particles with no measurable internal structure; that is, it is unknown whether they are composed of other particles. They are the fundamental ob ...
s called
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite particle, composite subatomic particle made of two or more quarks bound state, held together by the strong interaction. They are analogous to molecules tha ...
s, the most stable of which are
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s and
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...
s, the components of
atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden experiments, Geiger–Marsden gold foil experiment. After th ...
. All commonly observable matter is composed of up quarks, down quarks and
electron The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electric charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are generally thought t ...
s. Owing to a phenomenon known as ''
color confinement 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, neu ...
'', quarks are never found in isolation; they can be found only within hadrons, which include
baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron list of particles, family of particles; hadrons are composed o ...
s (such as protons and neutrons) and
meson In particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles in the universe are ...
s, or in
quark–gluon plasma Quark–gluon plasma (QGP) or quark soup is an interacting localized assembly of quarks and gluons at Thermodynamic equilibrium#Local and global equilibrium, thermal (local kinetic) and (close to) chemical (abundance) equilibrium. The word ''plasm ...
s. more exotic phases of quark matter. For this reason, much of what is known about quarks has been drawn from observations of hadrons. Quarks have various
intrinsic In science Science is a systematic endeavor that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about the universe. Science may be as old as the human species, and ...
properties Property is the ownership of land, resources, improvements or other tangible objects, or intellectual property. Property may also refer to: Mathematics * Property (mathematics) Philosophy and science * Property (philosophy), in philosophy and ...
, including
electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electron ...
,
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a body. It was traditionally believed to be related to the physical quantity, quantity of matter in a Physical object, physical body, until the discovery of the atom and par ...
,
color charge Color charge is a property of 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 comp ...
, and
spin Spin or spinning most often refers to: * Spinning (textiles) Spinning is a twisting technique to form yarn from fiber Fiber or fibre (from la, fibra, links=no) is a #Natural fibers, natural or Fiber#Artificial fibers, artificial subst ...
. They are the only elementary particles in the
Standard Model The Standard Model of particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles ...
of
particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standa ...
to experience all four
fundamental interaction In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist: the gravitational and elect ...
s, also known as ''fundamental forces'' (
electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
,
gravitation In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
,
strong interaction The strong interaction or strong force is a fundamental interaction that confines Quark, quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is calle ...
, and
weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, ...
), as well as the only known particles whose electric charges are not
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language of ...
multiples of the
elementary charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...
. There are six types, known as ''
flavors Flavor or flavour is either the sensory perception of taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a ...
'', of quarks: up, down, charm, strange,
top A spinning top, or simply a top, is a toy with a squat body and a sharp point at the bottom, designed to be spun on its vertical axis, balancing on the tip due to the gyroscopic effect. Once set in motion, a top will usually wobble for a f ...
, and bottom. Up and down quarks have the lowest
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a body. It was traditionally believed to be related to the physical quantity, quantity of matter in a Physical object, physical body, until the discovery of the atom and par ...
es of all quarks. The heavier quarks rapidly change into up and down quarks through a process of
particle decay In particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles in the universe are cl ...
: the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in the
universe The universe is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmology, cosmological description of the development of ...
, whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions (such as those involving
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s and in
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies, and to contain them in well-defined particle beam, beams. Large accelerators are used for fun ...
s). For every quark flavor there is a corresponding type of
antiparticle In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite charge (physics), physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also ...
, known as an antiquark, that differs from the quark only in that some of its properties (such as the electric charge) have equal magnitude but opposite sign. The
quark model In particle 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. The quark model underlies Flavour (particle physics), ...
was independently proposed by physicists
Murray Gell-Mann Murray Gell-Mann (; September 15, 1929 – May 24, 2019) was an American physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scale ...
and
George Zweig George Zweig (; born May 30, 1937) is a Russian-American physicist. He was trained as a particle physicist under Richard Feynman. He introduced, independently of Murray Gell-Mann, the quark model (although he named it "aces"). He later turned his ...
in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until
deep inelastic scattering Deep inelastic scattering is the name given to a process used to probe the insides of hadrons (particularly the baryons, such as protons and neutrons), using electrons, muons and neutrinos. It provided the first convincing evidence of the reality ...
experiments at the
Stanford Linear Accelerator Center SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University Stanford University, officially Leland Stanfor ...
in 1968. Accelerator program experiments have provided evidence for all six flavors. The top quark, first observed at
Fermilab Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy United States Department of Energy National Labs, national laboratory specializing in high-energy parti ...
in 1995, was the last to be discovered.


Classification

The
Standard Model The Standard Model of particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles ...
is the theoretical framework describing all the known
elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, antiqu ...
s. This model contains six
flavors Flavor or flavour is either the sensory perception of taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a ...
of quarks (), named up (), down (), strange (), charm (), bottom (), and
top A spinning top, or simply a top, is a toy with a squat body and a sharp point at the bottom, designed to be spun on its vertical axis, balancing on the tip due to the gyroscopic effect. Once set in motion, a top will usually wobble for a f ...
().
Antiparticle In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite charge (physics), physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also ...
s of quarks are called ''antiquarks'', and are denoted by a bar over the symbol for the corresponding quark, such as for an up antiquark. As with
antimatter In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding subatomic particle, particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and som ...
in general, antiquarks have the same mass,
mean lifetime A quantity is subject to exponential decay if it decreases at a rate Proportionality (mathematics), proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and ...
, and spin as their respective quarks, but the electric charge and other charges have the opposite sign. Quarks are spin- particles, which means they are
fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks ...
s according to the
spin–statistics theorem In quantum mechanics Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quant ...
. They are subject to the
Pauli exclusion principle In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spin (physics), spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously. This principle ...
, which states that no two identical fermions can simultaneously occupy the same
quantum state In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement in quantum mechanics, measurement on a system. Knowledge of the quantum state together with the rul ...
. This is in contrast to
boson In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0,1,2 ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have odd half-integer ...
s (particles with integer spin), of which any number can be in the same state. Unlike
lepton In particle physics, a lepton is an elementary particle of half-integer spin (spin (physics), spin ) that does not undergo strong interactions. Two main classes of leptons exist: electric charge, charged leptons (also known as the electron-li ...
s, quarks possess
color charge Color charge is a property of 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 comp ...
, which causes them to engage in the
strong interaction The strong interaction or strong force is a fundamental interaction that confines Quark, quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is calle ...
. The resulting attraction between different quarks causes the formation of composite particles known as ''
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite particle, composite subatomic particle made of two or more quarks bound state, held together by the strong interaction. They are analogous to molecules tha ...
s'' (see " Strong interaction and color charge" below). The quarks that determine the
quantum number In Quantum mechanics, quantum physics and chemistry, quantum numbers describe values of conserved quantity, conserved quantities in the dynamics of a quantum system. Quantum numbers correspond to eigenvalues of Operator (quantum mechanics), ope ...
s of hadrons are called ''valence quarks''; apart from these, any hadron may contain an indefinite number of virtual "
sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of Saline water, salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Water distributio ...
" quarks, antiquarks, and
gluon A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong interaction, strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged partic ...
s, which do not influence its quantum numbers. There are two families of hadrons:
baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron list of particles, family of particles; hadrons are composed o ...
s, with three valence quarks, and
meson In particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles in the universe are ...
s, with a valence quark and an antiquark. The most common baryons are the proton and the neutron, the building blocks of the
atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden experiments, Geiger–Marsden gold foil experiment. After th ...
. A great number of hadrons are known (see list of baryons and
list of mesons :''This list is of all known and predicted scalar, pseudoscalar and vector meson In particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, for ...
), most of them differentiated by their quark content and the properties these constituent quarks confer. The existence of "exotic" hadrons with more valence quarks, such as
tetraquark 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 ...
s () and
pentaquark A pentaquark is a human-made subatomic particle, consisting of four quarks and one antiquark bound state, bound together; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them. As quarks h ...
s (), was conjectured from the beginnings of the quark model but not discovered until the early 21st century. Elementary fermions are grouped into three generations, each comprising two leptons and two quarks. The first generation includes up and down quarks, the second strange and charm quarks, and the third bottom and top quarks. All searches for a fourth generation of quarks and other elementary fermions have failed, and there is strong indirect evidence that no more than three generations exist.The main evidence is based on the
resonance width In physics and engineering, the quality factor or ''Q'' factor is a Dimensionless quantity, dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the initial energy stored in the res ...
of the boson, which constrains the 4th generation neutrino to have a mass greater than ~. This would be highly contrasting with the other three generations' neutrinos, whose masses cannot exceed .
Particles in higher generations generally have greater mass and less stability, causing them to decay into lower-generation particles by means of
weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, ...
s. Only first-generation (up and down) quarks occur commonly in nature. Heavier quarks can only be created in high-energy collisions (such as in those involving
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s), and decay quickly; however, they are thought to have been present during the first fractions of a second after the
Big Bang The Big Bang event is a physical theory that describes how the Expansion of the universe, universe expanded from an initial state of high Energy density, density and temperature. Various Physical cosmology, cosmological models of the Big Ba ...
, when the universe was in an extremely hot and dense phase (the
quark epoch In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the unive ...
). Studies of heavier quarks are conducted in artificially created conditions, such as in
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies, and to contain them in well-defined particle beam, beams. Large accelerators are used for fun ...
s. Having electric charge, mass, color charge, and flavor, quarks are the only known elementary particles that engage in all four
fundamental interaction In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist: the gravitational and elect ...
s of contemporary physics: electromagnetism, gravitation, strong interaction, and weak interaction. Gravitation is too weak to be relevant to individual particle interactions except at extremes of energy ( Planck energy) and distance scales ( Planck distance). However, since no successful
quantum theory of gravity Quantum gravity (QG) is a field of theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict List of natural phenomena, ...
exists, gravitation is not described by the Standard Model. See the table of properties below for a more complete overview of the six quark flavors' properties.


History

The
quark model In particle 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. The quark model underlies Flavour (particle physics), ...
was independently proposed by physicists
Murray Gell-Mann Murray Gell-Mann (; September 15, 1929 – May 24, 2019) was an American physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scale ...
and
George Zweig George Zweig (; born May 30, 1937) is a Russian-American physicist. He was trained as a particle physicist under Richard Feynman. He introduced, independently of Murray Gell-Mann, the quark model (although he named it "aces"). He later turned his ...
in 1964. The proposal came shortly after Gell-Mann's 1961 formulation of a particle classification system known as the '' Eightfold Way'' – or, in more technical terms,
SU(3) In mathematics, the special unitary group of degree , denoted , is the Lie group of Unitary matrix, unitary Matrix (mathematics), matrices with determinant 1. The more general Unitary group, unitary matrices may have complex determinants with ...
flavor symmetry, streamlining its structure. Physicist
Yuval Ne'eman Yuval Ne'eman ( he, יובל נאמן, 14 May 1925 – 26 April 2006) was an Israeli Theoretical physics, theoretical physicist, military scientist, and politician. He was Ministry of Science, Technology and Space, Minister of Science and D ...
had independently developed a scheme similar to the Eightfold Way in the same year. An early attempt at constituent organization was available in the
Sakata model In particle physics, the Sakata model of hadrons was a precursor to the quark model. It proposed that the proton, neutron, and Lambda baryon were elementary particles (sometimes referred to as sakatons ), and that all other known hadrons were made ...
. At the time of the quark theory's inception, the " particle zoo" included a multitude of
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite particle, composite subatomic particle made of two or more quarks bound state, held together by the strong interaction. They are analogous to molecules tha ...
s, among other particles. Gell-Mann and Zweig posited that they were not elementary particles, but were instead composed of combinations of quarks and antiquarks. Their model involved three flavors of quarks, up, down, and strange, to which they ascribed properties such as spin and electric charge. The initial reaction of the physics community to the proposal was mixed. There was particular contention about whether the quark was a physical entity or a mere abstraction used to explain concepts that were not fully understood at the time. In less than a year, extensions to the Gell-Mann–Zweig model were proposed.
Sheldon Glashow Sheldon Lee Glashow (, ; born December 5, 1932) is a Nobel Prize in Physics, Nobel Prize-winning United States, American Theoretical physics, theoretical physicist. He is the Metcalf Center for Science and Engineering, Metcalf Professor of Mathem ...
and James Bjorken predicted the existence of a fourth flavor of quark, which they called ''charm''. The addition was proposed because it allowed for a better description of the
weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, ...
(the mechanism that allows quarks to decay), equalized the number of known quarks with the number of known
lepton In particle physics, a lepton is an elementary particle of half-integer spin (spin (physics), spin ) that does not undergo strong interactions. Two main classes of leptons exist: electric charge, charged leptons (also known as the electron-li ...
s, and implied a mass formula that correctly reproduced the masses of the known
meson In particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles in the universe are ...
s.
Deep inelastic scattering Deep inelastic scattering is the name given to a process used to probe the insides of hadrons (particularly the baryons, such as protons and neutrons), using electrons, muons and neutrinos. It provided the first convincing evidence of the reality ...
experiments conducted in 1968 at the
Stanford Linear Accelerator Center SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University Stanford University, officially Leland Stanfor ...
(SLAC) and published on October 20, 1969, showed that the proton contained much smaller, point-like objects and was therefore not an elementary particle. Physicists were reluctant to firmly identify these objects with quarks at the time, instead calling them " partons" – a term coined by
Richard Feynman Richard Phillips Feynman (; May 11, 1918 – February 15, 1988) was an American theoretical physicist, known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, the physics of the superflu ...
. The objects that were observed at SLAC would later be identified as up and down quarks as the other flavors were discovered. Nevertheless, "parton" remains in use as a collective term for the constituents of hadrons (quarks, antiquarks, and
gluon A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong interaction, strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged partic ...
s). The strange quark's existence was indirectly validated by SLAC's scattering experiments: not only was it a necessary component of Gell-Mann and Zweig's three-quark model, but it provided an explanation for the
kaon KAON (Karlsruhe ontology) is an Ontology engineering, ontology infrastructure developed by the University of Karlsruhe and the Forschungszentrum Informatik, Research Center for Information Technologies in Karlsruhe. Its first incarnation was deve ...
() and
pion In particle physics, a pion (or a pi meson, denoted with the Greek alphabet, Greek letter pi (letter), pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the li ...
() hadrons discovered in cosmic rays in 1947. In a 1970 paper, Glashow,
John Iliopoulos John (Jean) Iliopoulos ( Greek: Ιωάννης Ηλιόπουλος; 1940, Kalamata, Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkan ...
and
Luciano Maiani Luciano Maiani (born 16 July 1941, in Rome) is a San Marino, Sammarinese physicist best known for his prediction of the charm quark with Sheldon Glashow and John Iliopoulos (the "GIM mechanism"). Academic history In 1964 Luciano Maiani received h ...
presented the
GIM mechanism In particle physics, the GIM mechanism (or Glashow–Iliopoulos–Maiani mechanism) is the mechanism through which Flavor-changing neutral current, flavour-changing neutral currents (FCNCs) are suppressed in One-loop_Feynman_diagram, loop diagrams. ...
(named from their initials) to explain the experimental non-observation of flavor-changing neutral currents. This theoretical model required the existence of the as-yet undiscovered
charm quark The charm quark, charmed quark or c quark (from its symbol, c) 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. Examples of hadrons containin ...
. The number of supposed quark flavors grew to the current six in 1973, when Makoto Kobayashi and Toshihide Maskawa noted that the experimental observation of
CP violation In particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles in the universe are ...
CP violation is a phenomenon that causes weak interactions to behave differently when left and right are swapped (
P symmetry In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical science ...
) and particles are replaced with their corresponding antiparticles ( C symmetry).
could be explained if there were another pair of quarks. Charm quarks were produced almost simultaneously by two teams in November 1974 (see November Revolution) – one at SLAC under
Burton Richter Burton Richter (March 22, 1931 – July 18, 2018) was an American physicist. He led the Stanford Linear Accelerator Center (SLAC) team which co-discovered the J/ψ meson in 1974, alongside the Brookhaven National Laboratory Brookhaven Natio ...
, and one at
Brookhaven National Laboratory Brookhaven National Laboratory (BNL) is a United States Department of Energy national laboratories, United States Department of Energy national laboratory located in Upton, New York, Upton, Long Island, and was formally established in 1947 at th ...
under Samuel Ting. The charm quarks were observed bound with charm antiquarks in mesons. The two parties had assigned the discovered meson two different symbols, and ; thus, it became formally known as the meson. The discovery finally convinced the physics community of the quark model's validity. In the following years a number of suggestions appeared for extending the quark model to six quarks. Of these, the 1975 paper by
Haim Harari use both this parameter and , birth_date to display the person's date of birth, date of death, and age at death) --> , death_place = , death_cause = , body_discovered = , resting_place = , resting_place_coordinates ...
was the first to coin the terms ''
top A spinning top, or simply a top, is a toy with a squat body and a sharp point at the bottom, designed to be spun on its vertical axis, balancing on the tip due to the gyroscopic effect. Once set in motion, a top will usually wobble for a f ...
'' and '' bottom'' for the additional quarks. In 1977, the bottom quark was observed by a team at
Fermilab Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy United States Department of Energy National Labs, national laboratory specializing in high-energy parti ...
led by Leon Lederman. This was a strong indicator of the top quark's existence: without the top quark, the bottom quark would have been without a partner. It was not until 1995 that the top quark was finally observed, also by the CDF and teams at Fermilab. It had a mass much larger than expected, almost as large as that of a
gold Gold is a chemical element with the Symbol (chemistry), symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a Brightness, bright, slightly orange-yellow, dense, s ...
atom.


Etymology

For some time, Gell-Mann was undecided on an actual spelling for the term he intended to coin, until he found the word ''quark'' in
James Joyce James Augustine Aloysius Joyce (2 February 1882 – 13 January 1941) was an Irish novelist, poet, and literary critic. He contributed to the Modernism, modernist avant-garde movement and is regarded as one of the most influential and important ...
's 1939 book ''
Finnegans Wake ''Finnegans Wake'' is a novel by Irish literature, Irish writer James Joyce. It is well known for its experimental style and reputation as one of the most difficult works of fiction in the Western canon. It has been called "a work of fiction whi ...
'': The word ''quark'' is an outdated English word meaning ''to croak'' and the above-quoted lines are about a bird choir mocking king
Mark of Cornwall Mark of Cornwall ( la, Marcus, kw, Margh, cy, March, br, Marc'h) was a sixth-century King of History of Cornwall, Kernow (Cornwall), possibly identical with King Conomor. He is best known for his appearance in King Arthur, Arthurian legend as ...
in the legend of
Tristan and Iseult Tristan and Iseult, also known as Tristan and Isolde and other names, is a medieval chivalric romance told in numerous variations since the 12th century. Based on a Celts, Celtic legend and possibly other sources, the tale is a tragedy about th ...
. Especially in the German-speaking parts of the world there is a widespread legend, however, that Joyce had taken it from the word , a German word of Slavic origin which denotes a curd cheese, but is also a colloquial term for "trivial nonsense". In the legend it is said that he had heard it on a journey to Germany at a
farmers' market A farmers' market (or farmers market according to the AP stylebook, also farmer's market in the Cambridge Dictionary) is a physical retail marketplace intended to sell foods directly by farmers to consumers. Farmers' markets may be indoors or o ...
in
Freiburg Freiburg im Breisgau (; abbreviated as Freiburg i. Br. or Freiburg i. B.; Low Alemannic German, Low Alemannic: ''Friburg im Brisgau''), commonly referred to as Freiburg, is an independent city in Baden-Württemberg, Germany. With a population o ...
. Some authors, however, defend a possible German origin of Joyce's word ''quark''. Gell-Mann went into further detail regarding the name of the quark in his 1994 book ''The Quark and the Jaguar'': Zweig preferred the name ''ace'' for the particle he had theorized, but Gell-Mann's terminology came to prominence once the quark model had been commonly accepted. The quark flavors were given their names for several reasons. The up and down quarks are named after the up and down components of
isospin In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions o ...
, which they carry. Strange quarks were given their name because they were discovered to be components of the
strange particle A strange particle is an elementary particle with a strangeness quantum number different from zero. Strange particles are members of a large family of Elementary particle, elementary particles carrying the quantum number of strangeness, including se ...
s discovered in cosmic rays years before the quark model was proposed; these particles were deemed "strange" because they had unusually long lifetimes. Glashow, who co-proposed charm quark with Bjorken, is quoted as saying, "We called our construct the 'charmed quark', for we were fascinated and pleased by the symmetry it brought to the subnuclear world." The names "bottom" and "top", coined by Harari, were chosen because they are "logical partners for up and down quarks". Alternative names for bottom and top quarks are "beauty" and "truth" respectively, but these names have somewhat fallen out of use. While "truth" never did catch on, accelerator complexes devoted to massive production of bottom quarks are sometimes called " beauty factories".


Properties


Electric charge

Quarks have fractional electric charge values – either (−) or (+) times the
elementary charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...
(e), depending on flavor. Up, charm, and top quarks (collectively referred to as ''up-type quarks'') have a charge of + e; down, strange, and bottom quarks (''down-type quarks'') have a charge of − e. Antiquarks have the opposite charge to their corresponding quarks; up-type antiquarks have charges of − e and down-type antiquarks have charges of + e. Since the electric charge of a
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite particle, composite subatomic particle made of two or more quarks bound state, held together by the strong interaction. They are analogous to molecules tha ...
is the sum of the charges of the constituent quarks, all hadrons have integer charges: the combination of three quarks (baryons), three antiquarks (antibaryons), or a quark and an antiquark (mesons) always results in integer charges. For example, the hadron constituents of atomic nuclei, neutrons and protons, have charges of 0 e and +1 e respectively; the neutron is composed of two down quarks and one up quark, and the proton of two up quarks and one down quark.


Spin

Spin is an intrinsic property of elementary particles, and its direction is an important degree of freedom. It is sometimes visualized as the rotation of an object around its own axis (hence the name "
spin Spin or spinning most often refers to: * Spinning (textiles) Spinning is a twisting technique to form yarn from fiber Fiber or fibre (from la, fibra, links=no) is a #Natural fibers, natural or Fiber#Artificial fibers, artificial subst ...
"), though this notion is somewhat misguided at subatomic scales because elementary particles are believed to be
point-like A point particle (ideal particle or point-like particle, often spelled pointlike particle) is an idealization of particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, ...
. Spin can be represented by a
vector Vector most often refers to: *Euclidean vector In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities ...
whose length is measured in units of the
reduced Planck constant The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivalenc ...
''ħ'' (pronounced "h bar"). For quarks, a measurement of the spin vector component along any axis can only yield the values + or −; for this reason quarks are classified as spin- particles. The component of spin along a given axis – by convention the ''z'' axis – is often denoted by an up arrow ↑ for the value + and down arrow ↓ for the value −, placed after the symbol for flavor. For example, an up quark with a spin of + along the ''z'' axis is denoted by u↑.


Weak interaction

A quark of one flavor can transform into a quark of another flavor only through the weak interaction, one of the four
fundamental interaction In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist: the gravitational and elect ...
s in particle physics. By absorbing or emitting a
W boson In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles force carrier, mediate the weak interaction; the respective symbo ...
, any up-type quark (up, charm, and top quarks) can change into any down-type quark (down, strange, and bottom quarks) and vice versa. This flavor transformation mechanism causes the
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nucl ...
process of
beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar (nuclide), isobar of that ...
, in which a neutron () "splits" into a proton (), an
electron The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electric charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are generally thought t ...
() and an
electron antineutrino The electron neutrino () is an elementary particle which has zero electric charge and a spin (physics), spin of . Together with the electron, it forms the first generation (physics), generation of Lepton, leptons, hence the name electron neutrino ...
() (see picture). This occurs when one of the down quarks in the neutron () decays into an up quark by emitting a virtual boson, transforming the neutron into a proton (). The boson then decays into an electron and an electron antineutrino. Both beta decay and the inverse process of ''
inverse beta decay Inverse beta decay, commonly abbreviated to IBD, is a nuclear reaction In nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of ...
'' are routinely used in medical applications such as
positron emission tomography Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in Metabolism, metabolic processes, and in other physiological activities including bl ...
(PET) and in experiments involving
neutrino detection A neutrino detector is a physics apparatus which is designed to study neutrino A neutrino ( ; denoted by the Greek letter Nu (letter), ) is a fermion (an elementary particle with spin-1/2, spin of ) that interacts only via the weak interactio ...
. While the process of flavor transformation is the same for all quarks, each quark has a preference to transform into the quark of its own generation. The relative tendencies of all flavor transformations are described by a mathematical table, called the
Cabibbo–Kobayashi–Maskawa matrix In the Standard Model of particle physics, the Cabibbo–Kobayashi–Maskawa matrix, CKM matrix, quark mixing matrix, or KM matrix is a unitary matrix which contains information on the strength of the Flavour (particle physics), flavour-changing ...
(CKM matrix). Enforcing unitarity, the approximate magnitudes of the entries of the CKM matrix are: : \begin , V_\mathrm , & , V_\mathrm , & , V_\mathrm , \\ , V_\mathrm , & , V_\mathrm , & , V_\mathrm , \\ , V_\mathrm , & , V_\mathrm , & , V_\mathrm , \end \approx \begin 0.974 & 0.225 & 0.003 \\ 0.225 & 0.973 & 0.041 \\ 0.009 & 0.040 & 0.999 \end, where ''V''''ij'' represents the tendency of a quark of flavor ''i'' to change into a quark of flavor ''j'' (or vice versa).The actual probability of decay of one quark to another is a complicated function of (among other variables) the decaying quark's mass, the masses of the
decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps (de ...
s, and the corresponding element of the CKM matrix. This probability is directly proportional (but not equal) to the magnitude squared (, ''V''''ij'' , 2) of the corresponding CKM entry.
There exists an equivalent weak interaction matrix for leptons (right side of the W boson on the above beta decay diagram), called the Pontecorvo–Maki–Nakagawa–Sakata matrix (PMNS matrix). Together, the CKM and PMNS matrices describe all flavor transformations, but the links between the two are not yet clear.


Strong interaction and color charge

According to
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 o ...
(QCD), quarks possess a property called ''
color charge Color charge is a property of 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 comp ...
''. There are three types of color charge, arbitrarily labeled ''blue'', ''green'', and ''red''.Despite its name, color charge is not related to the color spectrum of visible light. Each of them is complemented by an anticolor – ''antiblue'', ''antigreen'', and ''antired''. Every quark carries a color, while every antiquark carries an anticolor. The system of attraction and repulsion between quarks charged with different combinations of the three colors is called
strong interaction The strong interaction or strong force is a fundamental interaction that confines Quark, quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is calle ...
, which is mediated by force carrying particles known as ''
gluon A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong interaction, strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged partic ...
s''; this is discussed at length below. The theory that describes strong interactions is called
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 o ...
(QCD). A quark, which will have a single color value, can form a bound system with an antiquark carrying the corresponding anticolor. The result of two attracting quarks will be color neutrality: a quark with color charge ''ξ'' plus an antiquark with color charge −''ξ'' will result in a color charge of 0 (or "white" color) and the formation of a
meson In particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The fundamental particles in the universe are ...
. This is analogous to the
additive color Additive color or additive mixing is a property of a color model that predicts the appearance of colors made by coincident component lights, i.e. the perceived color can be predicted by summing the numeric representations of the component colo ...
model in basic
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...
. Similarly, the combination of three quarks, each with different color charges, or three antiquarks, each with different anticolor charges, will result in the same "white" color charge and the formation of a
baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron list of particles, family of particles; hadrons are composed o ...
or
antibaryon In particle physics, a baryon is a type of composite particle, composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron list of particles, family of particles; hadrons are composed o ...
. In modern particle physics, gauge symmetries – a kind of
symmetry group In group theory, the symmetry group of a geometric object is the group (mathematics), group of all Transformation (geometry), transformations under which the object is invariant (mathematics), invariant, endowed with the group operation of Fu ...
– relate interactions between particles (see
gauge theories In physics, a gauge theory is a type of Field theory (physics), field theory in which the Lagrangian (field theory), Lagrangian (and hence the dynamics of the system itself) does not change (is Invariant (physics), invariant) under local symmetry, ...
). Color
SU(3) In mathematics, the special unitary group of degree , denoted , is the Lie group of Unitary matrix, unitary Matrix (mathematics), matrices with determinant 1. The more general Unitary group, unitary matrices may have complex determinants with ...
(commonly abbreviated to SU(3)c) is the gauge symmetry that relates the color charge in quarks and is the defining symmetry for quantum chromodynamics.Part III of Just as the laws of physics are independent of which directions in space are designated ''x'', ''y'', and ''z'', and remain unchanged if the coordinate axes are rotated to a new orientation, the physics of quantum chromodynamics is independent of which directions in three-dimensional color space are identified as blue, red, and green. SU(3)c color transformations correspond to "rotations" in color space (which, mathematically speaking, is a complex space). Every quark flavor ''f'', each with subtypes ''f''B, ''f''G, ''f''R corresponding to the quark colors, forms a triplet: a three-component quantum field that transforms under the fundamental representation of SU(3)c. The requirement that SU(3)c should be local – that is, that its transformations be allowed to vary with space and time – determines the properties of the strong interaction. In particular, it implies the existence of eight gluon types to act as its force carriers.


Mass

Two terms are used in referring to a quark's mass: '' current quark mass'' refers to the mass of a quark by itself, while ''
constituent quark A constituent quark is a current quark with a notional "covering" induced by the renormalization group In theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects an ...
mass'' refers to the current quark mass plus the mass of the
gluon A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong interaction, strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged partic ...
particle field surrounding the quark. These masses typically have very different values. Most of a hadron's mass comes from the gluons that bind the constituent quarks together, rather than from the quarks themselves. While gluons are inherently massless, they possess energy – more specifically, quantum chromodynamics binding energy (QCBE) – and it is this that contributes so greatly to the overall mass of the hadron (see
mass in special relativity The word "mass" has two meanings in special relativity: ''invariant mass'' (also called rest mass) is an invariant quantity which is the same for all Observer (special relativity), observers in all reference frames, while the relativistic mass is ...
). For example, a proton has a mass of approximately , of which the rest mass of its three valence quarks only contributes about ; much of the remainder can be attributed to the field energy of the gluons (see
chiral symmetry breaking In particle physics, chiral symmetry breaking is the spontaneous symmetry breaking of a chiral symmetry – usually by a gauge theory such as quantum chromodynamics, the quantum field theory of the strong interaction. Yoichiro Nambu was award ...
). The Standard Model posits that elementary particles derive their masses from the
Higgs mechanism In the Standard Model The Standard Model of particle physics Particle physics or high energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. T ...
, which is associated to the
Higgs boson The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the excited state, quantum excitation of the Higgs field, one of the field (physics), fields in particl ...
. It is hoped that further research into the reasons for the top quark's large mass of ~, almost the mass of a gold atom, might reveal more about the origin of the mass of quarks and other elementary particles.


Size

In QCD, quarks are considered to be point-like entities, with zero size. As of 2014, experimental evidence indicates they are no bigger than 10−4 times the size of a proton, i.e. less than 10−19 metres.


Table of properties

The following table summarizes the key properties of the six quarks. Flavor quantum numbers (
isospin In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions o ...
(''I''3), charm (''C''),
strangeness In particle physics, strangeness ("''S''") is a Physical property, property of Particle, particles, expressed as a quantum number, for describing Particle decay, decay of particles in Strong interaction, strong and Electromagnetic interaction, ele ...
(''S'', not to be confused with spin), topness (''T''), and
bottomness In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical science ...
(''B''′)) are assigned to certain quark flavors, and denote qualities of quark-based systems and hadrons. The
baryon number In particle physics, the baryon number is a Conservation law (physics), strictly conserved additive quantum number of a system. It is defined as ::B = \frac\left(n_\text - n_\bar\right), where ''n''q is the number of quarks, and ''n'' is the n ...
(''B'') is + for all quarks, as baryons are made of three quarks. For antiquarks, the electric charge (''Q'') and all flavor quantum numbers (''B'', ''I''3, ''C'', ''S'', ''T'', and ''B''′) are of opposite sign. Mass and
total angular momentum In quantum mechanics, the total angular momentum quantum number parametrises the total angular momentum of a given Subatomic particle, particle, by combining its angular momentum operator#Orbital angular momentum, orbital angular momentum and its ...
(''J''; equal to spin for point particles) do not change sign for the antiquarks.


Interacting quarks

As described by
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 o ...
, the
strong interaction The strong interaction or strong force is a fundamental interaction that confines Quark, quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is calle ...
between quarks is mediated by gluons, massless
vector Vector most often refers to: *Euclidean vector In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities ...
gauge boson In particle physics, a gauge boson is a bosonic elementary particle that acts as the force carrier for elementary fermions. Elementary particles, whose interactions are described by a gauge theory, interact with each other by the exchange of gauge ...
s. Each gluon carries one color charge and one anticolor charge. In the standard framework of particle interactions (part of a more general formulation known as
perturbation theory In mathematics and applied mathematics, perturbation theory comprises methods for finding an approximation theory, approximate solution to a problem, by starting from the exact solution (equation), solution of a related, simpler problem. A crit ...
), gluons are constantly exchanged between quarks through a virtual emission and absorption process. When a gluon is transferred between quarks, a color change occurs in both; for example, if a red quark emits a red–antigreen gluon, it becomes green, and if a green quark absorbs a red–antigreen gluon, it becomes red. Therefore, while each quark's color constantly changes, their strong interaction is preserved. Since gluons carry color charge, they themselves are able to emit and absorb other gluons. This causes ''
asymptotic freedom In quantum field theory, asymptotic freedom is a property of some gauge theory, gauge theories that causes interactions between particles to become asymptotically weaker as the energy scale increases and the corresponding length scale decreases. A ...
'': as quarks come closer to each other, the chromodynamic binding force between them weakens. Conversely, as the distance between quarks increases, the binding force strengthens. The color field becomes stressed, much as an elastic band is stressed when stretched, and more gluons of appropriate color are spontaneously created to strengthen the field. Above a certain energy threshold, pairs of quarks and antiquarks are created. These pairs bind with the quarks being separated, causing new hadrons to form. This phenomenon is known as ''
color confinement 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, neu ...
'': quarks never appear in isolation. This process of hadronization occurs before quarks, formed in a high energy collision, are able to interact in any other way. The only exception is the top quark, which may decay before it hadronizes.


Sea quarks

Hadrons contain, along with the ''
valence quark In particle 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. The quark model underlies Flavour (particle physics), ...
s'' () that contribute to their
quantum number In Quantum mechanics, quantum physics and chemistry, quantum numbers describe values of conserved quantity, conserved quantities in the dynamics of a quantum system. Quantum numbers correspond to eigenvalues of Operator (quantum mechanics), ope ...
s, virtual quark–antiquark () pairs known as ''sea quarks'' (). Sea quarks form when a gluon of the hadron's color field splits; this process also works in reverse in that the
annihilation In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy a ...
of two sea quarks produces a gluon. The result is a constant flux of gluon splits and creations colloquially known as "the sea". Sea quarks are much less stable than their valence counterparts, and they typically annihilate each other within the interior of the hadron. Despite this, sea quarks can hadronize into baryonic or mesonic particles under certain circumstances.


Other phases of quark matter

Under sufficiently extreme conditions, quarks may become "deconfined" out of bound states and propagate as thermalized "free" excitations in the larger medium. In the course of
asymptotic freedom In quantum field theory, asymptotic freedom is a property of some gauge theory, gauge theories that causes interactions between particles to become asymptotically weaker as the energy scale increases and the corresponding length scale decreases. A ...
, the strong interaction becomes weaker at increasing temperatures. Eventually, color confinement would be effectively lost in an extremely hot
plasma Plasma or plasm may refer to: Science * Plasma (physics) Plasma () 1, where \nu_ is the electron gyrofrequency and \nu_ is the electron collision rate. It is often the case that the electrons are magnetized while the ions are not. Magnetized ...
of freely moving quarks and gluons. This theoretical phase of matter is called
quark–gluon plasma Quark–gluon plasma (QGP) or quark soup is an interacting localized assembly of quarks and gluons at Thermodynamic equilibrium#Local and global equilibrium, thermal (local kinetic) and (close to) chemical (abundance) equilibrium. The word ''plasm ...
. The exact conditions needed to give rise to this state are unknown and have been the subject of a great deal of speculation and experimentation. An estimate puts the needed temperature at
kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its metric prefix, prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based eng ...
. While a state of entirely free quarks and gluons has never been achieved (despite numerous attempts by
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
in the 1980s and 1990s), recent experiments at the
Relativistic Heavy Ion Collider The Relativistic Heavy Ion Collider (RHIC ) is the first and one of only two operating heavy-ion An ion () is an atom Every atom is composed of a atomic nucleus, nucleus and one or more electrons bound to the nucleus. The nucleus is m ...
have yielded evidence for liquid-like quark matter exhibiting "nearly perfect" fluid motion. The quark–gluon plasma would be characterized by a great increase in the number of heavier quark pairs in relation to the number of up and down quark pairs. It is believed that in the period prior to 10−6 seconds after the
Big Bang The Big Bang event is a physical theory that describes how the Expansion of the universe, universe expanded from an initial state of high Energy density, density and temperature. Various Physical cosmology, cosmological models of the Big Ba ...
(the
quark epoch In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the unive ...
), the universe was filled with quark–gluon plasma, as the temperature was too high for hadrons to be stable. Given sufficiently high baryon densities and relatively low temperatures – possibly comparable to those found in
neutron star A neutron star is the Gravitational collapse, collapsed Stellar structure, core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially Metallicity, metal-rich. Except fo ...
s – quark matter is expected to degenerate into a
Fermi liquid Fermi liquid theory (also known as Landau's Fermi-liquid theory) is a theoretical model of interacting fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: s ...
of weakly interacting quarks. This liquid would be characterized by a
condensation Condensation is the change of the state of matter from the gas, gas phase into the liquid, liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of w ...
of colored quark
Cooper pair In condensed matter physics, a Cooper pair or BCS pair (Bardeen–Cooper–Schrieffer pair) is a pair of electrons (or other fermions) bound together at low temperatures in a certain manner first described in 1956 by American physicist Leon Cooper. ...
s, thereby breaking the local SU(3)c symmetry. Because quark Cooper pairs harbor color charge, such a phase of quark matter would be color superconductive; that is, color charge would be able to pass through it with no resistance.


See also

* Color–flavor locking * Koide formula *
Nucleon magnetic moment The nucleon magnetic moments are the intrinsic magnetic dipole moment In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have ...
* Preons *
Quarkonium In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavor (physics), flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle. Light quarks ...
*
Quark star A quark star is a hypothetical type of compact, exotic star, where extremely high core temperature and pressure has forced nuclear particles to form quark matter, a continuous state of matter In physics, a state of matter is one of the ...
*
Quark–lepton complementarity The quark–lepton complementarity (QLC) is a possible fundamental symmetry between quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the mo ...


Explanatory notes


References


Further reading

* * * * * * * * *


External links


1969 Physics Nobel Prize lecture by Murray Gell-Mann









The Top Quark And The Higgs Particle by T.A. Heppenheimer
nbsp;– A description of
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
's experiment to count the families of quarks.
Think Big website, Quarks and GluonsThink Big website, Quarks 2019
{{Authority control Elementary particles Finnegans Wake