A
quark is an
elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
and a fundamental constituent of
matterThe term matter traditionally refers to the substance that all objects are made of. One common way to identify this "substance" is through its physical properties; a common definition of matter is anything that has mass and occupies a volume...
. Quarks combine to form composite particles called
hadronIn particle physics, a hadron is a particle made of quarks held together by the strong force . Hadrons are either mesons or baryons...
s, the most stable of which are
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
s and
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
s, the components of
atomic nucleiThe nucleus is the very dense region consisting of nucleons at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons....
. Due to a phenomenon known as
color confinement, quarks are never found in isolation; they can only be found within hadrons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.
There are six types of quarks, known as
flavorIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics flavour is a global symmetry...
s:
upThe up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, form the neutrons and protons of atomic nuclei...
,
downThe down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
,
charmThe charm quark is a second-generation quark with an electric charge of + e. It is the third most massive of the quarks, at about . The existence of a fourth quark had been speculated by a number of authors around 1964...
,
strangeThe strange quark is a second-generation quark with a charge of − e and a strangeness of −1. It is the third-lightest quark after the up and down quarks, with a mass of somewhere between 80 and...
,
topThe top quark or t quark is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions...
, and
bottomThe bottom quark is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large mass , a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements...
. Up and down quarks have the lowest
massIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
es of all quarks. The heavier quarks rapidly change into up and down quarks through a process of
particle decayParticle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...
: the transformation from a higher mass state to a lower mass state. For this reason, up and down quarks are generally stable and the most common in the
universeThe Universe comprises everything that physically exists, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them...
, whereas charm, strange, top, and bottom quarks can only be produced in high energy collisions (such as those involving
cosmic rayCosmic rays are energetic particles originating from outer space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, almost 10% are helium nuclei , and slightly under 1% are heavier elements and electrons...
s and in
particle acceleratorA particle accelerator is a device that uses electric fields to propel ions or charged subatomic particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator...
s).
Quarks have various intrinsic properties, including
electric chargeElectric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields...
,
color chargeIn particle physics, color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics . Color charge has analogies with the notion of electric charge of particles, but because of the mathematical complications of QCD,...
,
spinIn particle physics and quantum mechanics, spin is a fundamental characteristic property of elementary particles including the force carriers , composite particles , and atomic nuclei....
, and
massIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
. Quarks are the only elementary particles in the
Standard ModelThe Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions. These particles make up all visible matter in the universe...
of
particle physicsParticle physics is a branch of physics that studies the elementary constituents of matter and radiation, and the interactions between them. It is also called high energy physics, because many elementary particles do not occur under normal circumstances in nature, but can be created and detected...
to experience all four
fundamental interactionIn physics, fundamental interactions are the ways that the simplest particles in the universe interact with one other...
s, also known as
fundamental forces (
electromagnetismElectromagnetism is the physics of the electromagnetic field, a field that exerts a force on particles with the property of electric charge and is reciprocally affected by the presence and motion of such particles....
,
gravitationGravitation is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass. Gravitation causes dispersed matter to coalesce, thus accounting for the existence of the Earth, the...
,
strong interactionIn particle physics, the strong interaction holds quarks and gluons together to form protons, neutrons and other particles. The strong interaction is one of the four fundamental interactions, along with gravitation, the electromagnetic force and the weak interaction...
, and
weak interactionThe weak interaction is one of the four fundamental interactions of nature. In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons...
), as well as the only known particles whose electric charges are not
integerThe integers are natural numbers including 0 and their negatives . They are numbers that can be written without a fractional or decimal component, and fall within the set {.....
multiples of the
elementary chargeThe elementary charge, usually denoted e, is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. This is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called the "elementary positive...
. For every quark flavor there is a corresponding type of
antiparticleCorresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
, known as
antiquark, that differs from the quark only in that some of its properties have
equal magnitude but opposite signIn mathematics, the additive inverse, or opposite, of a number a is the number that, when added to a, yields zero.The additive inverse of F is denoted −F....
.
The
quark modelIn 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. These quantum numbers are labels identifying the hadrons, and are of two kinds...
was independently proposed by physicists
Murray Gell-MannMurray Gell-Mann is an American physicist who received the 1969 Nobel Prize in physics for his work on the theory of elementary particles....
and
George ZweigGeorge Zweig was originally trained as a particle physicist under Richard Feynman and later turned his attention to neurobiology...
in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until 1968. All six flavors of quark have since been observed in accelerator experiments; the top quark, first observed at
FermilabFermi National Accelerator Laboratory , located in Batavia near Chicago, Illinois, is a U.S. Department of Energy national laboratory specializing in high-energy particle physics. As of January 1, 2007, Fermilab is operated by the Fermi Research Alliance, a joint venture of the University of...
in 1995, was the last to be discovered.
Classification
The
Standard ModelThe Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions. These particles make up all visible matter in the universe...
is the theoretical framework describing all the currently known
elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s, as well as the unobserved
[.] Higgs bosonThe Higgs boson is a massive scalar elementary particle predicted to exist by the Standard Model in particle physics. At present there are no known fundamental scalar particles in nature....
. This model contains six
flavorIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics flavour is a global symmetry...
s of quarks , named
upThe up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, form the neutrons and protons of atomic nuclei...
,
downThe down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
,
charmThe charm quark is a second-generation quark with an electric charge of + e. It is the third most massive of the quarks, at about . The existence of a fourth quark had been speculated by a number of authors around 1964...
,
strangeThe strange quark is a second-generation quark with a charge of − e and a strangeness of −1. It is the third-lightest quark after the up and down quarks, with a mass of somewhere between 80 and...
,
topThe top quark or t quark is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions...
, and
bottomThe bottom quark is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large mass , a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements...
.
AntiparticleCorresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
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
antimatterIn particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
in general, antiquarks have the same mass, mean lifetime, and spin as their respective quarks, but the electric charge and other
chargesIn physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
have the opposite sign.
Quarks are
spin-
{{about|the type of particle}}
A quark ({{pron-en|kwɔrk}}, {{respell|kwork}} or {{IPA-en|kwɑrk|}}, {{respell|kwark}}) is an elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
and a fundamental constituent of
matterThe term matter traditionally refers to the substance that all objects are made of. One common way to identify this "substance" is through its physical properties; a common definition of matter is anything that has mass and occupies a volume...
. Quarks combine to form composite particles called
hadronIn particle physics, a hadron is a particle made of quarks held together by the strong force . Hadrons are either mesons or baryons...
s, the most stable of which are
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
s and
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
s, the components of
atomic nucleiThe nucleus is the very dense region consisting of nucleons at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons....
. Due to a phenomenon known as
color confinement, quarks are never found in isolation; they can only be found within hadrons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.
There are six types of quarks, known as
flavorIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics flavour is a global symmetry...
s:
upThe up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, form the neutrons and protons of atomic nuclei...
,
downThe down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
,
charmThe charm quark is a second-generation quark with an electric charge of + e. It is the third most massive of the quarks, at about . The existence of a fourth quark had been speculated by a number of authors around 1964...
,
strangeThe strange quark is a second-generation quark with a charge of − e and a strangeness of −1. It is the third-lightest quark after the up and down quarks, with a mass of somewhere between 80 and...
,
topThe top quark or t quark is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions...
, and
bottomThe bottom quark is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large mass , a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements...
. Up and down quarks have the lowest
massIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
es of all quarks. The heavier quarks rapidly change into up and down quarks through a process of
particle decayParticle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...
: the transformation from a higher mass state to a lower mass state. For this reason, up and down quarks are generally stable and the most common in the
universeThe Universe comprises everything that physically exists, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them...
, whereas charm, strange, top, and bottom quarks can only be produced in high energy collisions (such as those involving
cosmic rayCosmic rays are energetic particles originating from outer space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, almost 10% are helium nuclei , and slightly under 1% are heavier elements and electrons...
s and in
particle acceleratorA particle accelerator is a device that uses electric fields to propel ions or charged subatomic particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator...
s).
Quarks have various intrinsic properties, including
electric chargeElectric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields...
,
color chargeIn particle physics, color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics . Color charge has analogies with the notion of electric charge of particles, but because of the mathematical complications of QCD,...
,
spinIn particle physics and quantum mechanics, spin is a fundamental characteristic property of elementary particles including the force carriers , composite particles , and atomic nuclei....
, and
massIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
. Quarks are the only elementary particles in the
Standard ModelThe Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions. These particles make up all visible matter in the universe...
of
particle physicsParticle physics is a branch of physics that studies the elementary constituents of matter and radiation, and the interactions between them. It is also called high energy physics, because many elementary particles do not occur under normal circumstances in nature, but can be created and detected...
to experience all four
fundamental interactionIn physics, fundamental interactions are the ways that the simplest particles in the universe interact with one other...
s, also known as
fundamental forces (
electromagnetismElectromagnetism is the physics of the electromagnetic field, a field that exerts a force on particles with the property of electric charge and is reciprocally affected by the presence and motion of such particles....
,
gravitationGravitation is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass. Gravitation causes dispersed matter to coalesce, thus accounting for the existence of the Earth, the...
,
strong interactionIn particle physics, the strong interaction holds quarks and gluons together to form protons, neutrons and other particles. The strong interaction is one of the four fundamental interactions, along with gravitation, the electromagnetic force and the weak interaction...
, and
weak interactionThe weak interaction is one of the four fundamental interactions of nature. In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons...
), as well as the only known particles whose electric charges are not
integerThe integers are natural numbers including 0 and their negatives . They are numbers that can be written without a fractional or decimal component, and fall within the set {.....
multiples of the
elementary chargeThe elementary charge, usually denoted e, is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. This is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called the "elementary positive...
. For every quark flavor there is a corresponding type of
antiparticleCorresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
, known as
antiquark, that differs from the quark only in that some of its properties have
equal magnitude but opposite signIn mathematics, the additive inverse, or opposite, of a number a is the number that, when added to a, yields zero.The additive inverse of F is denoted −F....
.
The
quark modelIn 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. These quantum numbers are labels identifying the hadrons, and are of two kinds...
was independently proposed by physicists
Murray Gell-MannMurray Gell-Mann is an American physicist who received the 1969 Nobel Prize in physics for his work on the theory of elementary particles....
and
George ZweigGeorge Zweig was originally trained as a particle physicist under Richard Feynman and later turned his attention to neurobiology...
in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until 1968. All six flavors of quark have since been observed in accelerator experiments; the top quark, first observed at
FermilabFermi National Accelerator Laboratory , located in Batavia near Chicago, Illinois, is a U.S. Department of Energy national laboratory specializing in high-energy particle physics. As of January 1, 2007, Fermilab is operated by the Fermi Research Alliance, a joint venture of the University of...
in 1995, was the last to be discovered.
Classification
{{See also|Standard Model}}
The
Standard ModelThe Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions. These particles make up all visible matter in the universe...
is the theoretical framework describing all the currently known
elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s, as well as the unobserved
[{{As of|2009|07}}.] Higgs bosonThe Higgs boson is a massive scalar elementary particle predicted to exist by the Standard Model in particle physics. At present there are no known fundamental scalar particles in nature....
. This model contains six
flavorIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics flavour is a global symmetry...
s of quarks ({{SubatomicParticle|quark}}), named
upThe up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, form the neutrons and protons of atomic nuclei...
({{SubatomicParticle|up quark}}),
downThe down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
({{SubatomicParticle|down quark}}),
charmThe charm quark is a second-generation quark with an electric charge of + e. It is the third most massive of the quarks, at about . The existence of a fourth quark had been speculated by a number of authors around 1964...
({{SubatomicParticle|charm quark}}),
strangeThe strange quark is a second-generation quark with a charge of − e and a strangeness of −1. It is the third-lightest quark after the up and down quarks, with a mass of somewhere between 80 and...
({{SubatomicParticle|strange quark}}),
topThe top quark or t quark is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions...
({{SubatomicParticle|top quark}}), and
bottomThe bottom quark is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large mass , a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements...
({{SubatomicParticle|bottom quark}}).
AntiparticleCorresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
s of quarks are called
antiquarks, and are denoted by a bar over the symbol for the corresponding quark, such as {{SubatomicParticle|Up antiquark}} for an up antiquark. As with
antimatterIn particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
in general, antiquarks have the same mass, mean lifetime, and spin as their respective quarks, but the electric charge and other
chargesIn physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
have the opposite sign.
Quarks are
spin-
{{about|the type of particle}}
A quark ({{pron-en|kwɔrk}}, {{respell|kwork}} or {{IPA-en|kwɑrk|}}, {{respell|kwark}}) is an elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
and a fundamental constituent of
matterThe term matter traditionally refers to the substance that all objects are made of. One common way to identify this "substance" is through its physical properties; a common definition of matter is anything that has mass and occupies a volume...
. Quarks combine to form composite particles called
hadronIn particle physics, a hadron is a particle made of quarks held together by the strong force . Hadrons are either mesons or baryons...
s, the most stable of which are
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
s and
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
s, the components of
atomic nucleiThe nucleus is the very dense region consisting of nucleons at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons....
. Due to a phenomenon known as
color confinement, quarks are never found in isolation; they can only be found within hadrons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.
There are six types of quarks, known as
flavorIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics flavour is a global symmetry...
s:
upThe up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, form the neutrons and protons of atomic nuclei...
,
downThe down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
,
charmThe charm quark is a second-generation quark with an electric charge of + e. It is the third most massive of the quarks, at about . The existence of a fourth quark had been speculated by a number of authors around 1964...
,
strangeThe strange quark is a second-generation quark with a charge of − e and a strangeness of −1. It is the third-lightest quark after the up and down quarks, with a mass of somewhere between 80 and...
,
topThe top quark or t quark is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions...
, and
bottomThe bottom quark is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large mass , a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements...
. Up and down quarks have the lowest
massIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
es of all quarks. The heavier quarks rapidly change into up and down quarks through a process of
particle decayParticle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...
: the transformation from a higher mass state to a lower mass state. For this reason, up and down quarks are generally stable and the most common in the
universeThe Universe comprises everything that physically exists, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them...
, whereas charm, strange, top, and bottom quarks can only be produced in high energy collisions (such as those involving
cosmic rayCosmic rays are energetic particles originating from outer space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, almost 10% are helium nuclei , and slightly under 1% are heavier elements and electrons...
s and in
particle acceleratorA particle accelerator is a device that uses electric fields to propel ions or charged subatomic particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator...
s).
Quarks have various intrinsic properties, including
electric chargeElectric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields...
,
color chargeIn particle physics, color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics . Color charge has analogies with the notion of electric charge of particles, but because of the mathematical complications of QCD,...
,
spinIn particle physics and quantum mechanics, spin is a fundamental characteristic property of elementary particles including the force carriers , composite particles , and atomic nuclei....
, and
massIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
. Quarks are the only elementary particles in the
Standard ModelThe Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions. These particles make up all visible matter in the universe...
of
particle physicsParticle physics is a branch of physics that studies the elementary constituents of matter and radiation, and the interactions between them. It is also called high energy physics, because many elementary particles do not occur under normal circumstances in nature, but can be created and detected...
to experience all four
fundamental interactionIn physics, fundamental interactions are the ways that the simplest particles in the universe interact with one other...
s, also known as
fundamental forces (
electromagnetismElectromagnetism is the physics of the electromagnetic field, a field that exerts a force on particles with the property of electric charge and is reciprocally affected by the presence and motion of such particles....
,
gravitationGravitation is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass. Gravitation causes dispersed matter to coalesce, thus accounting for the existence of the Earth, the...
,
strong interactionIn particle physics, the strong interaction holds quarks and gluons together to form protons, neutrons and other particles. The strong interaction is one of the four fundamental interactions, along with gravitation, the electromagnetic force and the weak interaction...
, and
weak interactionThe weak interaction is one of the four fundamental interactions of nature. In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons...
), as well as the only known particles whose electric charges are not
integerThe integers are natural numbers including 0 and their negatives . They are numbers that can be written without a fractional or decimal component, and fall within the set {.....
multiples of the
elementary chargeThe elementary charge, usually denoted e, is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. This is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called the "elementary positive...
. For every quark flavor there is a corresponding type of
antiparticleCorresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
, known as
antiquark, that differs from the quark only in that some of its properties have
equal magnitude but opposite signIn mathematics, the additive inverse, or opposite, of a number a is the number that, when added to a, yields zero.The additive inverse of F is denoted −F....
.
The
quark modelIn 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. These quantum numbers are labels identifying the hadrons, and are of two kinds...
was independently proposed by physicists
Murray Gell-MannMurray Gell-Mann is an American physicist who received the 1969 Nobel Prize in physics for his work on the theory of elementary particles....
and
George ZweigGeorge Zweig was originally trained as a particle physicist under Richard Feynman and later turned his attention to neurobiology...
in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until 1968. All six flavors of quark have since been observed in accelerator experiments; the top quark, first observed at
FermilabFermi National Accelerator Laboratory , located in Batavia near Chicago, Illinois, is a U.S. Department of Energy national laboratory specializing in high-energy particle physics. As of January 1, 2007, Fermilab is operated by the Fermi Research Alliance, a joint venture of the University of...
in 1995, was the last to be discovered.
Classification
{{See also|Standard Model}}
The
Standard ModelThe Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions. These particles make up all visible matter in the universe...
is the theoretical framework describing all the currently known
elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s, as well as the unobserved
[{{As of|2009|07}}.] Higgs bosonThe Higgs boson is a massive scalar elementary particle predicted to exist by the Standard Model in particle physics. At present there are no known fundamental scalar particles in nature....
. This model contains six
flavorIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics flavour is a global symmetry...
s of quarks ({{SubatomicParticle|quark}}), named
upThe up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, form the neutrons and protons of atomic nuclei...
({{SubatomicParticle|up quark}}),
downThe down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
({{SubatomicParticle|down quark}}),
charmThe charm quark is a second-generation quark with an electric charge of + e. It is the third most massive of the quarks, at about . The existence of a fourth quark had been speculated by a number of authors around 1964...
({{SubatomicParticle|charm quark}}),
strangeThe strange quark is a second-generation quark with a charge of − e and a strangeness of −1. It is the third-lightest quark after the up and down quarks, with a mass of somewhere between 80 and...
({{SubatomicParticle|strange quark}}),
topThe top quark or t quark is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions...
({{SubatomicParticle|top quark}}), and
bottomThe bottom quark is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large mass , a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements...
({{SubatomicParticle|bottom quark}}).
AntiparticleCorresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
s of quarks are called
antiquarks, and are denoted by a bar over the symbol for the corresponding quark, such as {{SubatomicParticle|Up antiquark}} for an up antiquark. As with
antimatterIn particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
in general, antiquarks have the same mass, mean lifetime, and spin as their respective quarks, but the electric charge and other
chargesIn physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
have the opposite sign.
Quarks are
spin-{{fracSpin-½ is the property of particles whose spin is ½. In quantum mechanics, spin is an intrinsic property of all elementary particles. Fermions, the particles that constitute ordinary matter, have half-integer spin. Spin-½ particles constitute an important subset of such fermions...
particles, implying that they are
fermionIn particle physics, fermions are particles which obey Fermi-Dirac statistics; they are named after Enrico Fermi. In contrast to bosons, which have Bose-Einstein statistics, only one fermion can occupy a quantum state at a given time; this is the Pauli Exclusion Principle. Thus, if more than one...
s according to the
spin-statistics theoremIn quantum mechanics, the spin-statistics theorem relates the spin of a particle to the particle statistics obeyed by it. The spin of a particle is its intrinsic angular momentum...
. They are subject to the
Pauli exclusion principleThe Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925. It states that no two identical fermions may occupy the same quantum state simultaneously. A more rigorous statement of this principle is that, for two identical fermions, the total wave function...
, which states that no two identical fermions can simultaneously occupy the same
quantum stateIn quantum physics, a quantum state is a mathematical object that fully describes a quantum system. One typically imagines some experimental apparatus and procedure which "prepares" this quantum state; the mathematical object then reflects the setup of the apparatus. Quantum states can be...
. This is in contrast to
bosonIn particle physics, bosons are particles which obey Bose–Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein. In contrast to fermions, which obey Fermi-Dirac statistics, several bosons can occupy the same quantum state. Thus, bosons with the same energy can occupy the...
s (particles with integer spin), of which any number can be in the same state.
Unlike
leptonLeptons are a family of elementary particles, alongside quarks and gauge bosons . Like quarks, leptons are fermions and are subject to the electromagnetic force, the gravitational force, and weak interaction, but unlike quarks, leptons do not participate in the strong interaction.There are six...
s, quarks possess
color chargeIn particle physics, color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics . Color charge has analogies with the notion of electric charge of particles, but because of the mathematical complications of QCD,...
, which causes them to engage in the
strong interactionIn particle physics, the strong interaction holds quarks and gluons together to form protons, neutrons and other particles. The strong interaction is one of the four fundamental interactions, along with gravitation, the electromagnetic force and the weak interaction...
. The resulting attraction between different quarks causes the formation of composite particles known as
hadronIn particle physics, a hadron is a particle made of quarks held together by the strong force . Hadrons are either mesons or baryons...
s (see "Strong interaction and color charge" below).
The quarks which determine the
quantum numberQuantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities. This is distinguished from classical mechanics where the values can range continuously...
s of hadrons are called
valence quarks; apart from these, any hadron may contain an indefinite number of
virtualIn physics, a virtual particle is a particle that exists for a limited time and space, introducing uncertainty in their energy and momentum due to the Heisenberg Uncertainty Principle...
(or
sea) quarks, antiquarks, and
gluonGluons are elementary expressions of quark interaction, and are indirectly involved with the binding of protons and neutrons together in atomic nuclei...
s which do not influence its quantum numbers. There are two families of hadrons:
baryonBaryons are the family of composite particles made of three quarks, as opposed to the mesons which are the family of composite particles made of one quark and one antiquark. Both baryons and mesons are part of the larger particle family comprising all particles made of quarks—the hadrons...
s, with three valence quarks, and
mesonIn particle physics, mesons are subatomic particles composed of one quark and one antiquark. They are part of the hadron particle family—particles made of quarks. The other members of the hadron family are the baryons—subatomic particles composed of three quarks...
s, with a valence quark and an antiquark. The most common baryons are the proton and the neutron, the building blocks of the
atomic nucleusThe nucleus is the very dense region consisting of nucleons at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons....
. A great number of hadrons are known (see
list of baryons and
list of mesons), most of them differentiated by their quark content and the properties these constituent quarks confer. The existence of
"exotic" hadronsExotic hadrons are subatomic particles made of quarks , but which do not fit into the usual scheme of hadrons. While bound by the strong interaction they are not predicted by the simple quark model...
with more valence quarks, such as
tetraquarkIn particle physics a tetraquark is a hypothetical meson composed of four valence quarks. In principle, a tetraquark state may be allowed in Quantum chromodynamics, the modern theory of strong interactions. However, there has been no confirmed report of a tetraquark state to date...
s ({{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|antiquark}}{{SubatomicParticle|antiquark}}) and
pentaquarkA pentaquark is a hypothetical subatomic particle consisting of a group of five quarks , or more specifically four quarks and one antiquark . It has therefore been assigned a new particle classification, called an exotic baryon...
s ({{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|antiquark}}), has been conjectured but not proven.
[Several research groups claimed to have proven the existence of tetraquarks and pentaquarks in the early 2000s. While the status of tetraquarks is still under debate, all known pentaquark candidates have since been established as non-existent.]
Elementary fermions are grouped into three
generationIn particle physics, a generation is a division of the elementary particles. Between generations, particles differ only by their mass. All interactions and quantum numbers are identical. There are three generations according to the Standard Model of particle physics.Each generation is divided into...
s, each comprising two leptons and two quarks. The first generation includes up and down quarks, the second charm and strange quarks, and the third top and bottom 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 of the {{SubatomicParticle]The W and Z⁰ bosons are the elementary particles that mediate the weak force. Their discovery has been heralded as a major success for the Standard Model of particle physics....
, which constrains the 4th generation neutrino to have a mass greater than ~{{val|45|u=GeV/c2}}. This would be highly contrasting with the other three generations' neutrinos, whose masses cannot exceed {{val|2|u=MeV/c2}}. Particles in higher generations generally have greater mass and lesser stability, causing them to
decayParticle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...
into lower-generation particles by means of
weak interactionThe weak interaction is one of the four fundamental interactions of nature. In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons...
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 rayCosmic rays are energetic particles originating from outer space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, almost 10% are helium nuclei , and slightly under 1% are heavier elements and electrons...
s), and decay quickly; however, they are thought to have been present during the first fractions of a second after the
Big BangThe Big Bang is the cosmological model of the initial conditions and subsequent development of the Universe that is supported by the most comprehensive and accurate explanations from current scientific evidence and observation...
, when the universe was in an extremely hot and dense phase (the
quark epochIn physical cosmology the quark epoch was the period in the evolution of the early universe when the fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction had taken their present forms, but the temperature of the universe was still too high to...
). Studies of heavier quarks are conducted in artificially created conditions, such as in particle accelerators.
Having electric charge, mass, color charge, and flavor, quarks are the only known elementary particles that engage in all four
fundamental interactionIn physics, fundamental interactions are the ways that the simplest particles in the universe interact with one other...
s of contemporary physics: electromagnetism, gravitation, strong interaction, and weak interaction. Gravitation, however, is usually irrelevant at subatomic scales, and 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 was independently proposed by physicists
Murray Gell-MannMurray Gell-Mann is an American physicist who received the 1969 Nobel Prize in physics for his work on the theory of elementary particles....
and
George ZweigGeorge Zweig was originally trained as a particle physicist under Richard Feynman and later turned his attention to neurobiology...
in 1964. The proposal came shortly after Gell-Mann's 1961 formulation of a particle classification system known as the
Eightfold WayIn physics, the Eightfold Way is a term coined by American physicist Murray Gell-Mann for a theory organizing subatomic baryons and mesons into octets...
—or, in more technical terms, SU(3) flavor symmetry. Physicist
Yuval Ne'emanYuval Ne'eman , was an Israeli soldier, physicist and politician, serving as a Minister during the 1980s and early 1990s. He was a self-declared atheist.-Background:...
had independently developed a scheme similar to the Eightfold Way in the same year.
At the time of the quark theory's inception, the "
particle zooIn particle physics, the term particle zoo is used colloquially to describe a relatively extensive list of the known elementary particles that almost look like hundreds of species in the zoo....
" included, amongst other particles, a multitude of
hadronIn particle physics, a hadron is a particle made of quarks held together by the strong force . Hadrons are either mesons or baryons...
s. 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—
upThe up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, form the neutrons and protons of atomic nuclei...
,
downThe down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
, and
strangeThe strange quark is a second-generation quark with a charge of − e and a strangeness of −1. It is the third-lightest quark after the up and down quarks, with a mass of somewhere between 80 and...
—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 an abstraction used to explain concepts that were not properly understood at the time.
In less than a year, extensions to the Gell-Mann–Zweig model were proposed.
Sheldon Lee GlashowSheldon Lee Glashow is a Nobel Prize winning American physicist. He is the Metcalf Professor of Mathematics and Physics at Boston University.-Birth and education:...
and
James BjorkenJames Daniel "Bj" Bjorken is one of the world's foremost theoretical physicists. He was a Putnam Fellow in 1954 and obtained his Ph.D. from Stanford University in 1959...
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 interactionThe weak interaction is one of the four fundamental interactions of nature. In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons...
(the mechanism that allows quarks to decay), equalized the number of known quarks with the number of known
leptonLeptons are a family of elementary particles, alongside quarks and gauge bosons . Like quarks, leptons are fermions and are subject to the electromagnetic force, the gravitational force, and weak interaction, but unlike quarks, leptons do not participate in the strong interaction.There are six...
s, and implied a
mass formulaA mass formula is an equation or set of equations in physics which attempts to predict the mass or mass ratios of the subatomic particles.An important step in high energy physics was the discovery of the Gell-Mann-Okubo mass formula predicting relationships between masses of the members of SU...
that correctly reproduced the masses of the known
mesonIn particle physics, mesons are subatomic particles composed of one quark and one antiquark. They are part of the hadron particle family—particles made of quarks. The other members of the hadron family are the baryons—subatomic particles composed of three quarks...
s.
In 1968,
deep inelastic scatteringDeep inelastic scattering is the name given to a process used to probe the insides of hadrons , using electrons, muons and neutrinos. It provided the first convincing evidence of the reality of quarks, which up until that point had been considered by many to be a purely mathematical phenomenon...
experiments at the
Stanford Linear Accelerator CenterThe SLAC National Accelerator Laboratory, originally named Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S...
(SLAC) showed that the proton contained much smaller,
point-like objectsA point particle is an idealized object heavily used in physics. Its defining feature is that it lacks spatial extension: being zero-dimensional, it does not take up space...
and was therefore not an elementary particle. Physicists were reluctant to identify these objects with quarks at the time, instead calling them "partons"—a term coined by
Richard FeynmanRichard Phillips Feynman was an American physicist known for the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as work in particle physics...
. The objects that were observed at the 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
gluonGluons are elementary expressions of quark interaction, and are indirectly involved with the binding of protons and neutrons together in atomic nuclei...
s).
The strange quark's existence was indirectly validated by the 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
kaonIn particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...
({{SubatomicParticle|Kaon}}) and
pionIn particle physics, a pion is any of three subatomic particles: , and . Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force.-Basic properties:...
({{SubatomicParticle|Pion}}) hadrons discovered in cosmic rays in 1947.
In a 1970 paper, Glashow,
John IliopoulosJohn Iliopoulos is a Greek physicist and the first person to present the Standard Model of particle physics in a single report. He is best known for his prediction of the charm quark with Sheldon Lee Glashow and Luciano Maiani ....
and
Luciano MaianiLuciano Maiani is a San Marino citizen physicist best known for his prediction of the charm quark with Sheldon Lee Glashow and John Iliopoulos .-Academic history:...
presented further reasoning for the existence of the as-yet undiscovered
charm quarkThe charm quark is a second-generation quark with an electric charge of + e. It is the third most massive of the quarks, at about . The existence of a fourth quark had been speculated by a number of authors around 1964...
. The number of supposed quark flavors grew to the current six in 1973, when
Makoto Kobayashiis a Japanese physicist known for his work on CP-violation who was awarded one quarter of the 2008 Nobel Prize in Physics "for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature."-Work:After completing his PhD at Nagoya...
and
Toshihide Maskawais a Japanese theoretical physicist known for his work on CP-violation who was awarded one quarter of the 2008 Nobel Prize in Physics "for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature."-Biography:A native of Aichi...
noted that the experimental observation of
CP violationIn particle physics, CP violation is a violation of the postulated CP symmetry, the combination of C symmetry and P symmetry. CP symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle , and left and right were swapped...
[CP violation is a phenomenon which causes weak interactions to behave differently when left and right are swapped (P symmetry) and particles are replaced with their corresponding antiparticles (C symmetry).] could be explained if there were another pair of quarks. The two new quarks were eventually named
topThe top quark or t quark is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions...
and
bottomThe bottom quark is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large mass , a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements...
.
Charm quarks were produced almost simultaneously by two teams in November 1974 (see November Revolution)—one at the SLAC under
Burton RichterBurton Richter is a Nobel Prize-winning American physicist.-Life:A native of New York City, Richter was born into a Jewish family in Brooklyn and was raised in the Queens neighborhood of Far Rockaway. He graduated from Far Rockaway High School, a school that also produced fellow laureates Baruch...
, and one at
Brookhaven National LaboratoryBrookhaven National Laboratory , is a United States national laboratory located in Upton, New York on Long Island, and was formally established in 1947 at the site of Camp Upton, a former U.S. Army base...
under
Samuel TingSamuel Chao Chung Ting is an American physicist who received the Nobel Prize in 1976, with Burton Richter, for discovering the subatomic J/ψ particle...
. The charm quarks were observed
boundIn physics, a bound state is a composite that behaves as a single object. In quantum mechanics , a bound state is a state in the Hilbert space that corresponds to two or more particles whose interaction energy is negative, and therefore these particles cannot be separated unless energy is...
with charm antiquarks in mesons. The two parties had assigned the discovered meson two different symbols, J and ψ; thus, it became formally known as the {{SubatomicParticle. The discovery finally convinced the physics community of the quark model's validity.
In 1977, the bottom quark was observed by a team at
FermilabFermi National Accelerator Laboratory , located in Batavia near Chicago, Illinois, is a U.S. Department of Energy national laboratory specializing in high-energy particle physics. As of January 1, 2007, Fermilab is operated by the Fermi Research Alliance, a joint venture of the University of...
led by
Leon LedermanLeon Max Lederman is an American experimental physicist and Nobel Prize in Physics laureate for his work with neutrinos. He is Director Emeritus of Fermi National Accelerator Laboratory in Batavia, Illinois...
. This was a strong indicator of the top quark's existence: without the top quark, the bottom quark would have been without a partner. However, it was not until 1995 that the top quark was finally observed, also by a team at Fermilab. It had a mass much greater than had been previously expected—almost as great as a
goldGold is a chemical element with the symbol Au and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is...
atom.
{{clr}}
Etymology
Gell-Mann originally named the quark after the sound made by ducks. For some time, he was undecided on an actual spelling for the term he intended to coin, until he found the word
quark in
James JoyceJames Augustine Aloysius Joyce was an Irish expatriate author, playwright and poet of the 20th century. He is known for his landmark novel Ulysses and its controversial successor Finnegans Wake , as well as the short story collection Dubliners and the semi-autobiographical novel A Portrait of...
's book
Finnegans WakeFinnegans Wake is a work of comic fiction by Irish author James Joyce, significant for an experimental style, and its resulting reputation as one of the most difficult works of fiction in the English language. Written in Paris over a period of 17 years, and published in 1939, two years before the...
:
{{epigraph|quote=
Three quarks for Muster Mark!
Sure he has not got much of a bark
And sure any he has it's all beside the mark.
|cite=James Joyce,
Finnegans Wake
}}
Gell-Mann went into further detail regarding the name of the quark in his book,
The Quark and the Jaguar:
{{quotebox|In 1963, when I assigned the name "quark" to the fundamental constituents of the nucleon, I had the sound first, without the spelling, which could have been "kwork". Then, in one of my occasional perusals of
Finnegans Wake, by James Joyce, I came across the word "quark" in the phrase "Three quarks for Muster Mark". Since "quark" (meaning, for one thing, the cry of the gull) was clearly intended to rhyme with "Mark", as well as "bark" and other such words, I had to find an excuse to pronounce it as "kwork". But the book represents the dream of a publican named Humphrey Chimpden Earwicker. Words in the text are typically drawn from several sources at once, like the "portmanteau" words in "Through the Looking-Glass". From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar. I argued, therefore, that perhaps one of the multiple sources of the cry "Three quarks for Muster Mark" might be "Three quarts for Mister Mark", in which case the pronunciation "kwork" would not be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.}}
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 a number of reasons. The up and down quarks are named after the up and down components of
isospinIn physics, and specifically, particle physics, isospin is a quantum number related to the strong interaction. This term was derived from isotopic spin, but the term is confusing as two isotopes of a nucleus have different numbers of nucleons; in contrast, rotations of isospin maintain the number...
, which they carry. Strange quarks were given their name because they were discovered to be components of the strange particles discovered in cosmic rays years before the quark model was proposed; these particles were deemed "strange" because they had unusually long lifetimes. Glashow, who coproposed 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 "top" and "bottom" were chosen because they are "logical partners for up and down quarks". In the past, top and bottom quarks were sometimes referred to as "truth" and "beauty" respectively, but these names have mostly fallen out of use.
Electric charge
{{See also|Electric charge}}
Quarks have
fractionA fraction is a number that can represent part of a whole.The earliest fractions were reciprocals of integers, symbols representing one half, one third, one quarter, and so on...
al electric charge values—either −{{frac|1|3}} or +{{frac|2|3}} times the
elementary chargeThe elementary charge, usually denoted e, is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. This is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called the "elementary positive...
, depending on flavor. Up, charm, and top quarks (collectively referred to as
up-type quarks) have a charge of +{{frac|2|3}}, while down, strange, and bottom quarks (
down-type quarks) have −{{frac|1|3}}. Antiquarks have the opposite charge to their corresponding quarks; up-type antiquarks have charges of −{{frac|2|3}} and down-type antiquarks have charges of +{{frac|1|3}}. Since the electric charge of a hadron 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 and +1 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
{{See also|Spin (physics)}}
Spin is an intrinsic property of elementary particles, and its direction is an important
degree of freedomDegrees of freedom is a general term used in explaining dependence on parameters, and implying the possibility of counting the number of those parameters...
. It is sometimes visualized as the rotation of an object around its own axis (hence the name "spin"), though this notion is somewhat misguided at subatomic scales because elementary particles are believed to be
point-likeA point particle is an idealized object heavily used in physics. Its defining feature is that it lacks spatial extension: being zero-dimensional, it does not take up space...
.
Spin can be represented by a vector whose length is measured in units of the reduced Planck constant
ħ (pronounced "h bar"). For quarks, a measurement of the spin vector component along any axis can only yield the values +
ħ/2 or −
ħ/2; for this reason quarks are classified as
spin-{{fracSpin-½ is the property of particles whose spin is ½. In quantum mechanics, spin is an intrinsic property of all elementary particles. Fermions, the particles that constitute ordinary matter, have half-integer spin. Spin-½ particles constitute an important subset of such fermions...
particles. The component of spin along a given axis—by convention the
z axis—is often denoted by an up arrow ↑ for the value +{{frac|1|2}} and down arrow ↓ for the value −{{frac|1|2}}, placed after the symbol for flavor. For example, an up quark with a spin of +{{frac|1|2}} along the
z axis is denoted by u↑.
Weak interaction
{{Main|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 interactionIn physics, fundamental interactions are the ways that the simplest particles in the universe interact with one other...
s in particle physics. By absorbing or emitting a W boson, 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
radioactiveRadioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter...
process of
beta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted. In the case of electron emission, it is referred to as beta minus , while in the case of a positron emission as beta plus...
, in which a neutron ({{SubatomicParticle|neutron}}) "splits" into a proton ({{SubatomicParticle|proton}}), an
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
({{SubatomicParticle|electron}}) and an electron antineutrino ({{SubatomicParticle|electron antineutrino}}) (see picture). This occurs when one of the down quarks in the neutron ({{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}}{{SubatomicParticle|down quark}}) decays into an up quark by emitting a
virtualIn physics, a virtual particle is a particle that exists for a limited time and space, introducing uncertainty in their energy and momentum due to the Heisenberg Uncertainty Principle...
{{SubatomicParticle|W boson-}} boson, transforming the neutron into a proton ({{SubatomicParticle|up quark}}{{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}}). The {{SubatomicParticle|W boson-}} boson then decays into an electron and an electron antineutrino.
| Neutron}} |
→ |
Proton}} + {{SubatomicParticle|electron}} + {{SubatomicParticle|electron antineutrino}} |
(Beta decay, hadron notation) |
| up quark}}{{SubatomicParticle|down quark}}{{SubatomicParticle|down quark}} |
→ |
up quark}}{{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}} + {{SubatomicParticle|electron}} + {{SubatomicParticle|electron antineutrino}} |
(Beta decay, quark notation) |
Both beta decay and the inverse process of
inverse beta decay are routinely used in medical applications such as
positron emission tomographyPositron emission tomography is a nuclear medicine imaging technique which produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
(PET) and in high-energy experiments such as
neutrino detectionA neutrino detector is a physics apparatus designed to study neutrinos. Because neutrinos are very weakly interacting, neutrino detectors must be very large in order to detect a significant number of neutrinos. Neutrino detectors are often built underground in order to isolate the detector from...
.
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 tableIn mathematics, a matrix is a rectangular array of numbers, such asEntries of a matrix are often denoted by a variable with two subscripts, as shown on the right. Matrices of the same size can be added and subtracted entrywise and matrices of compatible size can be multiplied...
, called the Cabibbo–Kobayashi–Maskawa matrix (or CKM matrix). The approximate
magnitudesIn mathematics, the absolute value of a real number is its numerical value without regard to its sign. So, for example, 3 is the absolute value of both 3 and −3.The absolute value of a number is denoted by ....
of the entries of the CKM matrix are:
\begin{bmatrix} |V_\mathrm {ud}| & |V_\mathrm {us}| & |V_\mathrm {ub}| \\ |V_\mathrm {cd}| & |V_\mathrm {cs}| & |V_\mathrm {cb}| \\ |V_\mathrm {td}| & |V_\mathrm {ts}| & |V_\mathrm {tb}| \end{bmatrix} \approx
\begin{bmatrix} 0.974 & 0.226 & 0.004 \\ 0.226 & 0.973 & 0.041 \\ 0.009 & 0.041 & 0.999 \end{bmatrix},
where
Vij 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 (amongst other variables) the decaying quark's mass, the masses of the decay product]In nuclear physics, a decay product is a nuclide produced by radioactive decay. Radioactive decay often involves a sequence of steps...
s, and the corresponding element of the CKM matrix. This probability is directly proportional (but not equal) to the magnitude squared 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.
{{clr}}
Strong interaction and color charge
{{See also|Color charge|Strong interaction}}
Quarks possess a property called
color chargeIn particle physics, color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics . Color charge has analogies with the notion of electric charge of particles, but because of the mathematical complications of QCD,...
. There are three types of color charge, arbitrarily labeled
blue,
green, and
red.
[Despite its name, color charge is not related with 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 interactionIn particle physics, the strong interaction holds quarks and gluons together to form protons, neutrons and other particles. The strong interaction is one of the four fundamental interactions, along with gravitation, the electromagnetic force and the weak interaction...
, which is mediated by
force carrying particlesIn particle physics, the quantum field theory called the Standard Model describes the strong, weak and electromagnetic fundamental forces.In such theories, each type of interaction has a characteristic set of force, or field carrier particles associated with quantum excitation of the force field...
known as
gluonGluons are elementary expressions of quark interaction, and are indirectly involved with the binding of protons and neutrons together in atomic nuclei...
s; this is discussed at length below. The theory that describes strong interactions is called
quantum chromodynamicsIn theoretical physics, Quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
(QCD). A quark charged with one color value can form a
bound systemIn physics, a bound state is a composite that behaves as a single object. In quantum mechanics , a bound state is a state in the Hilbert space that corresponds to two or more particles whose interaction energy is negative, and therefore these particles cannot be separated unless energy is...
with an antiquark carrying the corresponding anticolor; three (anti)quarks, one of each (anti)color, will similarly be bound together. 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. Analogous to the
additive colorAn additive color model involves light emitted directly from a source or illuminant of some sort. The additive reproduction process usually uses red, green and blue light to produce the other colors. Combining one of these additive primary colors with another in equal amounts produces the...
model in basic
opticsOptics is the branch of physics which studies the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
, the combination of three quarks or three antiquarks, each with different color charges, will result in the same "white" color charge and the formation of a baryon or antibaryon.
In modern particle physics,
gauge symmetriesIn gauge symmetry, 'gauge' means 'measure', and symmetry means 'stays the same'. Geometry is the study of the properties of objects that do not change when they move around. An object is symmetric if some motion leaves it looking the same, for instance, rotating an equilateral triangle through 120...
—a kind of
symmetry groupThe symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...
—relate interactions between particles (see gauge theories). Color SU(3) (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. 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
fB,
fG,
fR corresponding to the quark colors, forms a triplet: a three-component quantum field which 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 the existence of eight gluon types to act as its force carriers.
Mass
{{See also|Mass}}
Two terms are used in referring to a quark's mass:
current quarkCurrent quarks and are defined as the constituent quark cores of a valence quark....
mass refers to the mass of a quark by itself, while
constituent quarkA constituent quark is a current quark with a covering.In the low energy limit of QCD, a description by means of perturbation theory is not possible. Here, no Asymptotic freedom exists, but the interactions between valence quarks and sea quarks gain strongly on significance...
mass refers to the current quark mass plus the mass of the
gluonGluons are elementary expressions of quark interaction, and are indirectly involved with the binding of protons and neutrons together in atomic nuclei...
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 relativityThe term mass in special relativity usually refers to the rest mass of the object, which is the Newtonian mass as measured by an observer moving along with the object. The invariant mass is another name for the rest mass of single particles...
). For example, a proton has a mass of approximately 938 MeV/c
2, of which the rest mass of its three valence quarks only contributes about 11 MeV/c
2; much of the remainder can be attributed to the gluons' QCBE.
The Standard Model posits that elementary particles derive their masses from the
Higgs mechanismIn the standard model of particle physics, the Higgs mechanism is a theoretical framework which explains how the masses of the W and Z bosons arise as a result of electroweak symmetry breaking....
, which is related to the unobserved
Higgs bosonThe Higgs boson is a massive scalar elementary particle predicted to exist by the Standard Model in particle physics. At present there are no known fundamental scalar particles in nature....
. Physicists hope that further research into the reasons for the top quark's large mass, which was found to be approximately equal to that of a gold nucleus (~171 GeV/c
2), might reveal more about the origin of the mass of quarks and other elementary particles.
Table of properties
{{See also|Flavor (physics)}}
The following table summarizes the key properties of the six quarks. Flavor quantum numbers (
isospinIn physics, and specifically, particle physics, isospin is a quantum number related to the strong interaction. This term was derived from isotopic spin, but the term is confusing as two isotopes of a nucleus have different numbers of nucleons; in contrast, rotations of isospin maintain the number...
(
Iz),
charmCharm is a flavour quantum number representing the difference between the number of charm quarks and charm antiquarks that are present in a particle:...
(
C), strangeness (
S, not to be confused with spin),
topnessTopness , a flavour quantum number, is the number of top quarks minus the number of top anti-quarks that are present in a particle:Top quarks have a topness of +1 and anti-top quarks have a topness of −1....
(
T), and
bottomnessIn physics, bottomness also formerly called beauty, is a flavour quantum number reflecting the difference between the number of bottom antiquarks and the number of bottom quarks that are present in a particle:Bottom quarks have a bottomness of −1 while bottom antiquarks have a bottomness...
(
B′)) are assigned to certain quark flavors, and denote qualities of quark-based systems and hadrons. The
baryon numberIn particle physics, the baryon number is an approximate conserved quantum number of a system. It is defined as:whereWhy one third? According to the laws of strong interaction there cannot be any bare color charge, i.e. the total color charge of a particle has to be zero ,...
(
B) is +{{frac|1|3}} for all quarks, as baryons are made of three quarks. For antiquarks, the electric charge (
Q) and all flavor quantum numbers (
B,
Iz,
C,
S,
T, and
B′) are of opposite sign. Mass and total angular momentum (
J; equal to spin for point particles) do not change sign for the antiquarks.
Quark flavor properties
| Name |
Symbol |
Mass (MeV/c2)* |
J |
B |
Q |
Iz |
C |
S |
T |
B′ |
Antiparticle |
Antiparticle symbol |
| First generation |
| Up |
Up quark}} |
1.5 to 3.3 |
1|2}} |
1|3}} |
2|3}} |
1|2}} |
0 |
0 |
0 |
0 |
Antiup |
Up antiquark}} |
| Down |
Down quark}} |
3.5 to 6.0 |
1|2}} |
1|3}} |
1|3}} |
1|2}} |
0 |
0 |
0 |
0 |
Antidown |
Down antiquark}} |
| Second generation |
| Charm |
Charm quark}} |
1270|+70|-110}} |
1|2}} |
1|3}} |
2|3}} |
0 |
+1 |
0 |
0 |
0 |
Anticharm |
Charm antiquark}} |
| Strange |
Strange quark}} |
104|+26|-34}} |
1|2}} |
1|3}} |
1|3}} |
0 |
0 |
−1 |
0 |
0 |
Antistrange |
Strange antiquark}} |
| Third generation |
| Top |
Top quark}} |
171200|2100}} |
1|2}} |
1|3}} |
2|3}} |
0 |
0 |
0 |
+1 |
0 |
Antitop |
Top antiquark}} |
| Bottom |
Bottom quark}} |
4200|+170|-70}} |
1|2}} |
1|3}} |
1|3}} |
0 |
0 |
0 |
0 |
−1 |
Antibottom |
Bottom antiquark}} |
J = total angular momentum, B = baryon numberIn particle physics, the baryon number is an approximate conserved quantum number of a system. It is defined as:whereWhy one third? According to the laws of strong interaction there cannot be any bare color charge, i.e. the total color charge of a particle has to be zero ,...
, Q = electric chargeElectric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields...
, Iz = isospinIn physics, and specifically, particle physics, isospin is a quantum number related to the strong interaction. This term was derived from isotopic spin, but the term is confusing as two isotopes of a nucleus have different numbers of nucleons; in contrast, rotations of isospin maintain the number...
, C = charmCharm is a flavour quantum number representing the difference between the number of charm quarks and charm antiquarks that are present in a particle:...
, S = strangeness, T = topnessTopness , a flavour quantum number, is the number of top quarks minus the number of top anti-quarks that are present in a particle:Top quarks have a topness of +1 and anti-top quarks have a topness of −1....
, B′ = bottomnessIn physics, bottomness also formerly called beauty, is a flavour quantum number reflecting the difference between the number of bottom antiquarks and the number of bottom quarks that are present in a particle:Bottom quarks have a bottomness of −1 while bottom antiquarks have a bottomness...
.
* Notation such as {{val|4200|+170|-70}} denotes measurement uncertaintyIn metrology, measurement uncertainty describes a region about an observed value of a physical quantity which is likely to enclose the true value of that quantity...
.
Interacting quarks
{{See also|Color confinement|Gluon}}
As described by
quantum chromodynamicsIn theoretical physics, Quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
, the
strong interactionIn particle physics, the strong interaction holds quarks and gluons together to form protons, neutrons and other particles. The strong interaction is one of the four fundamental interactions, along with gravitation, the electromagnetic force and the weak interaction...
between quarks is mediated by gluons, massless
vectorIn particle physics, a vector boson is a boson with the spin quantum number equal to 1.The vector bosons considered to be elementary particles are the gauge bosons or, the force carriers of fundamental interactions: the photon of electromagnetism, the W and Z bosons of the weak interaction, and the...
gauge bosons. 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 theoryIn quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler one. The idea is to start with a simple system for which a mathematical solution is known, and add an...
), gluons are constantly exchanged between quarks through a
virtualIn physics, a virtual particle is a particle that exists for a limited time and space, introducing uncertainty in their energy and momentum due to the Heisenberg Uncertainty Principle...
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 freedomIn physics, asymptotic freedom is a property of some gauge theories that causes interactions between particles, such as quarks, to become arbitrarily weak at shorter distances, i.e...
: 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: quarks never appear in isolation. This process of
hadronizationIn particle physics, hadronization is the process of the formation of hadrons out of quarks and gluons. This occurs after high-energy collisions in a particle collider in which free quarks or gluons are created. Due to colour confinement, these cannot exist individually. In the Standard Model they...
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, along with the
valence quarks ({{SubatomicParticle|valence quark}}) that contribute to their
quantum numberQuantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities. This is distinguished from classical mechanics where the values can range continuously...
s, contain
virtualIn physics, a virtual particle is a particle that exists for a limited time and space, introducing uncertainty in their energy and momentum due to the Heisenberg Uncertainty Principle...
quark–antiquark ({{SubatomicParticle|quark}}{{SubatomicParticle|antiquark}}) pairs known as
sea quarks ({{SubatomicParticle|sea quark}}). Sea quarks form when a gluon of the hadron's color field splits; this process also works in reverse in that the
annihilationAnnihilation is defined as "total destruction" or "complete obliteration" of an object; having its root in the Latin nihil . A literal translation is "to make into nothing"....
of two sea quarks produces a gluon. The result is a constant flux of gluon splits and creation 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.
Virtual quark–antiquark pairs have a tendency to form what can be described as a kind of "cloud" or "shield" around the valence quarks in hadrons. This cloud is complemented by another layer of virtual gluons lying beyond it. These layers adopt color charges based on the valence quark they surround. Quantum chromodynamics causes the virtual antiquarks in the cloud—which exhibit the anticolor of the valence quark—to be closer to the inside of the field. In the case of a red quark, for instance, the antired virtual antiquarks would tend to be closer to the red valence quark than their red virtual quark partners. This disparity in proximity and the anticolor barrier it creates has the effect of "de-amplifying" the color charge of the valence quark. However, this anticolor tilt is neutralized by the virtual gluon field beyond, which carries the original color charge and "re-amplifies" the valence quark's color. The canceling of these two influences on the valence quark results in the balanced color charge that valence quarks have been observed to possess.
Other phases of quark matter
{{Main|QCD matter}}
Under sufficiently extreme conditions, quarks may become deconfined and exist as free particles. In the course of
asymptotic freedomIn physics, asymptotic freedom is a property of some gauge theories that causes interactions between particles, such as quarks, to become arbitrarily weak at shorter distances, i.e...
, the strong interaction becomes weaker at higher temperatures. Eventually, color confinement would be lost and an extremely hot
plasmaIn physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it...
of freely moving quarks and gluons would be formed. This theoretical phase of matter is called quark–gluon plasma. 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. A recent estimate puts the needed temperature at {{val|1.90|0.02|e=12}}
kelvinThe kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
s. While a state of entirely free quarks and gluons has never been achieved (despite numerous attempts by
CERNThe European Organization for Nuclear Research , known as CERN , , is the world's largest particle physics laboratory, situated in the northwest suburbs of Geneva on the Franco–Swiss border, established in 1954...
in the 1980s and 1990s), recent experiments at the
Relativistic Heavy Ion ColliderThe Relativistic Heavy Ion Collider is a heavy-ion collider located at and operated by Brookhaven National Laboratory in Upton, New York...
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 BangThe Big Bang is the cosmological model of the initial conditions and subsequent development of the Universe that is supported by the most comprehensive and accurate explanations from current scientific evidence and observation...
(the
quark epochIn physical cosmology the quark epoch was the period in the evolution of the early universe when the fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction had taken their present forms, but the temperature of the universe was still too high to...
), 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 starA neutron star is a type of remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and roughly the same mass as protons...
s—quark matter is expected to degenerate into a
Fermi liquidFermi liquid is a generic term for a quantum mechanical liquid of fermions that arises under certain physical conditions when the temperature is sufficiently low. The interaction between the particles of the many-body system does not need to be small...
of weakly interacting quarks. This liquid would be characterized by a
condensationCondensation is the change of the physical state of aggregation of matter from gaseous phase into liquid phase and the reverse of evaporation. When the transition happens from the gaseous phase into the solid phase directly, bypassing the liquid phase, the change is called deposition...
of colored quark
Cooper pairIn condensed matter physics, a Cooper pair is the name given to electrons that are 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 symmetryIn physics, spontaneous symmetry breaking occurs when a system that is symmetric with respect to some symmetry group goes into a vacuum state that is not symmetric. When that happens, the system no longer appears to behave in a symmetric manner...
. Because quark Cooper pairs harbor color charge, such a phase of quark matter would be
color superconductiveColor superconductivity is a phenomenon predicted to occur in quark matter if the baryon density is sufficiently high and the temperature is not too high...
; that is, color charge would be able to pass through it with no resistance.
See also
{{Portal|Physics|Stylised_Lithium_Atom.svg}}
- Color–flavor locking
- Quarkonium
In particle physics, quarkonium designates a flavorless meson whose constituents are a quark and its own antiquark. Examples of quarkonia are the J/ψ and the...
– Mesons made of a quark and antiquark of the same flavor
- Preon
In particle physics, preons are postulated "point-like" particles, conceived to be subcomponents of quarks and leptons. The word was coined by Jogesh Pati and Abdus Salam in 1974...
s – Hypothetical particles which were once postulated to be subcomponents of quarks and leptons
- Quark–lepton complementarity – Possible fundamental relation between quarks and leptons
- Quark star
A quark star or strange star is a hypothetical type of exotic star composed of quark matter, or strange matter. These are ultra-dense phases of degenerate matter theorized to form inside particularly massive neutron stars....
– A hypothetical degenerate neutron starA neutron star is a type of remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and roughly the same mass as protons...
with extreme density
External links
{{Commons|Quark}}
{{Particles}}
{{Composition}}
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