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| Meta Title | Quark | Definition, Flavors, & Colors | Britannica |
| Meta Description | Quark, any member of a group of elementary subatomic particles that are believed to be among the fundamental constituents of matter. |
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| Boilerpipe Text | Top Questions
What is a quark?
Where are quarks found inside an atom?
How many types of quarks are there?
How do quarks combine to make protons and neutrons?
What is the strong force and what does it do with quarks?
Why can't we see quarks alone in nature?
quark
,
any member of a group of elementary
subatomic particles
that interact by means of the
strong force
and are believed to be among the fundamental
constituents
of
matter
. Quarks associate with one another via the strong
force
to make up
protons
and
neutrons
, in much the same way that the latter particles combine in various proportions to make up atomic nuclei. There are six types, or
flavours
, of quarks that differ from one another in their
mass
and charge characteristics. These six quark flavours can be grouped in three pairs: up and down, charm and strange, and top and bottom. Quarks appear to be true elementary particles; that is, they have no apparent structure and cannot be resolved into something smaller. In addition, however, quarks always seem to occur in combination with other quarks or with
antiquarks
, their
antiparticles
, to form all
hadrons
—the so-called strongly interacting particles that
encompass
both
baryons
and
mesons
.
Quarks*
quark type
baryon number
charge
strangeness**
charm**
bottom**
top**
mass (MeV)
*Note that antiquarks exist for all flavours of quark and have opposite values for all the quantum numbers listed here.
**These are quantum numbers that must be assigned to the quarks to differentiate the various flavours.
down (
d
)
1/3
−(1/3)
e
0
0
0
0
5–15
up (
u
)
1/3
+(2/3)
e
0
0
0
0
2–8
strange (
s
)
1/3
−(1/3)
e
−1
0
0
0
100–300
charm (
c
)
1/3
+(2/3)
e
0
1
0
0
1,000–1,600
bottom (
b
)
1/3
−(1/3)
e
0
0
−1
0
4,100–4,500
top (
t
)
1/3
+(2/3)
e
0
0
0
1
180,000
Quark “flavours”
Throughout the 1960s theoretical physicists, trying to account for the ever-growing number of subatomic particles observed in experiments, considered the possibility that protons and neutrons were composed of smaller units of matter. In 1961 two physicists,
Murray Gell-Mann
of the
United States
and
Yuval Neʾeman of Israel, proposed a particle classification scheme called the
Eightfold Way
, based on the mathematical
symmetry
group SU(3), which described strongly interacting particles in terms of building blocks. In 1964 Gell-Mann introduced the concept of quarks as a physical basis for the scheme, having adopted the fanciful term from a passage in
James Joyce
’s novel
Finnegans Wake
. (The American physicist
George Zweig
developed a similar theory independently that same year and called his fundamental particles “aces.”) Gell-Mann’s model provided a simple picture in which all mesons are shown as consisting of a quark and an antiquark and all baryons as composed of three quarks. It
postulated
the existence of three types of quarks, distinguished by unique “flavours.” These three quark types are now commonly designated as “up” (
u
), “down” (
d
), and “strange” (
s
). Each carries a fractional value of the
electron charge
(i.e., a charge less than that of the
electron
,
e
). The
up quark
(charge
2
/
3
e
) and
down quark
(charge −
1
/
3
e
) make up protons and neutrons and are thus the ones observed in ordinary matter.
Strange quarks
(charge −
1
/
3
e
) occur as components of
K
mesons
and various other extremely short-lived subatomic particles that were first observed in
cosmic rays
but that play no part in ordinary matter.
Quark “
colours
”
The interpretation of quarks as actual physical
entities
initially posed two major problems. First, quarks had to have half-integer
spin
(intrinsic angular momentum) values for the model to
work
, but at the same time they seemed to violate the
Pauli exclusion principle
, which governs the behaviour of all particles (called
fermions
) having odd half-integer spin. In many of the
baryon
configurations constructed of quarks, sometimes two or even three identical quarks had to be set in the same
quantum
state—an arrangement prohibited by the exclusion principle. Second, quarks appeared to defy being freed from the particles they made up. Although the forces binding quarks were strong, it seemed improbable that they were powerful enough to withstand bombardment by high-energy particle
beams
from accelerators.
Britannica Quiz
How Much Do You Know About Physics?
These problems were resolved by the introduction of the concept of
colour
, as formulated in
quantum chromodynamics
(QCD). In this theory of strong interactions, whose breakthrough ideas were published in 1973, colour has nothing to do with the colours of the everyday world but rather represents a property of quarks that is the source of the strong force. The colours
red,
green, and
blue are ascribed to quarks, and their opposites,
antired,
antigreen, and
antiblue, are ascribed to antiquarks. According to QCD, all combinations of quarks must contain mixtures of these imaginary colours that cancel out one another, with the resulting particle having no net colour. A baryon, for example, always consists of a combination of one red, one green, and one blue quark and so never violates the exclusion principle. The property of colour in the strong force plays a role
analogous
to that of
electric charge
in the
electromagnetic force
, and just as charge implies the exchange of
photons
between charged particles, so does colour involve the exchange of massless particles called
gluons
among quarks. Just as photons carry electromagnetic force, gluons transmit the forces that bind quarks together. Quarks change their colour as they emit and absorb gluons, and the exchange of gluons maintains proper quark colour distribution.
Binding forces and “massive” quarks
The binding forces carried by the gluons tend to be weak when quarks are close together. Within a
proton
(or other hadron), at distances of less than 10
−15
metre, quarks behave as though they were nearly free. This condition is called
asymptotic freedom
. When one begins to draw the quarks apart, however, as when attempting to knock them out of a proton, the effect of the force grows stronger. This is because, as explained by QCD, gluons have the ability to create other gluons as they move between quarks. Thus, if a quark starts to speed away from its companions after being struck by an
accelerated
particle, the gluons utilize
energy
that they draw from the quark’s
motion
to produce more gluons. The larger the number of gluons exchanged among quarks, the stronger the effective binding forces become. Supplying additional energy to extract the quark only results in the conversion of that energy into new quarks and antiquarks with which the first quark combines. This phenomenon is observed at high-energy particle accelerators in the production of “jets” of new particles that can be associated with a single quark.
The discovery in the 1970s of the “
charm
” (
c
) and “
bottom
” (
b
) quarks and their associated antiquarks, achieved through the creation of mesons, strongly suggests that quarks occur in pairs. This
speculation
led to efforts to find a sixth type of quark called “
top
” (
t
), after its proposed
flavour
. According to theory, the top quark carries a charge of
2
/
3
e
; its partner, the bottom quark, has a charge of −
1
/
3
e
. In 1995 two independent groups of scientists at the
Fermi National Accelerator Laboratory
reported that they had found the top quark. Their results give the top quark a mass of 173.8 ± 5.2
gigaelectron volts
(GeV; 10
9
eV). (The next heaviest quark, the bottom, has a mass of about 4.2 GeV.) It has yet to be explained why the top quark is so much more massive than the other elementary particles, but its existence completes the
Standard Model
, the
prevailing
theoretical scheme of nature’s fundamental building blocks.
This article was most recently revised and updated by
Adam Augustyn
. |
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[quark](https://www.britannica.com/science/quark)
- [Introduction](https://www.britannica.com/science/quark)
- [Quark “flavours”](https://www.britannica.com/science/quark#ref284649)
- [Quark “colours”](https://www.britannica.com/science/quark#ref284650)
- [Binding forces and “massive” quarks](https://www.britannica.com/science/quark#ref284651)
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# quark
subatomic particle
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Written and fact-checked by
[Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....](https://www.britannica.com/editor/The-Editors-of-Encyclopaedia-Britannica/4419)
Britannica Editors
Last updated
Mar. 24, 2026
•[History](https://www.britannica.com/science/quark/additional-info#history)
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Quick Summary
Table of Contents
Table of Contents
Quick Summary
Ask Anything
Top Questions
- What is a quark?
- Where are quarks found inside an atom?
- How many types of quarks are there?
- How do quarks combine to make protons and neutrons?
- What is the strong force and what does it do with quarks?
- Why can't we see quarks alone in nature?
Show more
Show less
**quark**, any member of a group of elementary [subatomic particles](https://www.britannica.com/science/subatomic-particle) that interact by means of the [strong force](https://www.britannica.com/science/strong-force) and are believed to be among the fundamental [constituents](https://www.merriam-webster.com/dictionary/constituents) of [matter](https://www.britannica.com/science/matter). Quarks associate with one another via the strong [force](https://www.britannica.com/science/force-physics) to make up [protons](https://www.britannica.com/science/proton-subatomic-particle) and [neutrons](https://www.britannica.com/science/neutron), in much the same way that the latter particles combine in various proportions to make up atomic nuclei. There are six types, or [flavours](https://www.britannica.com/science/flavour-particle-physics), of quarks that differ from one another in their [mass](https://www.britannica.com/science/mass-physics) and charge characteristics. These six quark flavours can be grouped in three pairs: up and down, charm and strange, and top and bottom. Quarks appear to be true elementary particles; that is, they have no apparent structure and cannot be resolved into something smaller. In addition, however, quarks always seem to occur in combination with other quarks or with [antiquarks](https://www.britannica.com/science/antiquark), their [antiparticles](https://www.britannica.com/science/antiparticle), to form all [hadrons](https://www.britannica.com/science/hadron)—the so-called strongly interacting particles that [encompass](https://www.merriam-webster.com/dictionary/encompass) both [baryons](https://www.britannica.com/science/baryon) and [mesons](https://www.britannica.com/science/meson).
Quarks\*
| quark type | baryon number | charge | strangeness\*\* | charm\*\* | bottom\*\* | top\*\* | mass (MeV) |
|---|---|---|---|---|---|---|---|
| \*Note that antiquarks exist for all flavours of quark and have opposite values for all the quantum numbers listed here. | | | | | | | |
| \*\*These are quantum numbers that must be assigned to the quarks to differentiate the various flavours. | | | | | | | |
| down (*d*) | 1/3 | −(1/3)*e* | 0 | 0 | 0 | 0 | 5–15 |
| up (*u*) | 1/3 | \+(2/3)*e* | 0 | 0 | 0 | 0 | 2–8 |
| strange (*s*) | 1/3 | −(1/3)*e* | −1 | 0 | 0 | 0 | 100–300 |
| charm (*c*) | 1/3 | \+(2/3)*e* | 0 | 1 | 0 | 0 | 1,000–1,600 |
| bottom (*b*) | 1/3 | −(1/3)*e* | 0 | 0 | −1 | 0 | 4,100–4,500 |
| top (*t*) | 1/3 | \+(2/3)*e* | 0 | 0 | 0 | 1 | 180,000 |
## Quark “flavours”
Throughout the 1960s theoretical physicists, trying to account for the ever-growing number of subatomic particles observed in experiments, considered the possibility that protons and neutrons were composed of smaller units of matter. In 1961 two physicists, [Murray Gell-Mann](https://www.britannica.com/biography/Murray-Gell-Mann) of the [United States](https://www.britannica.com/place/United-States) and Yuval Neʾeman of Israel, proposed a particle classification scheme called the [Eightfold Way](https://www.britannica.com/topic/Eightfold-Way), based on the mathematical [symmetry](https://www.britannica.com/science/symmetry-physics) group SU(3), which described strongly interacting particles in terms of building blocks. In 1964 Gell-Mann introduced the concept of quarks as a physical basis for the scheme, having adopted the fanciful term from a passage in [James Joyce](https://www.britannica.com/biography/James-Joyce)’s novel *Finnegans Wake*. (The American physicist [George Zweig](https://www.britannica.com/biography/George-Zweig) developed a similar theory independently that same year and called his fundamental particles “aces.”) Gell-Mann’s model provided a simple picture in which all mesons are shown as consisting of a quark and an antiquark and all baryons as composed of three quarks. It [postulated](https://www.britannica.com/dictionary/postulated) the existence of three types of quarks, distinguished by unique “flavours.” These three quark types are now commonly designated as “up” (*u*), “down” (*d*), and “strange” (*s*). Each carries a fractional value of the [electron charge](https://www.britannica.com/science/electron-charge) (i.e., a charge less than that of the [electron](https://www.britannica.com/science/electron), *e*). The [up quark](https://www.britannica.com/science/up-quark) (charge 2/3*e*) and [down quark](https://www.britannica.com/science/down-quark) (charge −1/3*e*) make up protons and neutrons and are thus the ones observed in ordinary matter. [Strange quarks](https://www.britannica.com/science/strange-quark) (charge −1/3*e*) occur as components of [K](https://www.britannica.com/science/K-meson) [mesons](https://www.britannica.com/science/meson) and various other extremely short-lived subatomic particles that were first observed in [cosmic rays](https://www.britannica.com/science/cosmic-ray) but that play no part in ordinary matter.
## Quark “[colours](https://www.britannica.com/science/color-quarks-and-antiquarks)”
The interpretation of quarks as actual physical [entities](https://www.britannica.com/dictionary/entities) initially posed two major problems. First, quarks had to have half-integer [spin](https://www.britannica.com/science/spin-atomic-physics) (intrinsic angular momentum) values for the model to [work](https://www.britannica.com/science/work-physics), but at the same time they seemed to violate the [Pauli exclusion principle](https://www.britannica.com/science/Pauli-exclusion-principle), which governs the behaviour of all particles (called [fermions](https://www.britannica.com/science/fermion)) having odd half-integer spin. In many of the [baryon](https://www.britannica.com/science/baryon) configurations constructed of quarks, sometimes two or even three identical quarks had to be set in the same [quantum](https://www.britannica.com/science/quantum) state—an arrangement prohibited by the exclusion principle. Second, quarks appeared to defy being freed from the particles they made up. Although the forces binding quarks were strong, it seemed improbable that they were powerful enough to withstand bombardment by high-energy particle [beams](https://www.britannica.com/dictionary/beams) from accelerators.
[ Britannica Quiz How Much Do You Know About Physics?](https://www.britannica.com/quiz/how-much-do-you-know-about-physics)
These problems were resolved by the introduction of the concept of [colour](https://www.britannica.com/science/color), as formulated in [quantum chromodynamics](https://www.britannica.com/science/quantum-chromodynamics) (QCD). In this theory of strong interactions, whose breakthrough ideas were published in 1973, colour has nothing to do with the colours of the everyday world but rather represents a property of quarks that is the source of the strong force. The colours red, green, and blue are ascribed to quarks, and their opposites, antired, antigreen, and antiblue, are ascribed to antiquarks. According to QCD, all combinations of quarks must contain mixtures of these imaginary colours that cancel out one another, with the resulting particle having no net colour. A baryon, for example, always consists of a combination of one red, one green, and one blue quark and so never violates the exclusion principle. The property of colour in the strong force plays a role [analogous](https://www.merriam-webster.com/dictionary/analogous) to that of [electric charge](https://www.britannica.com/science/electric-charge) in the [electromagnetic force](https://www.britannica.com/science/electromagnetism), and just as charge implies the exchange of [photons](https://www.britannica.com/science/photon) between charged particles, so does colour involve the exchange of massless particles called [gluons](https://www.britannica.com/science/gluon) among quarks. Just as photons carry electromagnetic force, gluons transmit the forces that bind quarks together. Quarks change their colour as they emit and absorb gluons, and the exchange of gluons maintains proper quark colour distribution.
## Binding forces and “massive” quarks
The binding forces carried by the gluons tend to be weak when quarks are close together. Within a [proton](https://www.britannica.com/science/proton-subatomic-particle) (or other hadron), at distances of less than 10−15 metre, quarks behave as though they were nearly free. This condition is called [asymptotic freedom](https://www.britannica.com/science/asymptotic-freedom). When one begins to draw the quarks apart, however, as when attempting to knock them out of a proton, the effect of the force grows stronger. This is because, as explained by QCD, gluons have the ability to create other gluons as they move between quarks. Thus, if a quark starts to speed away from its companions after being struck by an [accelerated](https://www.britannica.com/dictionary/accelerated) particle, the gluons utilize [energy](https://www.britannica.com/science/energy) that they draw from the quark’s [motion](https://www.britannica.com/science/motion-mechanics) to produce more gluons. The larger the number of gluons exchanged among quarks, the stronger the effective binding forces become. Supplying additional energy to extract the quark only results in the conversion of that energy into new quarks and antiquarks with which the first quark combines. This phenomenon is observed at high-energy particle accelerators in the production of “jets” of new particles that can be associated with a single quark.
Key People:
[Murray Gell-Mann](https://www.britannica.com/biography/Murray-Gell-Mann)
[Frank Wilczek](https://www.britannica.com/biography/Frank-Wilczek)
[Henry Way Kendall](https://www.britannica.com/biography/Henry-Way-Kendall)
[David Gross](https://www.britannica.com/biography/David-Gross)
[Gerardus ’t Hooft](https://www.britannica.com/biography/Gerardus-t-Hooft)
*(Show more)*
Related Topics:
[asymptotic freedom](https://www.britannica.com/science/asymptotic-freedom)
[confinement](https://www.britannica.com/science/confinement)
[delta state](https://www.britannica.com/science/delta-state)
[bottom quark](https://www.britannica.com/science/bottom-quark)
[antiquark](https://www.britannica.com/science/antiquark)
*(Show more)*
On the Web:
[K12 LibreTexts - Quarks](https://k12.libretexts.org/Bookshelves/Science_and_Technology/Physics/20%3A_Modern_Physics/20.05%3A_Quarks) (Mar. 24, 2026)
*(Show more)*
[See all related content](https://www.britannica.com/facts/quark)
The discovery in the 1970s of the “[charm](https://www.britannica.com/science/charm-quark)” (*c*) and “[bottom](https://www.britannica.com/science/bottom-quark)” (*b*) quarks and their associated antiquarks, achieved through the creation of mesons, strongly suggests that quarks occur in pairs. This [speculation](https://www.britannica.com/dictionary/speculation) led to efforts to find a sixth type of quark called “[top](https://www.britannica.com/science/top-quark)” (*t*), after its proposed [flavour](https://www.britannica.com/science/flavour-particle-physics). According to theory, the top quark carries a charge of 2/3*e*; its partner, the bottom quark, has a charge of −1/3*e*. In 1995 two independent groups of scientists at the [Fermi National Accelerator Laboratory](https://www.britannica.com/topic/Fermi-National-Accelerator-Laboratory) reported that they had found the top quark. Their results give the top quark a mass of 173.8 ± 5.2 [gigaelectron volts](https://www.britannica.com/science/electron-volt) (GeV; 109 eV). (The next heaviest quark, the bottom, has a mass of about 4.2 GeV.) It has yet to be explained why the top quark is so much more massive than the other elementary particles, but its existence completes the [Standard Model](https://www.britannica.com/science/standard-model), the [prevailing](https://www.britannica.com/dictionary/prevailing) theoretical scheme of nature’s fundamental building blocks.
This article was most recently revised and updated by [Adam Augustyn](https://www.britannica.com/editor/Adam-Augustyn/6394).
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# standard model
physics
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Written by
[Christine Sutton Science writer. Research Associate, Department of Nuclear Physics, University of Oxford. Author of *The Particle Connection* and *Spaceship Neutrino.*](https://www.britannica.com/contributor/Christine-Sutton/2900)
Christine Sutton
Fact-checked by
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[](https://www.britannica.com/video/introduction-model-particle-physics/-204026)
View and understand the standard model of particle physicsA brief introduction to the standard model of particle physics.
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**standard model**, the combination of two theories of [particle physics](https://www.britannica.com/science/subatomic-particle) into a single framework to describe all interactions of subatomic particles, except those due to gravity. The two components of the standard model are [electroweak theory](https://www.britannica.com/science/electroweak-theory), which describes interactions via the electromagnetic and weak forces, and [quantum chromodynamics](https://www.britannica.com/science/quantum-chromodynamics), the theory of the [strong nuclear force](https://www.britannica.com/science/strong-force). Both these theories are [gauge field theories](https://www.britannica.com/science/gauge-theory), which describe the interactions between particles in terms of the exchange of intermediary “messenger” particles that have one unit of [intrinsic](https://www.merriam-webster.com/dictionary/intrinsic) [angular momentum](https://www.britannica.com/science/angular-momentum), or [spin](https://www.britannica.com/science/spin-atomic-physics).
In addition to these force-carrying particles, the standard model [encompasses](https://www.merriam-webster.com/dictionary/encompasses) two families of subatomic particles that build up matter and that have spins of one-half unit. These particles are the [quarks](https://www.britannica.com/science/quark) and the [leptons](https://www.britannica.com/science/lepton), and there are six varieties, or “flavours,” of each, related in pairs in three “generations” of increasing mass. Everyday matter is built from the members of the lightest generation: the “up” and “down” quarks that make up the protons and neutrons of atomic nuclei; the [electron](https://www.britannica.com/science/electron) that orbits within atoms and participates in binding atoms together to make molecules and more complex structures; and the electron-neutrino that plays a role in radioactivity and so influences the [stability](https://www.britannica.com/dictionary/stability) of matter. Heavier types of [quark](https://www.britannica.com/science/quark) and [lepton](https://www.britannica.com/science/lepton) have been discovered in studies of high-energy particle interactions, both at scientific laboratories with particle accelerators and in the natural reactions of high-energy cosmic-ray particles in the atmosphere.
Related Topics:
[subatomic particle](https://www.britannica.com/science/subatomic-particle)
[scientific modeling](https://www.britannica.com/science/scientific-modeling)
[particle physics](https://www.britannica.com/science/particle-physics)
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On the Web:
[National Center for Biotechnology Information - PubMed Central - Study of the standard model with weak decays of charmed hadrons at BESIII](https://pmc.ncbi.nlm.nih.gov/articles/PMC8645065/) (Apr. 10, 2026)
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[See all related content](https://www.britannica.com/facts/standard-model)
The standard model has proved a highly successful framework for predicting the interactions of quarks and leptons with great accuracy. Yet it has a number of weaknesses that lead physicists to search for a more complete theory of subatomic particles and their interactions. The present standard model, for example, cannot explain why there are three generations of quarks and leptons. It makes no [predictions](https://www.britannica.com/dictionary/predictions) of the masses of the quarks and the leptons nor of the strengths of the various interactions. Physicists hope that, by probing the standard model in detail and making highly accurate measurements, they will discover some way in which the model begins to break down and thereby find a more complete theory. This may prove to be what is known as a [grand unified theory](https://www.britannica.com/science/unified-field-theory), which uses a single theoretical structure to describe the strong, weak, and electromagnetic forces.
[ More From Britannica subatomic particle: Testing the Standard Model](https://www.britannica.com/science/subatomic-particle/Current-research-in-particle-physics#ref893172)
[Christine Sutton](https://www.britannica.com/contributor/Christine-Sutton/2900)
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| Readable Markdown | Top Questions
- What is a quark?
- Where are quarks found inside an atom?
- How many types of quarks are there?
- How do quarks combine to make protons and neutrons?
- What is the strong force and what does it do with quarks?
- Why can't we see quarks alone in nature?
**quark**, any member of a group of elementary [subatomic particles](https://www.britannica.com/science/subatomic-particle) that interact by means of the [strong force](https://www.britannica.com/science/strong-force) and are believed to be among the fundamental [constituents](https://www.merriam-webster.com/dictionary/constituents) of [matter](https://www.britannica.com/science/matter). Quarks associate with one another via the strong [force](https://www.britannica.com/science/force-physics) to make up [protons](https://www.britannica.com/science/proton-subatomic-particle) and [neutrons](https://www.britannica.com/science/neutron), in much the same way that the latter particles combine in various proportions to make up atomic nuclei. There are six types, or [flavours](https://www.britannica.com/science/flavour-particle-physics), of quarks that differ from one another in their [mass](https://www.britannica.com/science/mass-physics) and charge characteristics. These six quark flavours can be grouped in three pairs: up and down, charm and strange, and top and bottom. Quarks appear to be true elementary particles; that is, they have no apparent structure and cannot be resolved into something smaller. In addition, however, quarks always seem to occur in combination with other quarks or with [antiquarks](https://www.britannica.com/science/antiquark), their [antiparticles](https://www.britannica.com/science/antiparticle), to form all [hadrons](https://www.britannica.com/science/hadron)—the so-called strongly interacting particles that [encompass](https://www.merriam-webster.com/dictionary/encompass) both [baryons](https://www.britannica.com/science/baryon) and [mesons](https://www.britannica.com/science/meson).
Quarks\*
| quark type | baryon number | charge | strangeness\*\* | charm\*\* | bottom\*\* | top\*\* | mass (MeV) |
|---|---|---|---|---|---|---|---|
| \*Note that antiquarks exist for all flavours of quark and have opposite values for all the quantum numbers listed here. | | | | | | | |
| \*\*These are quantum numbers that must be assigned to the quarks to differentiate the various flavours. | | | | | | | |
| down (*d*) | 1/3 | −(1/3)*e* | 0 | 0 | 0 | 0 | 5–15 |
| up (*u*) | 1/3 | \+(2/3)*e* | 0 | 0 | 0 | 0 | 2–8 |
| strange (*s*) | 1/3 | −(1/3)*e* | −1 | 0 | 0 | 0 | 100–300 |
| charm (*c*) | 1/3 | \+(2/3)*e* | 0 | 1 | 0 | 0 | 1,000–1,600 |
| bottom (*b*) | 1/3 | −(1/3)*e* | 0 | 0 | −1 | 0 | 4,100–4,500 |
| top (*t*) | 1/3 | \+(2/3)*e* | 0 | 0 | 0 | 1 | 180,000 |
## Quark “flavours”
Throughout the 1960s theoretical physicists, trying to account for the ever-growing number of subatomic particles observed in experiments, considered the possibility that protons and neutrons were composed of smaller units of matter. In 1961 two physicists, [Murray Gell-Mann](https://www.britannica.com/biography/Murray-Gell-Mann) of the [United States](https://www.britannica.com/place/United-States) and Yuval Neʾeman of Israel, proposed a particle classification scheme called the [Eightfold Way](https://www.britannica.com/topic/Eightfold-Way), based on the mathematical [symmetry](https://www.britannica.com/science/symmetry-physics) group SU(3), which described strongly interacting particles in terms of building blocks. In 1964 Gell-Mann introduced the concept of quarks as a physical basis for the scheme, having adopted the fanciful term from a passage in [James Joyce](https://www.britannica.com/biography/James-Joyce)’s novel *Finnegans Wake*. (The American physicist [George Zweig](https://www.britannica.com/biography/George-Zweig) developed a similar theory independently that same year and called his fundamental particles “aces.”) Gell-Mann’s model provided a simple picture in which all mesons are shown as consisting of a quark and an antiquark and all baryons as composed of three quarks. It [postulated](https://www.britannica.com/dictionary/postulated) the existence of three types of quarks, distinguished by unique “flavours.” These three quark types are now commonly designated as “up” (*u*), “down” (*d*), and “strange” (*s*). Each carries a fractional value of the [electron charge](https://www.britannica.com/science/electron-charge) (i.e., a charge less than that of the [electron](https://www.britannica.com/science/electron), *e*). The [up quark](https://www.britannica.com/science/up-quark) (charge 2/3*e*) and [down quark](https://www.britannica.com/science/down-quark) (charge −1/3*e*) make up protons and neutrons and are thus the ones observed in ordinary matter. [Strange quarks](https://www.britannica.com/science/strange-quark) (charge −1/3*e*) occur as components of [K](https://www.britannica.com/science/K-meson) [mesons](https://www.britannica.com/science/meson) and various other extremely short-lived subatomic particles that were first observed in [cosmic rays](https://www.britannica.com/science/cosmic-ray) but that play no part in ordinary matter.
## Quark “[colours](https://www.britannica.com/science/color-quarks-and-antiquarks)”
The interpretation of quarks as actual physical [entities](https://www.britannica.com/dictionary/entities) initially posed two major problems. First, quarks had to have half-integer [spin](https://www.britannica.com/science/spin-atomic-physics) (intrinsic angular momentum) values for the model to [work](https://www.britannica.com/science/work-physics), but at the same time they seemed to violate the [Pauli exclusion principle](https://www.britannica.com/science/Pauli-exclusion-principle), which governs the behaviour of all particles (called [fermions](https://www.britannica.com/science/fermion)) having odd half-integer spin. In many of the [baryon](https://www.britannica.com/science/baryon) configurations constructed of quarks, sometimes two or even three identical quarks had to be set in the same [quantum](https://www.britannica.com/science/quantum) state—an arrangement prohibited by the exclusion principle. Second, quarks appeared to defy being freed from the particles they made up. Although the forces binding quarks were strong, it seemed improbable that they were powerful enough to withstand bombardment by high-energy particle [beams](https://www.britannica.com/dictionary/beams) from accelerators.
[ Britannica Quiz How Much Do You Know About Physics?](https://www.britannica.com/quiz/how-much-do-you-know-about-physics)
These problems were resolved by the introduction of the concept of [colour](https://www.britannica.com/science/color), as formulated in [quantum chromodynamics](https://www.britannica.com/science/quantum-chromodynamics) (QCD). In this theory of strong interactions, whose breakthrough ideas were published in 1973, colour has nothing to do with the colours of the everyday world but rather represents a property of quarks that is the source of the strong force. The colours red, green, and blue are ascribed to quarks, and their opposites, antired, antigreen, and antiblue, are ascribed to antiquarks. According to QCD, all combinations of quarks must contain mixtures of these imaginary colours that cancel out one another, with the resulting particle having no net colour. A baryon, for example, always consists of a combination of one red, one green, and one blue quark and so never violates the exclusion principle. The property of colour in the strong force plays a role [analogous](https://www.merriam-webster.com/dictionary/analogous) to that of [electric charge](https://www.britannica.com/science/electric-charge) in the [electromagnetic force](https://www.britannica.com/science/electromagnetism), and just as charge implies the exchange of [photons](https://www.britannica.com/science/photon) between charged particles, so does colour involve the exchange of massless particles called [gluons](https://www.britannica.com/science/gluon) among quarks. Just as photons carry electromagnetic force, gluons transmit the forces that bind quarks together. Quarks change their colour as they emit and absorb gluons, and the exchange of gluons maintains proper quark colour distribution.
## Binding forces and “massive” quarks
The binding forces carried by the gluons tend to be weak when quarks are close together. Within a [proton](https://www.britannica.com/science/proton-subatomic-particle) (or other hadron), at distances of less than 10−15 metre, quarks behave as though they were nearly free. This condition is called [asymptotic freedom](https://www.britannica.com/science/asymptotic-freedom). When one begins to draw the quarks apart, however, as when attempting to knock them out of a proton, the effect of the force grows stronger. This is because, as explained by QCD, gluons have the ability to create other gluons as they move between quarks. Thus, if a quark starts to speed away from its companions after being struck by an [accelerated](https://www.britannica.com/dictionary/accelerated) particle, the gluons utilize [energy](https://www.britannica.com/science/energy) that they draw from the quark’s [motion](https://www.britannica.com/science/motion-mechanics) to produce more gluons. The larger the number of gluons exchanged among quarks, the stronger the effective binding forces become. Supplying additional energy to extract the quark only results in the conversion of that energy into new quarks and antiquarks with which the first quark combines. This phenomenon is observed at high-energy particle accelerators in the production of “jets” of new particles that can be associated with a single quark.
The discovery in the 1970s of the “[charm](https://www.britannica.com/science/charm-quark)” (*c*) and “[bottom](https://www.britannica.com/science/bottom-quark)” (*b*) quarks and their associated antiquarks, achieved through the creation of mesons, strongly suggests that quarks occur in pairs. This [speculation](https://www.britannica.com/dictionary/speculation) led to efforts to find a sixth type of quark called “[top](https://www.britannica.com/science/top-quark)” (*t*), after its proposed [flavour](https://www.britannica.com/science/flavour-particle-physics). According to theory, the top quark carries a charge of 2/3*e*; its partner, the bottom quark, has a charge of −1/3*e*. In 1995 two independent groups of scientists at the [Fermi National Accelerator Laboratory](https://www.britannica.com/topic/Fermi-National-Accelerator-Laboratory) reported that they had found the top quark. Their results give the top quark a mass of 173.8 ± 5.2 [gigaelectron volts](https://www.britannica.com/science/electron-volt) (GeV; 109 eV). (The next heaviest quark, the bottom, has a mass of about 4.2 GeV.) It has yet to be explained why the top quark is so much more massive than the other elementary particles, but its existence completes the [Standard Model](https://www.britannica.com/science/standard-model), the [prevailing](https://www.britannica.com/dictionary/prevailing) theoretical scheme of nature’s fundamental building blocks.
This article was most recently revised and updated by [Adam Augustyn](https://www.britannica.com/editor/Adam-Augustyn/6394). |
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