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Quark
A quark (/kwɔːrk, kwɑːrk/) is a type of
Quark
elementary particle and a fundamental
constituent of matter. Quarks combine to form
composite particles called hadrons, the most
stable of which are protons and neutrons, the
components of atomic nuclei.[1] All commonly
observable matter is composed of up quarks,
down quarks and electrons. Owing to a
phenomenon known as color confinement, quarks
are never found in isolation; they can be found
only within hadrons, which include baryons (such
as protons and neutrons) and mesons, or in
quark–gluon plasmas.[2][3][nb 1] For this reason,
much of what is known about quarks has been A proton is composed of two up quarks, one
drawn from observations of hadrons. down quark, and the gluons that mediate the
forces "binding" them together. The color
Quarks have various intrinsic properties, assignment of individual quarks is arbitrary, but
including electric charge, mass, color charge, and all three colors must be present; red, blue and
spin. They are the only elementary particles in the green are used as an analogy to the primary
Standard Model of particle physics to experience colors that together produce a white color.
all four fundamental interactions, also known as Composition elementary particle
fundamental forces (electromagnetism,
Statistics fermionic
gravitation, strong interaction, and weak
interaction), as well as the only known particles Generation 1st, 2nd, 3rd
whose electric charges are not integer multiples of Interactions strong, weak,
the elementary charge. electromagnetic, gravitation
Symbol q
There are six types, known as flavors, of quarks:
up, down, charm, strange, top, and bottom.[4] Up Antiparticle antiquark (q)
and down quarks have the lowest masses of all Theorized Murray Gell-Mann (1964)
quarks. The heavier quarks rapidly change into up
George Zweig (1964)
and down quarks through a process of particle
decay: the transformation from a higher mass Discovered SLAC (c. 1968)
state to a lower mass state. Because of this, up Types 6 (up, down, strange, charm,
and down quarks are generally stable and the bottom, and top)
most common in the universe, whereas strange,

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charm, bottom, and top quarks can only be Electric charge + 2 e, − 1 e

3 3
produced in high energy collisions (such as those
involving cosmic rays and in particle
Color charge yes
accelerators). For every quark flavor there is a
1
Spin
corresponding type of antiparticle, known as an 2 ħ
antiquark, that differs from the quark only in Baryon number 1
3
that some of its properties (such as the electric
charge) have equal magnitude but opposite sign.

The quark model was independently proposed by physicists Murray Gell-Mann and George
Zweig in 1964.[5] Quarks were introduced as parts of an ordering scheme for hadrons, and there
was little evidence for their physical existence until deep inelastic scattering experiments at the
Stanford Linear Accelerator Center in 1968.[6][7] Accelerator program experiments have
provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995, was the
last to be discovered.[5]

Classification
The Standard Model is the
theoretical framework describing
all the known elementary particles.
This model contains six flavors of
quarks (q), named up (u), down (
d), strange (s), charm (c), bottom (
b), and top (t).[4] Antiparticles of
quarks are called antiquarks, and
are denoted by a bar over the
symbol for the corresponding
quark, such as u for an up
antiquark. As with antimatter in
general, antiquarks have the same
mass, mean lifetime, and spin as
their respective quarks, but the
electric charge and other charges
have the opposite sign.[8]

1
Quarks are spin- 2 particles, which Six of the particles in the Standard Model are quarks (shown in
purple). Each of the first three columns forms a generation of matter.
means they are fermions according
to the spin–statistics theorem.
They are subject to the Pauli exclusion principle, which states that no two identical fermions can
simultaneously occupy the same quantum state. This is in contrast to bosons (particles with

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integer spin), of which any number can be in the same state.[9] Unlike leptons, quarks possess
color charge, which causes them to engage in the strong interaction. The resulting attraction
between different quarks causes the formation of composite particles known as hadrons (see
§ Strong interaction and color charge below).

The quarks that determine the quantum numbers of hadrons are called valence quarks; apart
from these, any hadron may contain an indefinite number of virtual "sea" quarks, antiquarks,
and gluons, which do not influence its quantum numbers.[10] There are two families of hadrons:
baryons, with three valence quarks, and mesons, with a valence quark and an antiquark.[11] The
most common baryons are the proton and the neutron, the building blocks of the atomic
nucleus.[12] 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" hadrons with more valence quarks, such as tetraquarks (qqqq)
and pentaquarks (qqqqq), was conjectured from the beginnings of the quark model[13] but not
discovered until the early 21st century.[14][15][16][17]

Elementary fermions are grouped into three generations, each comprising two leptons and two
quarks. The first generation includes up and down quarks, the second strange and charm
quarks, and the third bottom and top quarks. All searches for a fourth generation of quarks and
other elementary fermions have failed,[18][19] and there is strong indirect evidence that no more
than three generations exist.[nb 2][20][21][22] Particles in higher generations generally have
greater mass and less stability, causing them to decay into lower-generation particles by means
of weak interactions. 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
rays), and decay quickly; however, they are thought to have been present during the first
fractions of a second after the Big Bang, when the universe was in an extremely hot and dense
phase (the quark epoch). Studies of heavier quarks are conducted in artificially created
conditions, such as in particle accelerators.[23]

Having electric charge, mass, color charge, and flavor, quarks are the only known elementary
particles that engage in all four fundamental interactions of contemporary physics:
electromagnetism, gravitation, strong interaction, and weak interaction.[12] Gravitation is too
weak to be relevant to individual particle interactions except at extremes of energy (Planck
energy) and distance scales (Planck distance). However, since no successful quantum theory of
gravity exists, gravitation is not described by the Standard Model.

See the table of properties below for a more complete overview of the six quark flavors'
properties.

History

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The quark model was independently proposed by physicists

Murray Gell-Mann[24] and George Zweig[25][26] in 1964.[5]
The proposal came shortly after Gell-Mann's 1961
formulation of a particle classification system known as the
Eightfold Way – or, in more technical terms, SU(3) flavor
symmetry, streamlining its structure.[27] Physicist Yuval
Ne'eman had independently developed a scheme similar to
the Eightfold Way in the same year.[28][29] An early attempt
at constituent organization was available in the Sakata
model.

At the time of the quark theory's inception, the "particle Murray Gell-Mann (2007)
zoo" included a multitude of hadrons, among other
particles. Gell-Mann and Zweig posited that they were not
elementary particles, but were instead composed of
combinations of quarks and antiquarks. Their model
involved three flavors of quarks, up, down, and strange, to
which they ascribed properties such as spin and electric
charge.[24][25][26] The initial reaction of the physics
community to the proposal was mixed. There was particular
contention about whether the quark was a physical entity or
a mere abstraction used to explain concepts that were not
George Zweig (2015)
fully understood at the time.[30]

In less than a year, extensions to the Gell-Mann–Zweig model were proposed. Sheldon Glashow
and James Bjorken predicted the existence of a fourth flavor of quark, which they called charm.
The addition was proposed because it allowed for a better description of the weak interaction
(the mechanism that allows quarks to decay), equalized the number of known quarks with the
number of known leptons, and implied a mass formula that correctly reproduced the masses of
the known mesons.[31]

Deep inelastic scattering experiments conducted in 1968 at the Stanford Linear Accelerator
Center (SLAC) and published on October 20, 1969, showed that the proton contained much
smaller, point-like objects and was therefore not an elementary particle.[6][7][32] Physicists were
reluctant to firmly identify these objects with quarks at the time, instead calling them "partons"
– a term coined by Richard Feynman.[33][34][35] The objects that were observed at SLAC would
later be identified as up and down quarks as the other flavors were discovered.[36] Nevertheless,
"parton" remains in use as a collective term for the constituents of hadrons (quarks, antiquarks,
and gluons). Richard Taylor, Henry Kendall and Jerome Friedman received the 1990 Nobel
Prize in physics for their work at SLAC.

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The strange quark's existence was indirectly validated by

SLAC's scattering experiments: not only was it a necessary
component of Gell-Mann and Zweig's three-quark model,
but it provided an explanation for the kaon (K) and pion (π)
hadrons discovered in cosmic rays in 1947.[37]

In a 1970 paper, Glashow, John Iliopoulos and Luciano

Maiani presented the GIM mechanism (named from their
initials) to explain the experimental non-observation of
flavor-changing neutral currents. This theoretical model
Photograph of the event that led to required the existence of the as-yet undiscovered charm
the discovery of the Σ++
c baryon, at quark.[38][39] The number of supposed quark flavors grew to
the Brookhaven National Laboratory
the current six in 1973, when Makoto Kobayashi and
in 1974
Toshihide Maskawa noted that the experimental
observation of CP violation[nb 3][40] could be explained if
there were another pair of quarks.

Charm quarks were produced almost simultaneously by two teams in November 1974 (see
November Revolution) – one at SLAC under Burton Richter, and one at Brookhaven National
Laboratory under Samuel Ting. The charm quarks were observed bound with charm antiquarks
in mesons. The two parties had assigned the discovered meson two different symbols, J and ψ;
thus, it became formally known as the J/ψ meson. The discovery finally convinced the physics
community of the quark model's validity.[35]

In the following years a number of suggestions appeared for extending the quark model to six
quarks. Of these, the 1975 paper by Haim Harari[41] was the first to coin the terms top and
bottom for the additional quarks.[42]

In 1977, the bottom quark was observed by a team at Fermilab led by Leon Lederman.[43][44]
This was a strong indicator of the top quark's existence: without the top quark, the bottom
quark would have been without a partner. It was not until 1995 that the top quark was finally
observed, also by the CDF[45] and DØ[46] teams at Fermilab.[5] It had a mass much larger than
expected,[47] almost as large as that of a gold atom.[48]

Etymology
For some time, Gell-Mann was undecided on an actual spelling for the term he intended to coin,
until he found the word quark in James Joyce's 1939 book Finnegans Wake:[49]

– Three quarks for Muster Mark!

Sure he hasn't got much of a bark
And sure any he has it's all beside the mark.

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The word quark is an outdated English word meaning to croak[50] and the above-quoted lines
are about a bird choir mocking king Mark of Cornwall in the legend of Tristan and Iseult.[51]
Especially in the German-speaking parts of the world there is a widespread legend, however,
that Joyce had taken it from the word Quark,[52] a German word of Slavic origin which denotes
a curd cheese,[53] but is also a colloquial term for "trivial nonsense".[54] In the legend it is said
that he had heard it on a journey to Germany at a farmers' market in Freiburg.[55][56] Some
authors, however, defend a possible German origin of Joyce's word quark.[57] Gell-Mann went
into further detail regarding the name of the quark in his 1994 book The Quark and the
Jaguar:[58]

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.[59]

The quark flavors were given their names for several reasons. The up and down quarks are
named after the up and down components of isospin, which they carry.[60] 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.[61] Glashow, who co-proposed the
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."[62] The
names "bottom" and "top", coined by Harari, were chosen because they are "logical partners for
up and down quarks".[41][42][61] Alternative names for bottom and top quarks are "beauty" and
"truth" respectively,[nb 4] but these names have somewhat fallen out of use.[66] While "truth"
never did catch on, accelerator complexes devoted to massive production of bottom quarks are
sometimes called "beauty factories".[67]

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Properties

Electric charge
1 2
Quarks have fractional electric charge values – either (− 3 ) or (+ 3 ) times the elementary charge
(e), depending on flavor. Up, charm, and top quarks (collectively referred to as up-type quarks)
2
have a charge of + 3 e; down, strange, and bottom quarks (down-type quarks) have a charge of
1
− 3 e. Antiquarks have the opposite charge to their corresponding quarks; up-type antiquarks
2 1
have charges of − 3 e and down-type antiquarks have charges of + 3 e. 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.[68] For example, the hadron
constituents of atomic nuclei, neutrons and protons, have charges of 0 e and +1 e respectively;
the neutron is composed of two down quarks and one up quark, and the proton of two up
quarks and one down quark.[12]

Spin
Spin is an intrinsic property of elementary particles, and its direction is an important degree of
freedom. It is sometimes visualized as the rotation of an object around its own axis (hence the
name "spin"), though this notion is somewhat misguided at subatomic scales because
elementary particles are believed to be point-like.[69]

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
ħ ħ 1
along any axis can only yield the values + 2 or − 2 ; for this reason quarks are classified as spin- 2
particles.[70] The component of spin along a given axis – by convention the z axis – is often
1 1
denoted by an up arrow ↑ for the value + 2 and down arrow ↓ for the value − 2 , placed after the
1
symbol for flavor. For example, an up quark with a spin of + 2 along the z axis is denoted by
u↑.[71]

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 interactions 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 radioactive process of beta decay, in which a neutron (n) "splits" into a

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−
proton (p), an electron (e ) and an electron antineutrino (νe)
(see picture). This occurs when one of the down quarks in the
−
neutron (udd) decays into an up quark by emitting a virtual W
−
boson, transforming the neutron into a proton (uud). The W
boson then decays into an electron and an electron
antineutrino.[72]

n → p + e− + ν (Beta decay, hadron notation)

e
−
udd → uud + e + νe (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 tomography (PET) and in experiments involving
neutrino detection.
Feynman diagram of beta decay
with time flowing upwards. The
CKM matrix (discussed below)
encodes the probability of this
and other quark decays.

While the process of flavor transformation is the

same for all quarks, each quark has a preference to
transform into the quark of its own generation. The
relative tendencies of all flavor transformations are
described by a mathematical table, called the
The strengths of the weak interactions
between the six quarks. The "intensities" of Cabibbo–Kobayashi–Maskawa matrix (CKM
the lines are determined by the elements of matrix). Enforcing unitarity, the approximate
the CKM matrix. magnitudes of the entries of the CKM matrix are:[73]

where Vij represents the tendency of a quark of flavor i to change into a quark of flavor j (or vice
versa).[nb 5]

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).[74] Together, the CKM and PMNS matrices describe all flavor transformations, but the
links between the two are not yet clear.[75]

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Strong interaction and color charge

According to quantum
chromodynamics (QCD), quarks
possess a property called color
charge. There are three types of
color charge, arbitrarily labeled
blue, green, and red.[nb 6] Each of
them is complemented by an
anticolor – antiblue, antigreen,
and antired. Every quark carries a
color, while every antiquark
The pattern of strong charges for carries an anticolor.[76]
the three colors of quark, three
antiquarks, and eight gluons (with
The system of attraction and
two of zero charge overlapping).
repulsion between quarks charged
with different combinations of the
three colors is called strong interaction, which is mediated by force
carrying particles known as gluons; this is discussed at length
below. The theory that describes strong interactions is called
quantum chromodynamics (QCD). A quark, which will have a
single color value, can form a bound system with an antiquark
carrying the corresponding anticolor. The result of two attracting
quarks will be color neutrality: a quark with color charge ξ plus an
antiquark with color charge −ξ will result in a color charge of 0 (or
"white" color) and the formation of a meson. This is analogous to
the additive color model in basic optics. Similarly, the combination
of three quarks, each with different color charges, or three All types of hadrons have
antiquarks, each with different anticolor charges, will result in the zero total color charge.
same "white" color charge and the formation of a baryon or
antibaryon.[77]

In modern particle physics, gauge symmetries – a kind of symmetry group – 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.[78] 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,[79] forms a triplet: a three-component quantum field that transforms under the
fundamental representation of SU(3)c.[80] The requirement that SU(3)c should be local – that

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is, that its transformations be allowed to vary with space and time – determines the properties
of the strong interaction. In particular, it implies the existence of eight gluon types to act as its
force carriers.[78][81]

Mass
Two terms are used in referring to a quark's mass: current
quark mass refers to the mass of a quark by itself, while
constituent quark mass refers to the current quark mass
plus the mass of the gluon particle field surrounding the
quark.[82] These masses typically have very different values.
Most of a hadron's mass comes from the gluons that bind
the constituent quarks together, rather than from the quarks
themselves. While gluons are inherently massless, they
possess energy – more specifically, quantum
chromodynamics binding energy (QCBE) – and it is this that
contributes so greatly to the overall mass of the hadron (see
mass in special relativity). For example, a proton has a mass Current quark masses for all six
flavors in comparison, as balls of
of approximately 938 MeV/c2, of which the rest mass of its proportional volumes. Proton (gray)
three valence quarks only contributes about 9 MeV/c2; and electron (red) are shown in
much of the remainder can be attributed to the field energy bottom left corner for scale.
[83][84]
of the gluons (see chiral symmetry breaking). The
Standard Model posits that elementary particles derive their
masses from the Higgs mechanism, which is associated to the Higgs boson. It is hoped that
further research into the reasons for the top quark's large mass of ~173 GeV/c2, almost the mass
of a gold atom,[83][85] might reveal more about the origin of the mass of quarks and other
elementary particles.[86]

Size
In QCD, quarks are considered to be point-like entities, with zero size. As of 2014, experimental
evidence indicates they are no bigger than 10−4 times the size of a proton, i.e. less than 10−19
metres.[87]

Table of properties
The following table summarizes the key properties of the six quarks. Flavor quantum numbers
(isospin (I3), charm (C), strangeness (S, not to be confused with spin), topness (T), and
bottomness (B′)) are assigned to certain quark flavors, and denote qualities of quark-based
1
systems and hadrons. The baryon number (B) is + 3 for all quarks, as baryons are made of three

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quarks. For antiquarks, the electric charge (Q) and all flavor quantum numbers (B, I3, 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[83]

Particle Mass* Q Antiparticle

J B I3 C S T B′
Name Symbol (MeV/c2) (e) Name Symbol

First generation
1 1 2 1
up u 2.3 ± 0.7 ± 0.5 2 +3 +3 +2 0 0 0 0 antiup u

1 1 1 1
down d 4.8 ± 0.5 ± 0.3 2 +3 −3 −2 0 0 0 0 antidown d

Second generation
1 1 2
charm c 1275 ± 25 2 +3 +3 0 +1 0 0 0 anticharm c

1 1 1
strange s 95 ± 5 2 +3 −3 0 0 −1 0 0 antistrange s

Third generation

173 210 ± 510 1 1 2

top t 2 +3 +3 0 0 0 +1 0 antitop t
± 710 *
1 1 1
bottom b 4180 ± 30 2 +3 −3 0 0 0 0 −1 antibottom b

J = total angular momentum, B = baryon number, Q = electric charge,

I3 = isospin, C = charm, S = strangeness, T = topness, B′ = bottomness.

* Notation such as 173 210 ± 510 ± 710, in the case of the top quark, denotes two types of measurement
uncertainty: The first uncertainty is statistical in nature, and the second is systematic.

Interacting quarks
As described by quantum chromodynamics, the strong interaction between quarks is mediated
by gluons, massless vector 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 theory), gluons are constantly exchanged between quarks through a
virtual emission and absorption process. When a gluon is transferred between quarks, a color
change occurs in both; for example, if a red quark emits a red–antigreen gluon, it becomes
green, and if a green quark absorbs a red–antigreen gluon, it becomes red. Therefore, while
each quark's color constantly changes, their strong interaction is preserved.[88][89][90]

Since gluons carry color charge, they themselves are able to emit and absorb other gluons. This
causes asymptotic freedom: as quarks come closer to each other, the chromodynamic binding
force between them weakens.[91] Conversely, as the distance between quarks increases, the

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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.[92][93] This process of
hadronization occurs before quarks formed in a high energy collision are able to interact in any
other way. The only exception is the top quark, which may decay before it hadronizes.[94]

Sea quarks
Hadrons contain, along with the valence quarks (qv) that contribute to their quantum numbers,
virtual quark–antiquark (qq) pairs known as sea quarks (qs). Sea quarks form when a gluon of
the hadron's color field splits; this process also works in reverse in that the annihilation of two
sea quarks produces a gluon. The result is a constant flux of gluon splits and creations
colloquially known as "the sea".[95] 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.[96]

Other phases of quark matter

Under sufficiently extreme conditions, quarks may
become "deconfined" out of bound states and propagate
as thermalized "free" excitations in the larger medium.
In the course of asymptotic freedom, the strong
interaction becomes weaker at increasing temperatures.
Eventually, color confinement would be effectively lost
in an extremely hot plasma of freely moving quarks and
gluons. This theoretical phase of matter is called quark–
gluon plasma.[99]
A qualitative rendering of the phase
The exact conditions needed to give rise to this state are diagram of quark matter. The precise
unknown and have been the subject of a great deal of details of the diagram are the subject of
ongoing research.[97][98]
speculation and experimentation. An estimate puts the
needed temperature at (1.90 ± 0.02) × 1012 kelvin.[100]
While a state of entirely free quarks and gluons has never been achieved (despite numerous
attempts by CERN in the 1980s and 1990s),[101] recent experiments at the Relativistic Heavy
Ion Collider have yielded evidence for liquid-like quark matter exhibiting "nearly perfect" fluid
motion.[102]

The quark–gluon plasma would be characterized by a great increase in the number of heavier
quark pairs in relation to the number of up and down quark pairs. It is believed that in the
period prior to 10−6 seconds after the Big Bang (the quark epoch), the universe was filled with
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quark–gluon plasma, as the temperature was too high for hadrons to be stable.[103]

Given sufficiently high baryon densities and relatively low temperatures – possibly comparable
to those found in neutron stars – quark matter is expected to degenerate into a Fermi liquid of
weakly interacting quarks. This liquid would be characterized by a condensation of colored
quark Cooper pairs, thereby breaking the local SU(3)c symmetry. Because quark Cooper pairs
harbor color charge, such a phase of quark matter would be color superconductive; that is, color
charge would be able to pass through it with no resistance.[104]

Physics portal
See also
Color–flavor locking
Koide formula
Nucleon magnetic moment
Preons
Quarkonium
Quark star
Quark–lepton complementarity

Explanatory notes
1. There is also the theoretical possibility of more exotic phases of quark matter.
0
2. The main evidence is based on the resonance width of the Z boson, which constrains the
4th generation neutrino to have a mass greater than ~45 GeV/c2. This would be highly
contrasting with the other three generations' neutrinos, whose masses cannot exceed
2 MeV/c2.
3. CP violation is a phenomenon that causes weak interactions to behave differently when left
and right are swapped (P symmetry) and particles are replaced with their corresponding
antiparticles (C symmetry).
4. "Beauty" and "truth" are contrasted in the last lines of Keats' 1819 poem "Ode on a Grecian
Urn" and may have been the origin of those names.[63][64][65]
5. The actual probability of decay of one quark to another is a complicated function of (among
other variables) the decaying quark's mass, the masses of the decay products, and the
corresponding element of the CKM matrix. This probability is directly proportional (but not
equal) to the magnitude squared (|Vij |2) of the corresponding CKM entry.
6. Despite its name, color charge is not related to the color spectrum of visible light.

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Further reading
A. Ali; G. Kramer (2011). "JETS and QCD: A Historical Review of the Discovery of the Quark
and Gluon Jets and Its Impact on QCD". European Physical Journal H. 36 (2): 245.
arXiv:1012.2288 (https://arxiv.org/abs/1012.2288). Bibcode:2011EPJH...36..245A (https://ui.
adsabs.harvard.edu/abs/2011EPJH...36..245A). doi:10.1140/epjh/e2011-10047-1 (https://doi
.org/10.1140%2Fepjh%2Fe2011-10047-1). S2CID 54062126 (https://api.semanticscholar.or
g/CorpusID:54062126).
R. Bowley; E. Copeland. "Quarks" (http://www.sixtysymbols.com/videos/quarks.htm). Sixty
Symbols. Brady Haran for the University of Nottingham.
D. J. Griffiths (2008). Introduction to Elementary Particles (2nd ed.). Wiley–VCH. ISBN 978-
3-527-40601-2.
I. S. Hughes (1985). Elementary Particles (https://archive.org/details/elementarypartic00hug
h) (2nd ed.). Cambridge University Press. ISBN 978-0-521-26092-3.
R. Oerter (2005). The Theory of Almost Everything: The Standard Model, the Unsung
Triumph of Modern Physics (https://archive.org/details/theoryofalmostev0000oert). Pi Press.
ISBN 978-0-13-236678-6.
A. Pickering (1984). Constructing Quarks: A Sociological History of Particle Physics. The
University of Chicago Press. ISBN 978-0-226-66799-7.
B. Povh (1995). Particles and Nuclei: An Introduction to the Physical Concepts. Springer-
Verlag. ISBN 978-0-387-59439-2.
M. Riordan (1987). The Hunting of the Quark: A True Story of Modern Physics (https://archiv
e.org/details/huntingofquarktr00mich). Simon & Schuster. ISBN 978-0-671-64884-8.

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B. A. Schumm (2004). Deep Down Things: The Breathtaking Beauty of Particle Physics (htt
ps://archive.org/details/deepdownthingsbr00schu). Johns Hopkins University Press.
ISBN 978-0-8018-7971-5.

External links
1969 Physics Nobel Prize lecture by Murray Gell-Mann (http://nobelprize.org/nobel_prizes/p
hysics/laureates/1969/index.html)
1976 Physics Nobel Prize lecture by Burton Richter (http://nobelprize.org/nobel_prizes/physi
cs/laureates/1976/richter-lecture.html)
1976 Physics Nobel Prize lecture by Samuel C.C. Ting (http://nobelprize.org/nobel_prizes/p
hysics/laureates/1976/ting-lecture.html)
2008 Physics Nobel Prize lecture by Makoto Kobayashi (http://nobelprize.org/nobel_prizes/p
hysics/laureates/2008/kobayashi-lecture.html)
2008 Physics Nobel Prize lecture by Toshihide Maskawa (http://nobelprize.org/nobel_prizes/
physics/laureates/2008/maskawa-lecture.html)
The Top Quark And The Higgs Particle by T.A. Heppenheimer (http://books.nap.edu/openbo
ok.php?isbn=0-309-04893-1&page=236) – A description of CERN's experiment to count the
families of quarks.
Think Big website, Quarks and Gluons (https://bigthink.com/starts-with-a-bang/what-rules-th
e-proton-quarks-or-gluons/)
Think Big website, Quarks 2019 (https://bigthink.com/starts-with-a-bang/there-are-no-free-q
uarks/)

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