A quark () is a type of elementary particle
and a fundamental constituent of matter. Quarks
combine to form composite particles called
hadrons, the most stable of which are protons
and neutrons, the components of atomic nuclei.
Due to a phenomenon known as color confinement,
quarks are never directly observed or found
in isolation; they can be found only within
hadrons, which include baryons (such as protons
and neutrons) and mesons. For this reason,
much of what is known about quarks has been
drawn from observations of hadrons.
Quarks have various intrinsic properties,
including electric charge, mass, color charge,
and spin. They are the only elementary particles
in the Standard Model of particle physics
to experience all four fundamental interactions,
also known as fundamental forces (electromagnetism,
gravitation, strong interaction, and weak
interaction), as well as the only known particles
whose electric charges are not integer multiples
of the elementary charge.
There are six types, known as flavors, of
quarks: up, down, strange, charm, bottom,
and top. Up and down quarks have the lowest
masses of all quarks. The heavier quarks rapidly
change into up and down quarks through a process
of particle decay: the transformation from
a higher mass state to a lower mass state.
Because of this, up and down quarks are generally
stable and the most common in the universe,
whereas strange, charm, bottom, and top quarks
can only be produced in high energy collisions
(such as those involving cosmic rays and in
particle accelerators). For every quark flavor
there is a corresponding type of antiparticle,
known as an antiquark, that differs from the
quark only in that some of its properties
(such as the electric charge) have equal magnitude
but opposite sign.
The quark model was independently proposed
by physicists Murray Gell-Mann and George
Zweig in 1964. Quarks were introduced as parts
of an ordering scheme for hadrons, and there
was little evidence for their physical existence
until deep inelastic scattering experiments
at the Stanford Linear Accelerator Center
in 1968. Accelerator experiments have provided
evidence for all six flavors. The top quark,
first observed at Fermilab in 1995, was the
last to be discovered.
== Classification ==
The Standard Model is the theoretical framework
describing all the currently 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). 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.Quarks
are spin-​1⁄2 particles, implying that
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 integer spin), of which any
number can be in the same state. 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. There are two families of hadrons:
baryons, with three valence quarks, and mesons,
with a valence quark and an antiquark. The
most common baryons are the proton and the
neutron, the building blocks of the atomic
nucleus. 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 but
not discovered until the early 21st century.Elementary
fermions are grouped into three generations,
each comprising two leptons and two quarks.
The first generation includes up and down
quarks, the second strange and charm quarks,
and the third bottom and top quarks. All searches
for a fourth generation of quarks and other
elementary fermions have failed, and there
is strong indirect evidence that no more than
three generations exist. 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.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. 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 ==
The quark model was independently proposed
by physicists Murray Gell-Mann and George
Zweig in 1964. The proposal came shortly after
Gell-Mann's 1961 formulation of a particle
classification system known as the Eightfold
Way—or, in more technical terms, SU(3) flavor
symmetry, streamlining its structure. Physicist
Yuval Ne'eman had independently developed
a scheme similar to the Eightfold Way in the
same year. An early attempt at constituent
organization was available in the Sakata model.
At the time of the quark theory's inception,
the "particle zoo" included, amongst other
particles, a multitude of hadrons. Gell-Mann
and Zweig posited that they were not elementary
particles, but were instead composed of combinations
of quarks and antiquarks. Their model involved
three flavors of quarks, up, down, and strange,
to which they ascribed properties such as
spin and electric charge. The initial reaction
of the physics community to the proposal was
mixed. There was particular contention about
whether the quark was a physical entity or
a mere abstraction used to explain concepts
that were not fully understood at the time.In
less than a year, extensions to the Gell-Mann–Zweig
model were proposed. Sheldon Lee 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.In 1968, deep inelastic
scattering experiments at the Stanford Linear
Accelerator Center (SLAC) showed that the
proton contained much smaller, point-like
objects and was therefore not an elementary
particle. Physicists were reluctant to firmly
identify these objects with quarks at the
time, instead calling them "partons"—a term
coined by Richard Feynman. The objects that
were observed at SLAC would later be identified
as up and down quarks as the other flavors
were discovered. Nevertheless, "parton" remains
in use as a collective term for the constituents
of hadrons (quarks, antiquarks, and gluons).
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.In
a 1970 paper, Glashow, John Iliopoulos and
Luciano Maiani presented the so-called GIM
mechanism to explain the experimental non-observation
of flavor-changing neutral currents. This
theoretical model required the existence of
the as-yet undiscovered charm quark. The number
of supposed quark flavors grew to the current
six in 1973, when Makoto Kobayashi and Toshihide
Maskawa noted that the experimental observation
of CP violation 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.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 was the
first to coin the terms top and bottom for
the additional quarks.In 1977, the bottom
quark was observed by a team at Fermilab led
by Leon Lederman. This was a strong indicator
of the top quark's existence: without the
top quark, the bottom quark would have been
without a partner. However, it was not until
1995 that the top quark was finally observed,
also by the CDF and DØ teams at Fermilab.
It had a mass much larger than had been previously
expected, almost as large as that of a gold
atom.
== Etymology ==
For some time, Gell-Mann was undecided on
an actual spelling for the term he intended
to coin, until he found the word quark in
James Joyce's book Finnegans Wake:
The word quark itself is a Slavic borrowing
in German and denotes a dairy product, but
is also a colloquial term for ″rubbish″.
Gell-Mann went into further detail regarding
the name of the quark in his book The Quark
and the Jaguar:
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 several reasons. The
up and down quarks are named after the up
and down components of isospin, 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 co-proposed charm quark with Bjorken,
is quoted as saying, "We called our construct
the 'charmed quark', for we were fascinated
and pleased by the symmetry it brought to
the subnuclear world." The names "bottom"
and "top", coined by Harari, were chosen because
they are "logical partners for up and down
quarks". In the past, bottom and top quarks
were sometimes referred to as "beauty" and
"truth" respectively, but these names have
somewhat fallen out of use. While "truth"
never did catch on, accelerator complexes
devoted to massive production of bottom quarks
are sometimes called "beauty factories".
== Properties ==
=== 
Electric charge ===
Quarks have fractional electric charge values
– either (−​1⁄3) or (+​2⁄3) times
the elementary charge (e), depending on flavor.
Up, charm, and top quarks (collectively referred
to as up-type quarks) have a charge of +​2⁄3
e, while down, strange, and bottom quarks
(down-type quarks) have −​1⁄3 e. Antiquarks
have the opposite charge to their corresponding
quarks; up-type antiquarks have charges of
−​2⁄3 e and down-type antiquarks have
charges of +​1⁄3 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. For example, the
hadron constituents of atomic nuclei, neutrons
and protons, have charges of 0 e and +1 e
respectively; the neutron is composed of two
down quarks and one up quark, and the proton
of two up quarks and one down quark.
=== Spin ===
Spin is an intrinsic property of elementary
particles, and its direction is an important
degree of freedom. It is sometimes visualized
as the rotation of an object around its own
axis (hence the name "spin"), though this
notion is somewhat misguided at subatomic
scales because elementary particles are believed
to be point-like.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-​1⁄2
particles. The component of spin along a given
axis – by convention the z axis – is often
denoted by an up arrow ↑ for the value +​1⁄2
and down arrow ↓ for the value −​1⁄2,
placed after the symbol for flavor. For example,
an up quark with a spin of +​1⁄2 along
the z axis is denoted by u↑.
=== Weak interaction ===
A quark of one flavor can transform into a
quark of another flavor only through the weak
interaction, one of the four fundamental 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 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.
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.
While the process of flavor transformation
is the same for all quarks, each quark has
a preference to transform into the quark of
its own generation. The relative tendencies
of all flavor transformations are described
by a mathematical table, called the Cabibbo–Kobayashi–Maskawa
matrix (CKM matrix). Enforcing unitarity,
the approximate magnitudes of the entries
of the CKM matrix are:
[
|
V
u
d
|
|
V
u
s
|
|
V
u
b
|
|
V
c
d
|
|
V
c
s
|
|
V
c
b
|
|
V
t
d
|
|
V
t
s
|
|
V
t
b
|
]
≈
[
0.974
0.225
0.003
0.225
0.973
0.041
0.009
0.040
0.999
]
,
{\displaystyle {\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&0.225&0.003\\0.225&0.973&0.041\\0.009&0.040&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).There exists an equivalent
weak interaction matrix for leptons (right
side of the W boson on the above beta decay
diagram), called the Pontecorvo–Maki–Nakagawa–Sakata
matrix (PMNS matrix). Together, the CKM and
PMNS matrices describe all flavor transformations,
but the links between the two are not yet
clear.
=== Strong interaction and color charge ===
According to quantum chromodynamics (QCD),
quarks possess a property called color charge.
There are three types of color charge, arbitrarily
labeled blue, green, and red. Each of them
is complemented by an anticolor – antiblue,
antigreen, and antired. Every quark carries
a color, while every antiquark carries an
anticolor.The system of attraction and repulsion
between quarks charged with different combinations
of the three colors is called strong interaction,
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 antiquarks, each with anticolor charges,
will result in the same "white" color charge
and the formation of a baryon or antibaryon.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. 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,
it implies the existence of eight gluon types
to act as its force carriers.
=== Mass ===
Two terms are used in referring to a quark's
mass: current quark mass refers to the mass
of a quark by itself, while constituent quark
mass refers to the current quark mass plus
the mass of the gluon particle field surrounding
the quark. These masses typically have very
different values. Most of a hadron's mass
comes from the gluons that bind the constituent
quarks together, rather than from the quarks
themselves. While gluons are inherently massless,
they possess energy – more specifically,
quantum chromodynamics binding energy (QCBE)
– and it is this that contributes so greatly
to the overall mass of the hadron (see mass
in special relativity). For example, a proton
has a mass of approximately 938 MeV/c2, of
which the rest mass of its three valence quarks
only contributes about 9 MeV/c2; much of the
remainder can be attributed to the field energy
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, might reveal more about the origin of
the mass of quarks and other elementary particles.
=== Size ===
Quarks are treated as zero-dimensional point-like
entities of zero size in QCD; the lack of
any detectable size in experiments puts an
upper bound on their size of 10^-4 the size
of a proton, i.e. less than 10^-19 metres
=== Table of properties ===
The following table summarizes the key properties
of the six quarks. Flavor quantum numbers
(isospin (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
systems and hadrons. The baryon number (B)
is +​1⁄3 for all quarks, as baryons are
made of three 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.
J = 
total angular momentum, B = baryon number,
Q = electric charge, I3 = isospin, C = charm,
S = strangeness, T = topness, B′ = bottomness.
* Notation such as 173210±510 ± 710 denotes
two types of measurement uncertainty. In the
case of the top quark, 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.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. 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 hadronization occurs before quarks, formed
in a high energy collision, are able to interact
in any other way. The only exception is the
top quark, which may decay before it hadronizes.
=== Sea quarks ===
Hadrons contain, along with the valence 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". Sea quarks are much less stable
than their valence counterparts, and they
typically annihilate each other within the
interior of the hadron. Despite this, sea
quarks can hadronize into baryonic or mesonic
particles under certain circumstances.
=== Other phases of quark matter ===
Under sufficiently extreme conditions, quarks
may become deconfined and exist as free particles.
In the course of asymptotic freedom, the strong
interaction becomes weaker at higher temperatures.
Eventually, color confinement would be lost
and an extremely hot plasma 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 (1.90±0.02)×1012 kelvin. While a state
of entirely free quarks and gluons has never
been achieved (despite numerous attempts by
CERN in the 1980s and 1990s), recent experiments
at the Relativistic Heavy Ion Collider have
yielded evidence for liquid-like quark matter
exhibiting "nearly perfect" fluid motion.The
quark–gluon plasma would be characterized
by a great increase in the number of heavier
quark pairs in relation to the number of up
and down quark pairs. It is believed that
in the period prior to 10−6 seconds after
the Big Bang (the quark epoch), 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 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.
== See also ==
Color–flavor locking
Neutron magnetic moment
Leptons
Preons
Quarkonium
Quark star
Quark–lepton complementarity
Sakata model
== Notes
