A quark is an 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, such
as baryons, and mesons. For this reason, much
of what is known about quarks has been drawn
from observations of the hadrons themselves.
There are six types of quarks, known as flavors:
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.
Quarks have various intrinsic properties,
including electric charge, mass, color charge
and spin. Quarks are the only elementary particles
in the Standard Model of particle physics
to experience all four fundamental interactions,
also known as fundamental forces, as well
as the only known particles whose electric
charges are not integer multiples of the elementary
charge. 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 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
was the last to be discovered at Fermilab
in 1995.
Classification
The Standard Model is the theoretical framework
describing all the currently known elementary
particles. This model contains six flavors
of quarks, named up, down, strange, charm,
bottom, and top. 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,
any number of which 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.
The quarks which determine the quantum numbers
of hadrons are called valence quarks; apart
from these, any hadron may contain an indefinite
number of virtual 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,
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
and pentaquarks, has been conjectured but
not proven.
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
quarks occur commonly in nature. Heavier quarks
can only be created in high-energy collisions,
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. 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 and distance
scales. 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. Physicist Yuval Ne'eman had independently
developed a scheme similar to the Eightfold
Way in the same year.
At the time of the quark theory's inception,
the "particle 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, 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
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.
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 and pion hadrons
discovered in cosmic rays in 1947.
In a 1970 paper, Glashow, John Iliopoulos
and Luciano Maiani presented further reasoning
for 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—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:
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" was clearly intended to rhyme
with "Mark", as well as "bark" and other such
words, I had to find an excuse to pronounce
it as "kwork". But the book represents the
dream of a publican named Humphrey Chimpden
Earwicker. Words in the text are typically
drawn from several sources at once, like the
"portmanteau" words in "Through the Looking-Glass".
From time to time, phrases occur in the book
that are partially determined by calls for
drinks at the bar. I argued, therefore, that
perhaps one of the multiple sources of the
cry "Three quarks for Muster Mark" might be
"Three quarts for Mister Mark", in which case
the pronunciation "kwork" would not be totally
unjustified. In any case, the number three
fitted perfectly the way quarks occur in nature.
Zweig preferred the name ace for the particle
he had theorized, but Gell-Mann's terminology
came to prominence once the quark model had
been commonly accepted.
The quark flavors were given their names for
a number of reasons. The up and down quarks
are named after the up and down components
of 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 coproposed charm
quark with Bjorken, is quoted as saying, "We
called our construct the 'charmed quark',
for we were fascinated and pleased by the
symmetry it brought to the subnuclear world."
The names "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, depending on flavor. Up, charm, and
top quarks have a charge of +2⁄3, while
down, strange, and bottom quarks have −1⁄3.
Antiquarks have the opposite charge to their
corresponding quarks; up-type antiquarks have
charges of −2⁄3 and down-type antiquarks
have charges of +1⁄3. Since the electric
charge of a hadron is the sum of the charges
of the constituent quarks, all hadrons have
integer charges: the combination of three
quarks, three antiquarks, or a quark and an
antiquark always results in integer charges.
For example, the hadron constituents of atomic
nuclei, neutrons and protons, have charges
of 0 and +1 respectively; the neutron is composed
of two down quarks and one up quark, and the
proton of two up quarks and one down quark.
Spin
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, 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. 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 can change into
any down-type quark and vice versa. This flavor
transformation mechanism causes the radioactive
process of beta decay, in which a neutron
"splits" into a proton, an electron and an
electron antineutrino (ν
e). This occurs when one of the down quarks
in the neutron decays into an up quark by
emitting a virtual W− boson, transforming
the neutron into a proton. 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
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. Enforcing unitarity, the approximate
magnitudes of the entries of the CKM matrix
are:
where Vij represents the tendency of a quark
of flavor i to change into a quark of flavor
j.
There exists an equivalent weak interaction
matrix for leptons, called the Pontecorvo–Maki–Nakagawa–Sakata
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 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. A quark
charged with one color value can form a bound
system with an antiquark carrying the corresponding
anticolor; threequarks, one of eachcolor,
will similarly be bound together. The result
of two attracting quarks will be color neutrality:
a quark with color charge ξ plus an antiquark
with color charge −ξ will result in a color
charge of 0 and the formation of a meson.
Analogous to the additive color model in basic
optics, the combination of three quarks or
three antiquarks, each with different color
charges, will result in the same "white" color
charge and the formation of a baryon or antibaryon.
In modern particle physics, gauge symmetries
– a kind of symmetry group – relate interactions
between particles. Color 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. Every quark
flavor f, each with subtypes fB, fG, fR corresponding
to the quark colors, forms a triplet: a three-component
quantum field which transforms under the fundamental
representation of SU(3)c. The requirement
that SU(3)c should be local – that is, that
its transformations be allowed to vary with
space and time – determines the properties
of the strong interaction, in particular the
existence of eight gluon types to act as its
force carriers.
Mass
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 – and
it is this that contributes so greatly to
the overall mass of the hadron. 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 11 MeV/c2;
much of the remainder can be attributed to
the gluons' QCBE.
The Standard Model posits that elementary
particles derive their masses from the Higgs
mechanism, which is related to the Higgs boson.
Physicists hope 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.
Table of properties
The following table summarizes the key properties
of the six quarks. Flavor quantum numbers,
charm, strangeness, topness, and bottomness)
are assigned to certain quark flavors, and
denote qualities of quark-based systems and
hadrons. The baryon number is +1⁄3 for all
quarks, as baryons are made of three quarks.
For antiquarks, the electric charge and all
flavor quantum numbers are of opposite sign.
Mass and total angular momentum 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 4190+180
−60 denotes 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, 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, along with the valence quarks (q
v) that contribute to their quantum numbers,
contain virtual quark–antiquark pairs known
as sea quarks (q
s). 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×1012 Kelvin. While a state of entirely
free quarks and gluons has never been achieved,
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 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 – Hypothetical particles which were
once postulated to be subcomponents of quarks
and leptons
Quarkonium – Mesons made of a quark and
antiquark of the same flavor
Quark star – A hypothetical degenerate neutron
star with extreme density
Quark–lepton complementarity – Possible
fundamental relation between quarks and leptons
Notes
^ Several research groups claimed to have
proven the existence of tetraquarks and pentaquarks
in the early 2000s. While the status of tetraquarks
is still under debate, all known pentaquark
candidates have since been established as
non-existent.
^ The main evidence is based on the resonance
width of the Z0 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.
^ CP violation is a phenomenon which causes
weak interactions to behave differently when
left and right are swapped and particles are
replaced with their corresponding antiparticles.
^ The actual probability of decay of one quark
to another is a complicated function of the
decaying quark's mass, the masses of the decay
products, and the corresponding element of
the CKM matrix. This probability is directly
proportional to the magnitude squared of the
corresponding CKM entry.
^ Despite its name, color charge is not related
to the color spectrum of visible light.
References
Further reading
A. Ali, G. Kramer. "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: 245. arXiv:1012.2288. Bibcode:2011EPJH...36..245A.
doi:10.1140e2011-10047-1. 
D.J. Griffiths. Introduction to Elementary
Particles. Wiley–VCH. ISBN 3-527-40601-8. 
I.S. Hughes. Elementary particles. Cambridge
University Press. ISBN 0-521-26092-2. 
R. Oerter. The Theory of Almost Everything:
The Standard Model, the Unsung Triumph of
Modern Physics. Pi Press. ISBN 0-13-236678-9. 
A. Pickering. Constructing Quarks: A Sociological
History of Particle Physics. The University
of Chicago Press. ISBN 0-226-66799-5. 
B. Povh. Particles and Nuclei: An Introduction
to the Physical Concepts. Springer–Verlag.
ISBN 0-387-59439-6. 
M. Riordan. The Hunting of the Quark: A true
story of modern physics. Simon & Schuster.
ISBN 0-671-64884-5. 
B.A. Schumm. Deep Down Things: The Breathtaking
Beauty of Particle Physics. Johns Hopkins
University Press. ISBN 0-8018-7971-X. 
External links
1969 Physics Nobel Prize lecture by Murray
Gell-Mann
1976 Physics Nobel Prize lecture by Burton
Richter
1976 Physics Nobel Prize lecture by Samuel
C.C. Ting
2008 Physics Nobel Prize lecture by Makoto
Kobayashi
2008 Physics Nobel Prize lecture by Toshihide
Maskawa
The Top Quark And The Higgs Particle by T.A.
Heppenheimer – A description of CERN's
experiment to count the families of quarks.
Bowley, Roger; Copeland, Ed. "Quarks". Sixty
Symbols. Brady Haran for the University of
Nottingham. 
