In physics, a subatomic particle is a particle smaller than an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a baryon, like a proton or a neutron, composed of three quarks; or a meson, composed of two quarks), or an elementary particle, which is not composed of other particles (for example, quarks; or electrons, muons, and tau particles, which are called leptons). Particle physics and nuclear physics study these particles and how they interact. Most force-carrying particles like photons or gluons are called bosons
and, although they have quanta of energy, do not have rest mass or
discrete diameters (other than pure energy wavelength) and are unlike
the former particles that have rest mass and cannot overlap or combine
which are called fermions. The W and Z bosons, however, are an exception to this rule and have relatively large rest masses at approximately 80 GeV/c2 and 90 GeV/c2 respectively.
Experiments show that light could behave like a stream of particles (called photons) as well as exhibiting wave-like properties. This led to the concept of wave–particle duality to reflect that quantum-scale particles behave both like particles and like waves; they are occasionally called wavicles to reflect this.
Even among particle physicists,
the exact definition of a particle has diverse descriptions. These
professional attempts at the definition of a particle include:
Subatomic
particles are either "elementary", i.e. not made of multiple other
particles, or "composite" and made of more than one elementary particle
bound together.
The word hadron comes from Greek and was introduced in 1962 by Lev Okun. Nearly all composite particles contain multiple quarks (and/or
antiquarks) bound together by gluons (with a few exceptions with no
quarks, such as positronium and muonium). Those containing few (≤ 5) quarks (including antiquarks) are called hadrons. Due to a property known as color confinement,
quarks are never found singly but always occur in hadrons containing
multiple quarks. The hadrons are divided by number of quarks (including
antiquarks) into the baryons containing an odd number of quarks (almost always 3), of which the proton and neutron (the two nucleons) are by far the best known; and the mesons containing an even number of quarks (almost always 2, one quark and one antiquark), of which the pions and kaons are the best known.
Except for the proton and neutron, all other hadrons are unstable
and decay into other particles in microseconds or less. A proton is
made of two up quarks and one down quark,
while the neutron is made of two down quarks and one up quark. These
commonly bind together into an atomic nucleus, e.g. a helium-4 nucleus
is composed of two protons and two neutrons. Most hadrons do not live
long enough to bind into nucleus-like composites; those that do (other
than the proton and neutron) form exotic nuclei.
Any subatomic particle, like any particle in the three-dimensional space that obeys the laws of quantum mechanics, can be either a boson (with integer spin) or a fermion (with odd half-integer spin).
In the Standard Model, all the elementary fermions have spin 1/2, and are divided into the quarks which carry color charge and therefore feel the strong interaction, and the leptons
which do not. The elementary bosons comprise the gauge bosons (photon, W
and Z, gluons) with spin 1, while the Higgs boson is the only
elementary particle with spin zero.
The hypothetical graviton is required theoretically to have spin
2, but is not part of the Standard Model. Some extensions such as supersymmetry predict additional elementary particles with spin 3/2, but none have been discovered as of 2023.
Due to the laws for spin of composite particles, the baryons (3
quarks) have spin either 1/2 or 3/2 and are therefore fermions; the
mesons (2 quarks) have integer spin of either 0 or 1 and are therefore
bosons.
All composite particles are massive. Baryons (meaning "heavy")
tend to have greater mass than mesons (meaning "intermediate"), which in
turn tend to be heavier than leptons (meaning "lightweight"), but the
heaviest lepton (the tau particle) is heavier than the two lightest flavours of baryons (nucleons). It is also certain that any particle with an electric charge is massive.
When originally defined in the 1950s, the terms baryons, mesons
and leptons referred to masses; however, after the quark model became
accepted in the 1970s, it was recognised that baryons are composites of
three quarks, mesons are composites of one quark and one antiquark,
while leptons are elementary and are defined as the elementary fermions
with no color charge.
All massless particles (particles whose invariant mass is zero) are elementary. These include the photon and gluon, although the latter cannot be isolated.
By decay
Most subatomic particles are not stable. All leptons, as well as baryons decay by either the strong force or weak force (except for the proton). Protons are not known to decay,
although whether they are "truly" stable is unknown, as some very
important Grand Unified Theories (GUTs) actually require it. The μ and τ
muons, as well as their antiparticles, decay by the weak force.
Neutrinos (and antineutrinos) do not decay, but a related phenomenon of neutrino oscillations is thought to exist even in vacuums. The electron and its antiparticle, the positron, are theoretically stable due to charge conservation unless a lighter particle having magnitude of electric charge ≤ e exists (which is unlikely). Its charge is not shown yet.
Other properties
All observable subatomic particles have their electric charge an integer multiple of the elementary charge. The Standard Model's quarks have "non-integer" electric charges, namely, multiple of 1/3e, but quarks (and other combinations with non-integer electric charge) cannot be isolated due to color confinement. For baryons, mesons, and their antiparticles the constituent quarks' charges sum up to an integer multiple of e.
Through the work of Albert Einstein, Satyendra Nath Bose, Louis de Broglie, and many others, current scientific theory holds that all particles also have a wave nature. This has been verified not only for elementary particles but also for
compound particles like atoms and even molecules. In fact, according to
traditional formulations of non-relativistic quantum mechanics,
wave–particle duality applies to all objects, even macroscopic ones;
although the wave properties of macroscopic objects cannot be detected
due to their small wavelengths.
Interactions between particles have been scrutinized for many
centuries, and a few simple laws underpin how particles behave in
collisions and interactions. The most fundamental of these are the laws
of conservation of energy and conservation of momentum, which let us make calculations of particle interactions on scales of magnitude that range from stars to quarks. These are the prerequisite basics of Newtonian mechanics, a series of statements and equations in Philosophiae Naturalis Principia Mathematica, originally published in 1687.
Dividing an atom
The negatively charged electron has a mass of about 1/1836 of that of a hydrogen atom. The remainder of the hydrogen atom's mass comes from the positively charged proton. The atomic number
of an element is the number of protons in its nucleus. Neutrons are
neutral particles having a mass slightly greater than that of the
proton. Different isotopes of the same element contain the same number of protons but different numbers of neutrons. The mass number of an isotope is the total number of nucleons (neutrons and protons collectively).
Chemistry concerns itself with how electron sharing binds atoms into structures such as crystals and molecules. The subatomic particles considered important in the understanding of chemistry are the electron, the proton, and the neutron. Nuclear physics
deals with how protons and neutrons arrange themselves in nuclei. The
study of subatomic particles, atoms and molecules, and their structure
and interactions, requires quantum mechanics. Analyzing processes that change the numbers and types of particles requires quantum field theory. The study of subatomic particles per se is called particle physics. The term high-energy physics is nearly synonymous to "particle physics" since creation of particles requires high energies: it occurs only as a result of cosmic rays, or in particle accelerators. Particle phenomenology systematizes the knowledge about subatomic particles obtained from these experiments.
The term "subatomic particle" is largely a retronym of the 1960s, used to distinguish a large number of baryons and mesons (which comprise hadrons) from particles that are now thought to be truly elementary. Before that hadrons were usually classified as "elementary" because their composition was unknown.
A proton is composed of two up quarks, one down quark, and the gluons that mediate the forces "binding" them together. The color assignment
of individual quarks is arbitrary, but all three colors must be
present; red, blue and green are used as an analogy to the primary
colors that together produce a white color.
A quark (/kwɔːrk,kwɑːrk/ⓘ) 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. 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. For this reason, much of what is known about quarks has been drawn from observations of hadrons.
There are six types, known as flavors, of quarks: up, down, charm, strange, top, and bottom. 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 program experiments have provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered.
Six of the particles in the Standard Model are quarks (shown in purple). Each of the first three columns forms a generation of matter.
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). 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.
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
Murray Gell-Mann (2007)George Zweig (2015)
The quark model was independently proposed by physicists Murray Gell-Mann[24] 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 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. 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 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.
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. 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). Richard Taylor, Henry Kendall and Jerome Friedman received the 1990 Nobel Prize in physics for their work at SLAC.
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.
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. 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 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 1939 book Finnegans Wake:
– 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.
The word quark is an outdated English word meaning to croak and the above-quoted lines are about a bird choir mocking king Mark of Cornwall in the legend of Tristan and Iseult. Especially in the German-speaking parts of the world there is a
widespread legend, however, that Joyce had taken it from the word Quark, a German word of Slavic origin which denotes a curd cheese, but is also a colloquial term for "trivial nonsense". In the legend it is said that he had heard it on a journey to Germany at a farmers' market in Freiburg. Some authors, however, defend a possible German origin of Joyce's word quark. Gell-Mann went into further detail regarding the name of the quark in his 1994 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 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." The names "top" and "bottom", coined by Harari, were chosen because they are "logical partners for up and down quarks". Alternative names for top and bottom quarks are "truth" and "beauty" 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".
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 +2/3e; down, strange, and bottom quarks (down-type quarks) have a charge of −1/3e. Antiquarks have the opposite charge to their corresponding quarks; up-type antiquarks have charges of −2/3e and down-type antiquarks have charges of +1/3e. 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 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 constantħ (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↑.
Feynman diagram of beta decay with time flowing upwards. The CKM matrix (discussed below) encodes the probability of this and other quark decays.
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 (ν e) (see picture). This occurs when one of the down quarks in the neutron (udd) decays into an up quark by emitting a virtualW− boson, transforming the neutron into a proton (uud). The W− boson then decays into an electron and an electron antineutrino.
The strengths of the weak interactions between the six quarks. The "intensities" of the lines are determined by the elements of the CKM matrix.
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:
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.
All types of hadrons have zero total color charge.The
pattern of strong charges for the three colors of quark, three
antiquarks, and eight gluons (with two of zero charge overlapping).
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 different 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 that 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
Current quark masses for all six flavors in comparison, as balls of proportional volumes. Proton (gray) and electron (red) are shown in bottom left corner for scale.
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 gluonparticle 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
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.
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.
As described by quantum chromodynamics, the strong interaction between quarks is mediated by gluons, massless vectorgauge 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 (q v) that contribute to their quantum numbers, virtual quark–antiquark (qq) 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.
A qualitative rendering of the phase diagram of quark matter. The precise details of the diagram are the subject of ongoing research.
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.
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. An estimate puts the needed temperature at (1.90±0.02)×1012kelvin. 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.
![A tree diagram consisting mostly of straight arrows. A down quark forks into an up quark and a wavy-arrow W[superscript minus] boson, the latter forking into an electron and reversed-arrow electron antineutrino.](https://en.wikipedia.org/wiki/File:Beta_Negative_Decay.svg)
