Quark: Difference between revisions

For other uses, see Quark (disambiguation).

A proton, composed of two up quarks and one down quark.

A quark (Template:Pron-en or Template:IPAlink-en) is a type of subatomic particle. In technical terms, quarks are elementary fermions and the only particles in the Standard Model to experience all four fundamental interactions. Because of a phenomenon known as color confinement, quarks are never found on their own in nature; they are always bound together in composite particles named hadrons. Because of this, much of what is known about quarks has been inferred from observations on the hadrons themselves. The most common hadrons are protons and neutrons, which compose atomic nuclei.

There are six different types of quarks, known as flavors: up (symbol:
u
), down (
d
), charm (
c
), strange (
s
), top (
t
) and bottom (
b
). The flavors with the lowest masses, the up quark and the down quark, are generally stable and are very common in the universe. The heavier charm, strange, top and bottom quarks are unstable and rapidly decay; these can only be produced in high energy collisions, such as in particle accelerators and in cosmic rays. Quarks have various properties that are dependent on flavor, such as electrical and color charge, spin and mass. For every quark flavor there is a corresponding antiparticle, called antiquark, that differs from the quark only in that some of its properties have the opposite sign.

The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. There was little evidence for the physical reality of quarks until 1968, when electron–proton scattering experiments indicated that the electrons were scattering off three point-like constituents inside the proton. By 1995, when the top quark was observed at Fermilab, all the six flavors had been observed.

Classification

Main article: Standard Model

Six of the particles in the Standard Model are quarks.

The Standard Model is the theoretical framework describing all the currently known elementary particles, plus the unobserved (as of 2008) Higgs boson. This model comprises six flavors of quarks, named up, down, charm, strange, top and bottom. The top and bottom flavors are sometimes known as truth and beauty, respectively. In this context, flavor is an arbitrarily chosen term referring to different kinds of particles, and has nothing to do with the everyday experience of flavor. All quarks of the same flavor are identical particles, meaning that all of their properties are the same.

In the Standard Model, particles of matter, including quarks and leptons, are fermions, meaning that their spin quantum number (a property related to their intrinsic angular momentum) is half-integer; as a consequence, they are subject to the Pauli exclusion principle, stating that no two fermions of the same flavor can ever simultaneously occupy the same state. This contrasts with particles mediating forces, or bosons, that have integer spin; the Pauli exclusion principle does not apply to them. Quarks, unlike leptons, have a color charge, a property causing them to engage in the strong interaction. This interaction is the reason why quarks attract each other into hadrons. Like the electric force is responsible for atoms attracting each other to form molecules, the strong interaction is responsible for protons and neutrons attracting each other to form atomic nuclei. But unlike the electric force which has infinite range, the strong interaction only acts at close distances, of order 10⁻¹⁵ m or less. See nuclear force for more details.

Elementary fermions are grouped into three generations, each one comprising two leptons and two quarks. The first generation includes up and down quarks, the second includes charm and strange quarks, and the third includes top and bottom quarks. All searches for a fourth generation of quarks and other elementary fermions have failed, and there is strong indirect evidence that more than three generations cannot exist: each generation comprises only one flavor of neutrino, and the existence of a fourth generation would imply different values of the lifetime of the Z boson and the abundance of helium-4 in the universe than the experimentally observed ones. Particles in higher generations generally have greater mass and are less stable, tending to decay into lower-generation, less massive particles by means of weak interactions. Only the first-generation up and down quarks occur commonly in nature; heavier quarks can only be created in high-energy collisions, such as in cosmic rays, and quickly decay. As a result, these particles play little part in the universe of today, but likely were much more prominent in an earlier, hotter universe. Most studies conducted on heavier quarks have been performed in artificially created conditions, such as in particle accelerators.

Antiparticles of quarks are called antiquarks, and are denoted by a bar over the letter for the quark, such as
u
for an up antiquark. As with antimatter in general, antiquarks have the same mass, lifetime and spin as their respective quarks, but the electric charge and other charges have the opposite sign.

Having electric charge, flavor, color charge and mass, quarks are the only known elementary particles that engage in all four fundamental interactions of contemporary physics: electromagnetism, weak interaction, strong interaction and gravitation. Gravitation, however, is usually irrelevant at subatomic scales, and is not described by the Standard Model.

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

History

Murray Gell-Mann in 2007. Gell-Mann and George Zweig first proposed the quark model in 1964.

The quark model was first postulated independently by physicists Murray Gell-Mann and George Zweig in 1964. At the time of the theory's initial proposal, the "particle zoo" consisted of a few leptons and a multitude of hadrons. Gell-Mann and Zweig posited that hadrons were not elementary particles, but instead composed of various combinations of quarks and antiquarks.

Gell-Mann and Zweig postulated just three flavors of quarks—up, down and strange—to which they at first ascribed such properties as spin and electric charge. The initial reaction of the physics community to the proposal was mixed, many having reservations regarding the actual physicality of the quark concept. Some believed the quark was merely an abstract concept that could be temporarily used to help explain certain concepts that were not well understood, while others believed that the quark was a physical entity

In less than a year, extensions to the Gell-Mann–Zweig model were proposed when another duo of physicists, Sheldon Lee Glashow and James Bjorken, predicted the existence of a fourth flavor of quark, which they referred to as charm. The addition was proposed because it expanded the power and self-consistency of the theory: it allowed a better description of the weak interaction (the mechanism that allows quarks to decay); equalized the number of quarks with the number of known leptons; and implied a mass formula that correctly reproduced the masses of the known mesons (hadrons with integer spin).

In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center showed that the proton was not an elementary particle, but contained smaller particles within it that made up an inner substructure. However, while the concept of hadron substructure had been proven, there was still apprehension towards the quark model: the substructures became known at the time as partons (a term proposed by Richard Feynman, and supported by some experimental project reports), but it "was unfashionable to identify them explicitly with quarks". These partons were later identified as up and down quarks when the other flavors were beginning to surface. Their discovery also validated the existence of the strange quark, because it was necessary to the model Gell-Mann and Zweig had proposed.

In a 1970 paper, Glashow, John Iliopoulos and Luciano Maiani gave more compelling theoretical arguments for the as-yet undiscovered charm quark. The number of supposed quark flavors grew to the current six in 1973, following proposition by Makoto Kobayashi and Toshihide Maskawa; the two had noted that the experimental observation of CP violation (a phenomenon that has been observed to cause changes in the way particles weakly interact when particle and antiparticle are swapped) could be explained if there were another pair of quarks. They named the two additional quarks top and bottom.

1974 discovery photograph of a possible charmed baryon, now identified as the
Σ⁺⁺
_c baryon

It was the observation of the charm quark that finally convinced the physics community of the quark model's correctness. Following a decade without empirical evidence supporting their existence, charm quarks were finally produced and observed almost simultaneously by two teams in November 1974 (see November Revolution): one at the Stanford Linear Accelerator Center under Samuel Ting and one at Brookhaven National Laboratory under Burton Richter. The two parties had assigned the discovered particle two different names, J and ψ. The particle hence became formally known as the J/ψ meson and it was considered a quark–antiquark pair of the charm flavor that Glashow and Bjorken had predicted, or the charmonium.

In 1977, the bottom quark was observed by Leon Lederman and a team at Fermilab. This indicated that a top quark probably existed, because the bottom quark would have been without a partner if it had not. However, it was not until eighteen years later, in 1995, that the top quark was finally observed. The top quark's discovery was quite important, because it proved to be significantly more massive than expected, almost as heavy as a gold atom. Reasons for the top quark's extremely large mass remain unclear.

Etymology

Gell-Mann originally named the quark after the sound ducks make. For some time, he 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:

Three quarks for Muster Mark!

Sure he has not got much of a bark

And sure any he has it's all beside the mark.

— James Joyce, Finnegans Wake

Gell-Mann went into further detail regarding the name of the quark in his book, The Quark and the Jaguar: Adventures in the Simple and the Complex:

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.

— The Quark and the Jaguar: Adventures in the Simple and the Complex , in x, x, Murray Gell-Mann

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.

Properties

Hadronization

Main article: Hadron

Various quark flavor combinations result in the formation of composite particles known as hadrons through the process of hadronization. There are two types of hadrons: baryons, formed of three quarks, and mesons, formed of a quark and an antiquark. The quarks (and antiquarks) which determines 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 their quantum numbers. Such virtual quarks are called sea quarks (see below).

The building blocks of the atomic nucleus—the proton and the neutron—are baryons. There are a great number of known hadrons, and most of them are differentiated by their quark content and the properties that these constituent quarks confer upon them. The existence of hadrons with more valence quarks, called exotic hadrons, such as the tetraquarks (
q

q

q

q
) and pentaquarks (
q

q

q

q

q
) has been postulated. Several experiments have been claimed to reveal the existence of tetraquarks and pentaquarks, in the early 2000s, but all the reported candidates have been established as being non-existent since.

Electric charge

Main article: Electric charge

A quark has a fractional electric charge value, either −1⁄3 or +2⁄3 (measured in elementary charges). Specifically, up, charm and top quarks (collectively referred to as up-type quarks) have a charge of +2⁄3 each, while down, strange and bottom quarks (down-type quarks) have −1⁄3. The antiquark have the opposite charge of their corresponding quark—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, the total is always an integer.

The electric charge of quarks is important in the construction of nuclei. The hadron constituents of the atom, the neutron and proton, 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. The total electric charge of a nucleus, that is, the number of protons in it, is known as the atomic number, and it is the main difference between atoms of different chemical elements.

Spin

Main article: Spin (physics)

Spin is a intrinsic property of quantum particles, whose direction is an important degree of freedom. Roughly speaking, the spin of a particle is a contribution to its angular momentum that is not due to its motion. It is sometimes visualized as the rotation of an object around its own axis (hence the name spin), but this description is somewhat misguided at subatomic scales, as elementary particles are believed to be point-like and so they cannot rotate around themselves.

Spin is measured in units of h/(2π), where h is the Planck constant. This unit is often denoted by ħ ("h bar"), and called the "reduced Planck constant". The result of a measurement of the component of the spin of a quark along any axis is always either ħ/2 or −ħ/2; for this reason quarks are classified as spin-1⁄2 particles, which means they are fermions. The component of spin along any given axis—by convention the z axis—is denoted by an up arrow ↑ for the value +1⁄2 and down arrow ↓ for −1⁄2, respectively, which follows the symbol for the flavor. For example, an up quark with a spin of +1⁄2 along the z axis is denoted by u↑.

The quark's spin value contributes to the overall spin of the parent hadron, much as quark's electrical charge does to the overall charge of the hadron. Varying combinations of quark spins result in the total spin value that can be assigned to the hadron. For example, the proton and the Delta baryon are both composed of two up quarks and one down quarks: in the
Δ⁺
their spins are all aligned in the same direction, yielding a total spin of 3⁄2, whereas in the proton one of them has the opposite direction, giving a total spin of 1⁄2.

Flavor quantum numbers

In order to explain the phenomenology of strong and weak interactions—readily distinguishable in experiments by their markedly different coupling strengths—particle physicists began to introduce quantum numbers to the known baryons and mesons. The first such quantum number is known as isospin, since its mathematical properties are the same as for ordinary spin, governed by the Lie group SU(2). It was introduced by Werner Heisenberg in 1932 as a way of expressing the remarkable similarity between protons and neutrons, and of the three versions of pions. Its z-component, commonly denoted I_z, is related to the electric charge Q and the baryon number (+1 for baryons, 0 for mesons) of these particles by

$Q=I_{z}+{\frac {1}{2}}B,$

and calculation of probabilities (cross sections) in part becomes a matter of simple group-theoretical calculations involving isospins.

An additional quantum number was introduced in 1954 to explain the unexpectedly long life-times of particles such as K mesons, Xi baryons and Omega baryons. Assigning a new quantum number, strangeness (S, not to be confused with spin), and taking this quantum number to be left unchanged by the strong, but not by the weak force, explained the anomalous life-times: the particles in question can be pair-produced by the strong force, but each particle can only decay via the weak force. The formula for the electric charge was modified to be

$Q=I_{z}+{\frac {1}{2}}(B+S)=I_{z}+{\frac {1}{2}}Y,$

where Y = =B + S is called the hypercharge. This relation is known as the (original) Gell-Mann–Nishijima formula. Again, with this scheme, the various allowed strong-force particle reactions and their relative probability can be determined using straightforward mathematics.

The connection with group theory was fully exploited when, in 1961, Gell-Mann and Ne'emann showed that all the proposed quantum numbers could be explained by assigning the particles in question to certain realizations of the group SU(3), which has since become known as the "flavor-SU(3)". It is a general result of group theory that such realizations, which mathematicians call representations, can be constructed from elementary building blocks known as basic multiplets. The physical counterpart of the basic SU(3) multiplet, a triplet, is the set of the lightest three quarks: up, down, and strange. This correspondence between quarks and the components of a basic SU(3) multiplet is the basis of much of the quark model's explanatory power.

Further advances in the phenomenology of weak interactions led to the introduction of a new flavor quantum number, charm, corresponding to a fourth quark and an enlarged flavor symmetry group SU(4) and, subsequently to two more quarks with their own flavor quantum numbers. All in all, there are the following flavor quantum numbers: All quarks have baryon number +1⁄3. Isospin I_z is +1⁄2 for up quarks, −1⁄2 for down quarks, and zero for all others. Strangeness is S = −1 for strange quarks and zero for all others, charm is non-zero, C = 1, only for charm quarks, topness is T = +1 only for top quarks, and bottmoness non-zero, B′ = −1 only for bottom quarks. The modified Gell-Mann–Nishijima formula

$Q=I_{z}+{\frac {1}{2}}(B+S+C+B^{\prime }+T)$

is the generalized relation of all the flavor quantum numbers to the electrical charge.

Weak interaction

Main article: Weak interaction

A pictorial representation of the six quarks' decay modes, with mass increasing from left to right.

A quark of one flavor can transform into a quark of a different flavor through the weak interaction, one of the four fundamental interactions through which all particles interact with each other. A quark can decay into a lighter quark by emitting a virtual W boson, or can absorb a virtual W boson to turn into a heavier quark. This mechanism causes the radioactive process of beta decay, in which a neutron "splits" into a proton, an electron and an antineutrino. This occurs when one of the down quarks in the neutron (composition
u

d

d
) decays into an up quark by emitting a virtual
W⁻
boson, transforming the neutron into a proton (composition
u

u

d
). The
W⁻
boson then decays into an electron (
e⁻
) and an electron antineutrino (
ν
_e). A quark can also emit or absorb virtual Z bosons.

Weak interactions can also allow quarks or hadrons to decay into completely different elementary particles through a process of annihiliation. Taking the pi meson (composition
u

d
), a decay into a corresponding quark–antiquark flavor pair such as
u

u
or
d

d
would result in an annihiliation of the quark–antiquark pair. The release of energy therein could effect the creation of the new leptons, such as muons or neutrinos.

As well as being the only interaction capable of causing flavor changes, the weak interaction is the only interaction violating parity symmetry. That is, the weak interaction is the only one which would not stay unchanged if left and right were swapped. It exclusively acts on left-handed quarks and leptons, and on right-handed antiquarks and antileptons.

Cabibbo angle and CKM matrix

Main article: Cabibbo–Kobayashi–Maskawa matrix

In 1963, Nicola Cabibbo introduced the Cabibbo angle (θ_c) to keep track of how often certain weak interaction decays occurred in nature. In light of current knowledge (quarks were not yet theorized), the Cabibbo angle is related to the probability that down and strange quarks decays into up quarks. In particle physics parlance, the d and s quarks were said to form a weak interaction eigenstate, a superposition of d and s quarks quantum states. Mathematically this can be represented as:

$|d^{\prime }\rangle =V_{ud}|d\rangle +V_{us}|s\rangle =\cos \theta _{c}|d\rangle +\sin \theta _{c}|s\rangle$

where d′ is the weak interaction eigenstate. |V_ud|² and |V_us|² represent the probability that d and s quarks decay into u quarks. Using the currently accepted values for |V_ud| and |V_us| (see below), the Cabbibo angle can be calculated using

$\tan \theta _{c}={\frac {|V_{us}|}{|V_{ud}|}}\approx 0.232\rightarrow \theta _{c}\approx 13.0^{\circ }$

The modern equivalent of the Cabibbo angle is the Cabibbo–Kobayashi–Maskawa matrix (or CKM matrix), developed by Makoto Kobayashi and Toshihide Maskawa in 1972. The CKM matrix keeps track of the weak decays of all six quarks.

${\begin{bmatrix}|d^{\prime }\rangle \\|s^{\prime }\rangle \\|b^{\prime }\rangle \end{bmatrix}}={\begin{bmatrix}V_{ud}&V_{us}&V_{ub}\\V_{cd}&V_{cs}&V_{cb}\\V_{td}&V_{ts}&V_{tb}\end{bmatrix}}{\begin{bmatrix}|d\rangle \\|s\rangle \\|b\rangle \end{bmatrix}}$

Currently, the best determination of the magnitude of the entries of the CKM matrix is approximately:

${\begin{bmatrix}|V_{ud}|&|V_{us}|&|V_{ub}|\\|V_{cd}|&|V_{cs}|&|V_{cb}|\\|V_{td}|&|V_{ts}|&|V_{tb}|\end{bmatrix}}\approx {\begin{bmatrix}0.974&0.226&0.004\\0.226&0.973&0.041\\0.009&0.041&0.999\end{bmatrix}}.$

Color charge

Main article: Color charge

All types of hadrons always have zero total color charge.

Quarks possess a property called color charge. Despite its name, this is not related to colors of visible light, like the property flavor is not related to tasting. There are three types of color charge, named blue, green and red; each of them is complemented by an anti-color: antiblue, antigreen and antired, respectively. Each quark carries a color, while each antiquark carries an anticolor.

The system of attraction and repulsion between quarks charged with any of the three colors is called strong interaction. The area of physics that studies strong interactions is called quantum chromodynamics (QCD). A quark charged with one color value can be bound with an antiquark carrying the corresponding anticolor, while three quarks all charged with different colors will similarly be bound together. In any other case, the resulting system will be unstable. Quarks obtain their color and interact in this way via force mediating particles known as gluons, a concept which is further discussed below.

The three color types play a role in the process of hadronization, which is is the process of hadron formation out of quarks and gluons. The result of two attracting quarks that form a stable quark–antiquark pair will be color neutrality: a quark with ξ color charge plus an antiquark of −ξ color charge will result in a color charge of 0, or "white" and in the formation of a meson. The combination of all three color charges will similarly result in the cancelling out of color charge, yielding the same "white" color charge and result in the formation of a baryon.

In modern particle physics, color interactions are directly related to symmetry groups known as gauge symmetries. Quantum chromodynamics corresponds to an SU(3) gauge symmetry (not to be confused with the SU(3) flavor symmetry), one of a class of symmetry-based physical theories known as Yang-Mills theories. From the non-trivial structure of the gauge group, it follows directly that quantum chromodynamics is a non-abelian gauge theory relating to an interaction where the carrier particles themselves carry non-zero charge. Each quark is in the basic triplet of SU(3), whose three components correspond to the three colors, red, green and blue. Gluons are in what is called the adjoint representation, which immediately leads to the fact of each gluon carrying one color and one anticolor. Also, the fact that colorless states (singlets, in the language of group theory), can be formed by combining three quarks, or one quark and an antiquark, follows directly from the basic group theory of SU(3).

Mass

Main article: Mass

There are two different terms used when describing 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 two values are typically very different in their relative size, for several reasons.

In a hadron most of the mass comes from the gluons that bind the constituent quarks together, rather than from the individual quarks; the mass of the quarks is almost negligible compared to the mass derived from the gluons' energy. While gluons are inherently massless, they possess energy, and it is this energy that contributes so greatly to the overall mass of the hadron (see "Mass in special relativity"). This is demonstrated by a common hadron—the proton. Composed of one
d
and two
u
quarks, the proton has an overall mass of approximately 938 MeV/c², of which the mass of three valence quarks contributes around 11 MeV/c², with the remainder coming from the quantum chromodynamics binding energy (QCBE) provided by sea quarks and gluons. This makes direct calculations of quark masses based on quantum chromodynamics quite difficult, and often unreliable, as quantum perturbation methods, which were very successful in quantum electrodynamics, fail most of the time in QCD.

Often, mass values can be derived after calculating the difference in mass between two related hadrons that have opposing or complementary quark components. For example, in comparing the proton to the neutron, where the difference between the two particles is one down quark to one up quark, the relative masses and the mass differences can be measured by the difference in the overall mass of the two hadrons.

The masses of most quarks were within predicted ranges at the time of their discovery, with the notable exception of the top quark, which was found to have a mass approximately equal to that of a gold nucleus, significantly heavier than expected. Various theories have been offered to explain this very large mass. The Standard Model posits that elementary particles derive their masses from the Higgs mechanism, related to the unobserved Higgs boson. Physicists hope that, in the next years, the detection of the Higgs boson in particle accelerators—such as the Large Hadron Collider—and the study of the top quark's interaction with the Higgs field might help answer the question.

Table of properties

The following table summarizes the key properties of the six known quarks:

Quark flavor properties

Name	Symbol	Mass (MeV/c2)	Q	J	Iz	S	C	B′	T	Antiparticle	Antiparticle symbol
First generation
Up	u	1.5 to 3.3	+2⁄3	1⁄2	1⁄2	0	0	0	0	Antiup	u
Down	d	3.5 to 6.0	−1⁄3	1⁄2	−1⁄2	0	0	0	0	Antidown	d
Second generation
Charm	c	1270+70 −110	+2⁄3	1⁄2	0	0	+1	0	0	Anticharm	c
Strange	s	104+26 −34	−1⁄3	1⁄2	0	−1	0	0	0	Antistrange	s
Third generation
Top	t	171200±2100	+2⁄3	1⁄2	0	0	0	0	+1	Antitop	t
Bottom	b	4200+170 −70	−1⁄3	1⁄2	0	0	0	−1	0	Antibottom	b

Q = electric charge, J = spin, I_z = isospin, S = strangeness, C = charmness, B′ = bottomness, T = topness.
Notation like 104+26
−34 denotes measurement uncertainty: the value is believed to be between 104 + 26 = 130 and 104 − 34 = 70, with 104 being the most likely value.

Color confinement and gluons

Main articles: Color confinement and Gluon

A key phenomenon called color confinement is thought to keep quarks within a hadron. This refers to a quark's inability to escape as a single particle from its parent hadron, thereby rendering impossible the actual observation of a single quark. Color confinement is primarily caused by interactions with the gluon color field and the gluon exchange between quarks.

Color confinement applies to all quarks, except for the case of the top quark where the actual escape mechanism at extremely high energies is still uncertain. Therefore, most of what is known experimentally about quarks has been inferred indirectly from the effects they have on their parent hadron's properties. The top quark is an exception because its lifetime is so short that it does not have a chance to hadronize. One method used is to compare two hadrons that have all but one quark in common. The properties of the differing quarks are then inferred from the difference in values between the two hadrons.

Quarks have an inherent relationship with the gluon, which is technically a massless vector gauge boson. Gluons are responsible for the color field, or the strong interaction, that ensures that quarks remain bound in hadrons and causes color confinement.

Gluons are constantly exchanged between quarks through a virtual emission and re-absorption process, with a timescale of the order of 10⁻²⁴ seconds. When a gluon is transferred between one quark and another, a color change occurs in the receiving and emitting quark; for example, if a red quark emits a red–antigreen gluon, it becomes green, and if a green quark absorbs it, it becomes red. Therefore, although the color of each quark is always changing, a bound hadron will constantly retain a set of colors that will preserve the force of attraction, therefore forever disallowing quarks to exist in isolation.

The color field carried by the gluon contributes most significantly to a hadron's indivisibility into single quarks, or color confinement. This is demonstrated by the varying strength of the chromodynamic binding force between the constituent quarks of a hadron; as quarks come closer to each other, the chromodynamic binding force actually weakens in a process called asymptotic freedom. However, when they drift further apart, the strength of the bind dramatically increases. The color field becomes stressed by the drifting away of a quark, much as an elastic band is stressed when pulled apart, and a proportionate and necessary multitude of gluons of appropriate color property are created to strengthen the stretched field. In this way, an infinite amount of energy would be required to wrench a quark from its hadronized state. In practice, as soon as enough energy has been spent to distance the quarks, a quark–antiquark pair would be produced so that two hadrons would exist at the end.

These strong interactions are highly non-linear, because gluons can emit gluons and exchange gluons with other gluons. This property has led to a postulate regarding the possible existence of a glueball—a particle that is purely made of gluons—despite previous observations indicating that gluons cannot exist without the 'attached' quarks.

Sea quarks

The quarks that contribute to the quantum numbers of the hadrons are called valence quarks (
q
_v). Hadrons also contain virtual quark–antiquark (
q

q
) pairs, known as sea quarks (
q
_s), originating from the gluons' strong interaction field. Such sea quarks are much less stable, and they annihilate each other very quickly within the interior of the hadron. When a gluon is split, sea quarks are formed, and this process also works in reverse in that the annihilation of two sea quarks will reproduce a gluon. In addition to this, sea quarks can hadronize into baryonic or mesonic particles under the right circumstances. There is a constant quantum flux of sea quarks that are born from the vacuum, and this allows for a steady cycle of gluon splits and rebirths. This flux is colloquially known as "the sea".

QCD matter and free quarks

Main article: QCD matter

A notion that has recently come to prominence is that of quark matter, or QCD matter, a number of theorized phases of matter containing free quarks and gluons. One of these is the quark-gluon plasma. This model posits that, at sufficiently high temperatures and densities, quarks and gluons could potentially become deconfined and degenerate into a fluid-like plasma consisting of a non-uniform mix of gluons and quarks. The precise extremity of the conditions needed to give rise to this state are unknown and have been the subject of a great deal of speculation; CERN made many attempts to produce such conditions in the 1980s and 1990's. The symptoms of the state would variously include a great increase in the number of "unnatural" quark pairs compared to the volume of up and down quark pairs. It is believed that, in the period between 10⁻¹² and 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. It is also hypothesized that strange matter, that is non-nuclear matter containing relatively equal numbers of up, down and strange quarks, might also be stable at "ordinary" temperatures and pressures, in atomic nucleus-sized strangelets and kilometer-sized quark stars.

See also

• Eightfold way (physics) – Gell-Mann's original classification of hadrons into octets.
• Gauge boson – Force mediating particles

• Gluons – Particles mediating the strong interaction
• W and Z bosons – Particles mediation the weak interaction

• Hadrons – Particles made of quarks

• Mesons – Particles made of a quark and an antiquark

• Quarkonium – Mesons made of a quark and antiquark of the same flavour

• Baryons – Particles made of three quarks

• Diquarks – A hypothetical state of two quarks grouped inside a baryon, treated as a single particle with which the third quark interacts

• Tetraquarks – Hypothetical exotic mesons, made of two quarks and two anti quarks
• Pentaquarks – Hypothetical exotic baryons, made of four quarks and two anti quarks

• Leptons – The other fundamental particles of matter
• Preons – Hypothetical particles which were once postulated to be subcomponents of quarks and leptons
• Quark–lepton complementarity
• Quark star – A hypothetical degenerate neutron star that is so dense it is considered a single giant hadron
• Strong interaction – The interaction responsible for the formation of hadrons

• Quantum chromodynamics – The study of the strong interaction

• Weak interaction – The interaction by which quarks decay into other quarks

Further reading

• D.J. Griffiths (1987). Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4.
• I.S. Hughes (1985). Elementary particles (2nd ed.). Cambridge University Press. ISBN 0-521-26092-2.
• A. Pickering (1984). Constructing Quarks: A Sociological History of Particle Physics. The University of Chicago Press. ISBN 0-226-66799-5.
• B. Povh (1995). Particles and Nuclei: An Introduction to the Physical Concepts. Springer-Verlag. ISBN 0-387-59439-6.

External links

• The Top Quark And The Higgs Particle by T. A. Heppenheimer – A description of CERN’s experiment to count the families of quarks.
• An elementary popular introduction
• Quark dance
• The original English word quark and its adaptation to particle physics
• 2008 Physics Nobel Prize lecture by Makoto Kobayashi
• 2008 Physics Nobel Prize lecture by Toshihide Maskawa
• 1976 Physics Nobel Prize lecture by Samuel C.C. Ting
• 1976 Physics Nobel Prize lecture by Burton Richter
• 1969 Physics Nobel Prize lecture by Murray Gell-Man