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For other uses, see Quark (disambiguation).
A proton, composed by two up quarks and one down quark.

In physics, a quark (/kwɔrk/, /kwɑːk/ or /kwɑːrk/) is a type of subatomic particle.[1] In technical terms, quarks are elementary fermions which strongly interact due to their color charge.[2] Because of the phenomenon of color confinement, quarks are never found on their own in nature: they are always bound together in composite particles named hadrons.[3] The most common hadrons are the proton and the neutron, which are the components of 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
).[4] The lightest flavors, the up quark and the down quark, are generally stable and are very common in the universe, as they are the constituents of protons and neutrons. The more massive charm, strange, top and bottom quarks are unstable and rapidly decay; these can only be produced under high energy conditions, such as in particle accelerators and in cosmic rays. 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.[5] There was little evidence for the theory until 1968, when electron–proton scattering experiments indicated the existence of substructure within the proton resembling three "sphere-like" regions within the proton.[6][7] By 1995, when the top quark was observed at Fermilab, all the six flavors had been observed.

Since quarks are not found in isolation, their properties can only be deduced from experiments on hadrons.[3] An exception to this is the top quark, which decays so rapidly that it does not produce hadrons at all, and instead is observed through the identification of the particles it has decayed into.[8] Furthermore, it has been theorized in some of the Big Bang theories, that in the very beginning, our extremely hot universe may have contained single quarks, including "free" top quarks, in a quark-gluon plasma.

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 as-yet-unobserved Higgs boson. This model comprises six flavors[note 1] of quarks,[9] named up, down, charm, strange, top and bottom; the top and bottom flavors are also known as truth and beauty, respectively.[3] Any two 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, are classified as 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, which are bosons: that is, they have integer spin, and hence the Pauli exclusion principle does not apply to them. Among elementary fermions, quarks differ from leptons (the best-known flavor of which is the electron), in that they, unlike leptons, have a color charge, a property causing them to engage in strong interaction, the force keeping quarks bound together in hadrons.

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 there cannot exist more than three generations.[note 2][10] 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 interaction. Typically, only the first-generation up and down quarks are in common natural occurrence; heavier quarks can only be created in high-energy conditions, such as in cosmic rays, and quickly decay. 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 denoted by a bar over the letter for the quark, such as
u
 for an up antiquark. Like antimatter in general, antiquarks have the same mass and spin of their respective quarks, but the electric charge and other charges have the opposite sign.[11]

Having electric charge, flavor, color charge and mass, quarks are the only known elementary particles engaging in all the four fundamental interactions of contemporary physics: respectively, electromagnetism, weak interaction, strong interaction and gravitation. The last 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. Nobel laureates Gell-Mann and George Zweig first proposed the quark model in 1964.

The quark theory was first postulated by physicists Murray Gell-Mann and George Zweig in 1964.[5] At the time of the theory's initial proposal, the "particle zoo" consisted of several leptons and many different hadrons. Gell-Mann and Zweig developed the quark theory to explain the hadrons; they proposed that various combinations of quarks and antiquarks were the components of the hadrons, which were at the time considered to be indivisible.[12]

The Gell-Mann–Zweig model predicted three quarks, which they named up, down and strange. At the time, the pair of physicists ascribed various properties and values to the three new proposed particles, such as electric charge and spin.[13] The initial reaction of the physics community to the proposal was mixed, many having reservations regarding the actual physicality of the quark concept. They believed the quark was merely an abstract concept that could be temporarily used to help explain certain concepts that were not well understood, rather than an actual entity that existed in the way that Gell-Mann and Zweig had envisioned.[12]

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.[14]

In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center showed that the proton had substructure.[15][6][7] However, whilst 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),[16][17] but it "was unfashionable to identify them explicitly with quarks".[18] These partons were later[when?] identified as up and down quarks.[19] Their discovery also validated the existence of the strange quark, because it was necessary to the model Gell-Mann and Zweig had proposed.[20]

In a 1970 paper,[21] Glashow, John Iliopoulos, and Luciano Maiani gave more compelling theoretical arguments for the as-yet undiscovered charm quark.[22] The number of proposed 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. They named the two additional quarks top and bottom.[13]

It was the observation of the charm quark that finally convinced the physics community of the quark model's correctness.[18] Following a decade without empirical evidence supporting the flavor's existence, it was created and observed almost simultaneously by two teams in November 1974: 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.[12]

In 1977, the bottom quark was observed by Leon Lederman and a team at Fermilab.[5] This indicated that a top quark probably existed, because the bottom quark was without a partner. However, it was not until eighteen years later, in 1995, that the top quark was finally observed. The top quark's discovery was quite significant, 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.[23]

Etymology

Gell-Mann originally named the quark after the sound ducks make.[24] 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:

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 [25], in x, x, Murray Gell-Mann

George Zweig, the co-proposer of the theory, 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.[26]

Properties

Hadronization

Various quark flavor combinations result in the formation of composite particles known as hadrons. There are two types of hadrons: baryons, made of three quarks and mesons made of a quark and an antiquark. (These numbers refer to "valence quarks", the quarks determining the charges of the hadron. Hadrons also contain an indeterminate number of virtual pairs of quarks and their respective antiquarks, called "sea quarks" (see below), whose charges cancel each other.)

The building blocks of the atomic nucleus—the proton and the neutron—are baryons.[27]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.[3]

The existence of hadrons with more valence quarks, called exotic hadrons, has been postulated. Several experiments since 2003 have been claimed to reveal a pentaquark, composed by four quarks and an antiquark (
u

u

d

d

s
), but other similar experiments yielded null results. Other examples include tetraquarks (with two quarks and two antiquarks) and dibaryons (with six quarks); some detected particles have been suggested as candidates to being identified as tetraquarks and dibaryons, but these have not be confirmed.

Weak interaction

Main article: Weak interaction
File:Quarks and decays.png
A pictorial representation of the six quarks' most likely decay modes, with mass increasing from left to right.

A quark of one flavor can transform into a quark of a different flavor by the weak interaction. A quark can decay into a lighter quark by emitting a W boson, or can absorb a 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 (composed by
u

d

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

u

d
). The
W−
 boson then decays into an electron (
e−
) and an electron antineutrino (
ν
e).[28] A quark can also emit or absorb a Z boson.

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 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.

Electric charge

Main article: Electric charge

A quark has a fractional electric charge value, either −1/3 or +2/3 (measured in elementary charges); correspondingly, the charge of an antiquark can be either +1/3 or −2/3. The up, charm and top quarks all have charge of +2/3, while the down, strange and bottom quarks have −1/3. The electric charge of a hadron is the sum of the charges of the constituent quarks;[29] 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.[30]

Spin

Main article: Spin (physics)

The term spin denotes 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. However, this description is incorrect, 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, 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.[31] 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 positive spin of 1/2 along the z axis is denoted by u↑.[32]

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.[33] For example, the proton and the
Δ+
 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. However, this view has been recently challenged in quantum chromodynamics by theories that include vacuum polarization and the coupling of quark hadrons to strange quarks in the vacuum.[citation needed][clarification needed]

Color charge

Main article: Color charge
All types of hadrons always have zero total color charge.

In addition to the electric charge, quarks possess a property called color charge. Despite its name, this is not related to colors of visible light.[34] There are three types of color charge a quark can carry, named blue, green and red; each of them is complemented by an anti-color: antiblue, antigreen and antired, respectively. While a quark can have red, green or blue charge, an antiquark can have antired, antigreen, or antiblue charge.

In quantum chromodynamics, the system of attraction and repulsion between quarks charged with any of the three colors is called strong interaction. A quark charged with one color value will be attracted to an antiquark carrying the corresponding anticolor, while three quarks all charged with different colors will similarly be forced together. In any other case, a force of repulsion will come into effect.[35] Quarks undergo such color interactions via the exchange of quantum field carrier 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 n color charge plus an antiquark of −n color charge will result in a color charge of 0, or "white". The combination of all three color charges will similarly result in the cancelling out of color charge, yielding the same "white" color charge.

These two methods of color neutral hadronization are the same as the two ways in which all hadrons are formed. Hence, all hadrons will be color neutral. A meson, comprised of two particles, is the result of the binding of a quark and antiquark that have opposite color charges, whereas a baryon, containing three particles, arises from the hadronization of three quarks, all charged with different colors.[36]

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.[37] 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 three valence quarks contribute around 10 MeV/c2, with the remainder coming from the quantum chromodynamics binding energy (QCBE) provided by sea quarks and gluons.[38][39] This makes direct calculations of quark masses based on quantum chromodynamics quite difficult, and often unreliable, as quantum perturbation methods, that were very successful in quantum electrodynamics, now fail most of the time.

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.[38]

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 it was expected.[40] Various theories have been offered to explain this very large mass. Common predictions assert that the answer to the abnormality will be found when more is known about the top quark's interaction with the Higgs (boson) field, and how the Higgs boson field adds very heavily to the total mass, and might also bring about the very existence of mass.[23]

Table of quark properties

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

Quark flavor properties[41]
Name Symbol Gen. Mass (MeV/c2) I J Q S C B′ T Antiparticle Antiparticle symbol
Up
u
 		1st 000002.81.5 to 3.3 1/2 1/2 +2/3 0 0 0 0 Antiup
u
Down
d
 		1st 000004.83.5 to 6.0 1/2 1/2 −1/3 0 0 0 0 Antidown
d
Charm
c
 		2nd 001270.01270+70
−110 		0 1/2 +2/3 0 +1 0 0 Anticharm
c
Strange
s
 		2nd 000104.0104+26
−34 		0 1/2 −1/3 −1 0 0 0 Antistrange
s
Top
t
 		3rd 171200.0171200±2100 0 1/2 +2/3 0 0 0 +1 Antitop
t
Bottom
b
 		3rd 004200.04200+170
−70 		0 1/2 −1/3 0 0 −1 0 Antibottom
b

(Key: Gen. = generation, I = isospin, J = spin, Q = electric charge, S = strangeness, C = charm, B′ = bottomness, T = topness. Notation like 104+26
−34 denotes measurement uncertainty: the value is between 104 + 26 = 130 and 104 − 34 = 70, with 104 being the most likely value.)

Color confinement and gluons

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.[42][43] The top quark is an exception because its lifetime is so short that it does not have a chance to hadronize.[8] 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, and are the subjects of the quantum chromodynamics research area.[44] Gluons, roughly speaking, carry both a color charge and an anti-color charge, for example red–antiblue.[45][46]

Gluons are constantly exchanged between quarks through a "virtual" emission and re-absorption process. These gluon exchange events between quarks are extremely frequent, occurring approximately 1024 times every second.[47] When a gluon is transferred between one quark and another, a color change occurs in the receiving and emitting quark;[38][48] for example, if a red quark emits a red–antigreen gluon, it becomes green, and if a green quark absorbs it, it becomes red.[49] These constant switches in color within quarks are mediated by the gluons in such a way that a bound hadron will constantly retain a dynamic and ever-changing set of color types that will preserve the force of attraction, therefore forever disallowing quarks to exist in isolation.[50]

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.[51] 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. However, this would violate the de Broglie quantum mechanism as applied to gluon color fields, because the "pure particles" with no associated 'wave' would then be the only exception to the wave-particle duality, or de Broglie model, which is known to be applicable to all other quantum particles.[clarification needed] The glueball postulate thus amounts to denying the existence of gluon color fields and of the color confinement mechanism discussed above.[52]

Sea quarks

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

q
) pairs, known as sea quarks, 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. They are thought to be born from the splitting of a gluon, and thus when a sea quark is annihilated, new gluons are produced.[53] 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".[54]

Notes

  1. ^ In this context, the term flavor simply refers to different types of particles, and is unrelated with the everyday concept of flavors of food. See Knowing (2005).
  2. ^ Each generation comprises exactly one flavor of neutrinos, and if there were more than three neutrino flavors, the abundance of helium-4 produced in Big Bang nucleosynthesis would be greater, and the lifetime of the Z boson would be shorter, than what is observed. See Barrow (1994).

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