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

In physics, a quark ( /kwɔrk/ or /kwɑrk/) is a type of subatomic particle. [1] In technical terms, quarks are elementary fermions which engage in the strong interaction due to their color charge. [2] 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; [3] the most common hadrons are protons and neutrons, that 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
). [4] The flavors with least masses, the up quark and the down quark, are generally stable and are very common in the universe, as they are found in protons and neutrons and are two of the primary building blocks of matter. 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. [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 usually only be deduced from experiments on hadrons. [3] An exception to this is the top quark, which decays so rapidly that it does not hadronize at all, and instead is observed through the identification of the particles it has decayed into. [8] Furthermore, in some of the Big Bang theories, it has been postulated that in the very beginning, the extremely hot early 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 unobserved (as of 2008) Higgs boson. This model comprises six flavors 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] 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. 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 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, which are bosons, and have integer spin; as a consequence, the Pauli exclusion principle does not apply to them. [10] Quarks, unlike leptons (the best-known flavor of which is the electron), have a color charge, a property causing them to engage in the strong interaction, which binds them 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 more than three generations can not exist. [note 1] [11] 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 occur commonly in nature; heavier quarks can only be created in high-energy conditions, such as in cosmic rays, and quickly decay. As a result, these particles play little part in the universe of today, but would likely have been 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. [12]

Antiparticles of quarks are called antiquarks, and 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 and spin of their respective quarks, but the electric charge and other charges have the opposite sign. [13]

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 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 theory was first postulated independently by physicists Murray Gell-Mann and George Zweig in 1964. [5] At the time of the theory's initial proposal, the " particle zoo", the list of particles considered to be indivisible, consisted of several leptons and numerous hadrons. Gell-Mann and Zweig posited that hadrons were not elementary particles, but instead composed of various combinations of quarks and antiquarks. [14]

The Gell-Mann–Zweig model predicted three quarks, which they named up, down and strange. [15] At the time, the pair of physicists ascribed various properties and values to the three new proposed particles, such as electric charge and spin. [16] 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. [14]

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). [17]

In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center showed that the proton had substructure. [18] [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), [19] [20] but it "was unfashionable to identify them explicitly with quarks". [21] These partons were later identified as up and down quarks when the other flavors were beginning to surface. [22] Their discovery also validated the existence of the strange quark, because it was necessary to the model Gell-Mann and Zweig had proposed. [23]

In a 1970 paper, [24] Glashow, John Iliopoulos, and Luciano Maiani gave more compelling theoretical arguments for the as-yet undiscovered charm quark. [25] 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. [16]


It was the observation of the charm quark that finally convinced the physics community of the quark model's correctness. [21] 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. [14]

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. [26]

Etymology

Gell-Mann originally named the quark after the sound ducks make. [27] 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.

— Murray Gell-Mann, The Quark and the Jaguar: Adventures in the Simple and the Complex [28]

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. [29]

Quarks in hadrons

Quarks are not normally found in isolation; they are usually bound by the strong interaction into composite particles known as hadrons. There are two types of hadrons: baryons, which get their quantum numbers from three quarks, and mesons, which get their quantum numbers from a quark and an antiquark. The quarks (and antiquarks) which impart quantum numbers to hadrons are called valence quarks. Hadrons also contain an indefinite number of virtual quark–antiquark (
q

q
) pairs, which together contribute nothing to their quantum numbers. These virtual quarks are known as sea quarks (
q
s).

The building blocks of the atomic nucleus—the proton and the neutron—are baryons. [30] 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, such as the tetraquarks (
q

q

q

q
) and pentaquarks (
q

q

q

q

q
) has been postulated. [31] Several experiments have been claimed to reveal the existence of tetraquarks and pentaquarks, [32] in the early 2000s, but all the reported candidates have been established as being non-existent since. [33]

Color charge and strong interaction

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

The force keeping quarks together in hadrons is the strong interaction, studied by quantum chromodynamics. It acts on particles, such as quarks, which carry color charge, a property which, despite its name, 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. 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] The result of two attracting quarks that form a bound quark–antiquark pair (a meson) 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 three quarks with different color charges (a baryon) will similarly result in the cancelling out of color charge, yielding the same "white" color charge. [36]

The force carrier particle mediating the strong interaction is the gluon, which is technically a massless vector gauge boson. [37] Gluons, roughly speaking, carry both a color charge and an anti-color charge, for example red–antiblue (but, for technical reasons, there are eight possible combinations, not nine). [38] [39] Because gluons themselves also have color charges, they can in turn emit gluons and exchange gluons with other gluons. For this reason, strong interactions are highly non-linear. 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. [40]

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. [41] When a gluon is transferred between one quark and another, a color change occurs in the receiving and emitting quark; [42] [43] for example, if a red quark emits a red–antigreen gluon, it becomes green, and if a green quark absorbs it, it becomes red. [44] 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. [45]

Sea quarks

Sea quarks originate from the gluons' strong interaction field; 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. [46] In addition to this, sea quarks can hadronize into baryonic or mesonic particles under the right circumstances. [47] 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". [48]

Asymptotic freedom and color confinement

Main articles: Asymptotic freedom and Color confinement

The strength of strong interactions varies with the distance between quarks in a particular way: as quarks come closer to each other, it 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. [49] This feature of strong interactions is called color confinement.

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. [50] [51] 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.

The top quark is an exception because its lifetime is so short that it does not have a chance to hadronize. [8]

Other properties

Weak interaction

Main article: Weak interaction
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). [52] A quark can also emit or absorb Z bosons.

Weak interaction can also allow quarks or hadrons to decay into completely different elementary particles through a process of annihiliation. Taking the
π+
 meson, of
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. [53]

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). 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 charge of an antiquark is the negative of the charge of the corresponding quark. The electric charge of a hadron is the sum of the charges of the constituent quarks; [54] 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. [55]

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. 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. [56] 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↑. [57]

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. [58] 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 notion 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. [59]

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. [60] 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. [42] [61] 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, fail most of the time in chromodynamics.

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. [42]

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. [62] Various theories have been offered to explain this very large mass. Since in the Standard Model it is theorized that elementary particles get their masses from the Higgs mechanism, it is hoped 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. [26]

Table of properties

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

Quark flavor properties [63]
Name Symbol Gen. Mass ( MeV/c2) Iz J Q S C B′ T Antiparticle Antiparticle symbol
Up
u
 		1st 1.5 to 3.3 1⁄2 1⁄2 +2⁄3 0 0 0 0 Antiup
u
Down
d
 		1st 3.5 to 6.0 −1⁄2 1⁄2 −1⁄3 0 0 0 0 Antidown
d
Charm
c
 		2nd 1270+70
−110 		0 1⁄2 +2⁄3 0 +1 0 0 Anticharm
c
Strange
s
 		2nd 104+26
−34 		0 1⁄2 −1⁄3 −1 0 0 0 Antistrange
s
Top
t
 		3rd 171200±2100 0 1⁄2 +2⁄3 0 0 0 +1 Antitop
t
Bottom
b
 		3rd 4200+170
−70 		0 1⁄2 −1⁄3 0 0 −1 0 Antibottom
b
Gen. = generation, Iz = isospin, J = spin, Q = electric charge, 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.


Notes

  1. ^ Each generation comprises exactly one flavor of neutrino, 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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