Quark | Home = to Symsite |= Atomic Model Views |<= span style=3D'mso-spacerun:yes'> Quantum Physics <= /span>|

From Wikipe= dia, the free encyclopedia

3D"Text A quark (pronounced /ˈkwɔrk/ or= /ˈkwɑrk/) is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, th= e most stable of which are protons and neutro= ns, the components of atomic nuclei.^[1] Due t= o a phenomenon known as color confinement, quarks are never fou= nd in isolation; they can only be found within hadrons.^[2][3] For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.

There are six types of qu= arks, known as flavors: up, down<= /a>, cha= rm, strange, top<= /a>, and [= 4] Up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change in= to up and down quarks through a process of particle decay: the transformation from a higher mass state to a lower mass stat= e. Because of this, up and down quarks are generally stable and the most commo= n in the univ= erse, whereas charm, strange, top, and bottom quarks can only be produced in high energy collisions (such as those inv= olving co= smic rays and in particle accelerators).

Quarks have various intri= nsic properties, including electric charge, c= olor charge, spin, and mass. Quarks are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known = as fundamental forces (electromagnetism, gravi= tation, strong interaction, and weak interaction), as well as the only known particles whose electric charges are not in= teger multiples of the elementary charge. For every quark flavor t= here is a corresponding type of antiparticle, known as antiquark, that diff= ers from the quark only in that some of its properties have equal magnitude but opposite sign.

Classification

See also: Standard Model

Six of the particles in the Standard Model are quarks (shown in purple). Each of the first three columns for= ms a generation of matter.<= /o:p>

The Standard Model is the theoretical framework describing all the currently known elementary particles, as well as the unob= served^{[nb 1]} = Higgs boson.^[8] This = model contains six flavors of quarks (q), named up (u= ), down<= /a> (d), = charm (c), strange (s), top (t), and bottom (b).^[=
4] = 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 = antimat= ter 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.^[9]<= /o:p>

Quarks are s= pin-¹⁄₂ particles, implying= that they are f= ermions according to the spin-statistics theorem. They are sub= ject to the Pauli exclusion principle, which st= ates that no two identical fermions can simultaneously occupy the same quantum state. This is in contrast to bosons (particles with integer spin), of which any number= can be in the same state.^[10] Unl= ike leptons, qu= arks possess color charge, which causes them to engage in the strong interaction. The resulting attracti= on between different quarks causes the formation of composite particles known = as hadrons= .

= &nb= sp; = &nb= sp; = &nb= sp; (see "Strong interaction and color charge" below)

The quarks which determin= e the quantum numbers of hadrons are called valence quarks; apart from these, = any hadron may contain an indefinite number of virtual (or sea) quarks, antiquarks, and gluons which do not influence its quantum numbers.^[11] There are two families of hadrons: baryons, with three valence quarks, and mesons, with a valence quark and an antiquark.^[12] The= most common baryons are the proton and the neutron, the building blocks of the atomic nucleus.^{[13] 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 qu=
arks,
such as tetraquarks
(qqqq) and pentaquarks
(qqqqq), has been conjectured^[14=
]
but not proven.^{[nb 2][14=
][15]}}

Elementary fermions are g= rouped into three generations, each comprising two leptons and two quarks. The first generation includes up and down quarks, t= he second charm and strange quarks, and the third top and bottom quarks. All searches for a fourth generation of quarks and other elementary fermions ha= ve failed,^[16] and= there is strong indirect evidence that no more than three generations exist.^{[nb
3][17]<=
/sup>
Particles in higher generations generally have greater mass and lesser
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 t=
he Big Bang<=
/a>,
when the universe was in an extremely hot and dense phase (the qua=
rk epoch).
Studies of heavier quarks are conducted in artificially created conditions,
such as in particle accelerators.^[18]}

Having electric charge, m= ass, color charge, and flavor, quarks are the only known elementary particles th= at engage in all four fundamental interactions of contempor= ary physics: electromagnetism, gravitation, strong interaction, and weak interaction.^{[13]
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 at TED in 2007. Gell-Mann and George Zweig proposed the quark model in 1964.

The quark model was indep= endently proposed by physicists Murray Gell-Mann^[19]<=
/a> and George Zweig^[20][21] in 1964.[5] 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.^[22] Phy= sicist Yuval Ne'eman had independently developed a scheme similar to the Eightfold W= ay in the same year.^[23][24]

At the time of the quark = theory's inception, the "particle zoo" included, amongst other parti= cles, a multitude of hadrons. Gell-Mann and Zweig posited that they were not elementary particles, but we= re instead composed of combinations of quarks and antiquarks. Their model invo= lved three flavors of quarks—up, down, and strange—to which they ascribed properties such as spin and electric charge.^{[19]<=
/a>[20][21]} The initial reaction of the physics community to the proposal was mixed. Th= ere was particular contention about whether the quark was a physical entity or = an abstraction used to explain concepts that were not properly understood at t= he time.[25]

In less than a year, exte= nsions to the Gell-Mann–Zweig model were proposed. Sheldon Lee Glashow and James Bjorken predicted the existence of a fourth flavor of quark, which they called charm. The addition was proposed because it allowed for a bet= ter 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 masse= s of the known meso= ns.^[26]

In 1968, deep inelastic scattering experimen= ts at the Stanford Linear Accelerator Ce= nter (SLAC) showed that the proton contained much smaller, point-like objects and was therefore not an elementary particle.^[6][7]=
[27] Physicists were reluctant to identify these objects with quarks at the time, instead calling them "partons"—a term coined b= y Richard Feynman.^{[28][29][30]=} The objects that were observed at the SLAC would later be identified as up = and down quarks as the other flavors were discovered.^[31] Nevertheless, "parton" remains in use as a collective term for the constituents of hadrons (quarks, antiquarks, and gluons).=

The strange quark's exist= ence was indirectly validated by the SLAC's scattering experiments: not only was it a necessary component of Gell-Mann and Zweig's three-quark model, but it prov= ided an explanation for the kaon (K) and pion (π) hadrons discovered in cosmic rays in 1947.^=
[32]

In a 1970 paper, Glashow,= John Iliopoulos and Luciano Maiani presented further reasoning for= the existence of the as-yet undiscovered cha= rm quark.^[33][34] The number of supposed quark flavors grew to the current six in 1973, when Makoto Kobayashi and Toshihide Maskawa noted that the experimental observation of CP violation^{[nb 4][35]} = could be explained if there were another pair of quarks.

Photograph of the event that led to the discovery of = the Σ++<= /span>c baryo= n, at the Brookhaven National Laboratory= in 1974

Charm quarks were produce= d almost simultaneously by two teams in November 1974 (see November Revolution)—one = at the SLAC under Burton Richter, and one at Brookhaven National Laboratory= under Samuel Ting. The charm quarks were observed= bou= nd 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 qua= rk model's validity.^[30]=

In the following years a = number of suggestions appeared for extending the quark model to six quarks. Of the= se, the 1975 paper by Haim Harari^{[36]
was the first to coin the terms top=
and
bottom
for the additional quarks.^[37]}

In 1977, the bottom quark= was observed by a team at Fermilab led by Leon Lederman.^[38][39] Thi= s was a strong indicator of the top quark's existence: without the top quark, the bottom quark would have been without a partner. However, it was not until 1= 995 that the top quark was finally observed, also by the CDF^{[40]<=
/sup>
and DØ^{[41]
teams at Fermilab.^{[5]
It had a mass much greater than had been previously expected^[42]—almost
as great as a go=
ld
atom.^[43]}}}

Etymology

Gell-Mann originally name= d the quark after the sound made by ducks.^[44] For= some time, he was undecided on an actual spelling for the term he intended to co= in, until he found the word quark in Jam= es Joyce's book Finnegans Wake:

The quark flavors were gi= ven their names for a number of reasons. The up and down quarks are named after= the up and down components of isospin, which they carry.^{[48]
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.^[49]=
Glashow, who coproposed charm quark with Bjorken, is quoted as saying, &quo=
t;We
called our construct the 'charmed quark', for we were fascinated and please=
d by
the symmetry it brought to the subnuclear world."^[50] The=
names
"top" and "bottom", coined by Harari, were chosen becau=
se
they are "logical partners for up and down quarks".^{[36][37][49]=}
In the past, top and bottom quarks were sometimes referred to as
"truth" and "beauty" respectively, but these names have
mostly fallen out of use.^[51]}

Properties

Electric charge

See also: Electric charge

Quarks have fractional electric charge values—either −¹⁄₃ or +²⁄₃ times the elementary charge, depending on flavor. Up, charm, and top quarks (collectively referred to as up-type quarks) have a charge of += ²⁄₃, while down, strange, and bottom quarks (down-type quarks) have ͨ= 2;¹⁄₃. Antiquarks have the op= posite charge to their corresponding quarks; up-type antiquarks have charges of &#= 8722;²⁄₃ and down-type antiquark= s have charges of +¹⁄₃. Sinc= e the electric charge of a hadron is the sum of the charges of the constituent quar= ks, 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.^[52] For example, the hadron constituents of atomic nuclei, neutrons and protons, ha= ve 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.^[13]

Spin

See also: Spin (physics)

Spin is an intrinsic prop= erty 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 elemen= tary particles are believed to be point-like.^[53]

Spin can be represented b= y a vector whose length is measured in units of the reduced Planck constant ħ (pronounced "h bar"). For quarks, a measurement of the spin vecto= r component along any axis can only yield the values +ħ/2 or −= 5;/2; for this reason quarks are classified as s= pin-¹⁄₂ particles.^[54] 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↑.^[55]

Weak interaction

Main article: Weak interaction

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 qua= rk (down, strange, and bottom quarks) and vice versa. This flavor transformati= on 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 virtual
W⁻ boson, transforming the neutron into a proton (uud). Th=
e W⁻
boson then decays into an electron and an electron antineutrino.^[56]}

Both beta decay and the i= nverse process of inverse beta decay are routinely used = in medical applications such as positron emission tomography (PE= T) and in high-energy experiments such as neutrino detection.

The strengths of the weak interactions between the six qua= rks. The "intensities" of the lines are determined by the elements of = the CKM m= atrix.

While the process of flav= or 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–Kobaya= shi–Maskawa matrix (CKM matrix). The approximate magnitudes of the entries of the CKM matrix are:^[57]

where V_ij represents the tendency of a quark of flavor i to change into a quar= k of flavor j (or vice versa).^{[nb 5]}

There exists an equivalen= t weak interaction matrix for leptons (right side of the W boson on the above beta decay diagram), called the Pontecorvo–Maki–Nakagawa–Sakata mat= rix (PMNS matrix).^[58] Tog= ether, the CKM and PMNS matrices describe all flavor transformations, but the links between the two are not yet clear.^[59]

Strong interaction and color charge=

See also: C= olor charge and Strong interaction

All types of hadrons have zero total color charge.

Quarks possess a property= called c= olor charge. There are three types of color charge, arbitrarily labeled blue, = green, and red.^{[nb 6]} E= ach of them is complemented by an anticolor—antiblue, antigreen, and antired. Every quark carries a color, while every antiquark carr= ies an anticolor.^[60]

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 charged with one color value can form a bou= nd system with an antiquark carrying the corresponding anticolor; three (anti)quarks, one of each (anti)color, will similarly be bound together. The result of two attracting quarks will be color neutrality: a quark with color charge ξ plus an antiquark with color charge −ξ will result in a color charge of 0 (or "white" color) and the formation of a meson. Analogous to the additive color model in basic optics, the combination of three quarks or three antiqua= rks, each with different color charges, will result in the same "white" color charge and the formation of a baryon or antibaryon.^[61]

In modern particle physic= s, 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 rela= tes the color charge in quarks and is the defining symmetry for quantum chromodynamics.^[6=
2] 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 f_{B<=
/sub>,
f_G, f_R corresponding to the quark color=
s,^[63]
forms a triplet: a three-component quantum
field which transforms under the fundamental representation of SU(3)_c.^[64]
The requirement that SU(3)_c should be local—that is, that =
its
transformations be allowed to vary with space and time—determines the
properties of the strong interaction, in particular the existence of eight
gluon types to act as its force carriers.^[6=
2][65]}

Mass

See also: Mass

Two terms are used in ref= erring to a quark's mass: current quark mass refers to the mass of a = quark by itself, while constituent quark mass refers to the cu= rrent quark mass plus the mass of the gluon particle field surrounding the quark.^[66] 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 poss= ess energy—more specifically, quantum chromodynamics bind= ing 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 11 MeV/c²; much= of the remainder can be attributed to the gluons' QCBE.^[67]=
[68]

The Standard Model posits= that elementary particles derive their masses from the Higgs mechanism, which is related to the unobserved Hig= gs boson. Physicists hope that further research into the reasons for the top quark's large mass, which was found to be approximately equal to that of a gold nuc= leus (~171 GeV/c²),^[67]=
[69] mig= ht reveal more about the origin of the mass of quarks and other elementary particles.^[70]

Interacting quarks

See also: Color confinement and Gluon

As described by quantum chromodynamics, the strong interaction between quarks is media= ted by gluons, massless vector gau= ge 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 co= lor constantly changes, their strong interaction is preserved.^[71][72][73]

Since gluons carry color = charge, they themselves are able to emit and absorb other gluons. This causes asymptotic freedom: as quarks come clo= ser to each other, the chromodynamic binding force between them weakens.^[74] 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 spontaneo= usly created to strengthen the field. Above a certain energy threshold, pairs of quarks and antiquarks are created. These pairs bind with the quarks b= eing separated, causing new hadrons to form. This phenomenon is known as color confinement: quarks never appear in isolation.^[72][75] This process of hadronization occurs before quarks formed in a hi= gh energy collision are able to interact in any other way. The only exception = is the top quark, which may decay before it hadronizes.^[76]=

Sea quarks

Hadrons, along with the <= i>valence quarks (q_v) that contribute to their quantum numbers, contain virtual quark–antiquark (qq) pairs kno= wn as sea quarks (q_s). Sea quarks form when a gluon of the hadron's co= lor field splits; this process also works in reverse in that the ann= ihilation of two sea quarks produces a gluon. The result is a constant flux of gluon splits and creation colloquially known as "the sea".^[77] 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 cer= tain circumstances.^[78]

Other phases of quark matter

Main article: QCD m= atter

A qualitative rendering of the phase diagram of quark matter. The precise details of the diagram are the sub= ject of ongoing research.^[79][80]

Under sufficiently extreme conditions, quarks may become deconfined and exist as free particles. In the course of asymptotic freedom, the strong interaction becomes weaker at higher temperatures. Eventually, color confinement would = be lost and an extremely hot plasma of freely moving quarks and gluons would be formed. This theoretical phase = of matter is called quark–gluon plasma.^[81] The exact conditions needed to give rise to this state are unknown and have been the subject of a great deal of speculation and experimentation. A rece= nt estimate puts the needed temperature at 1.90±0.02×10^{12 kelvins.^[82]
While a state of entirely free quarks and gluons has never been achieved
(despite numerous attempts by CERN in the 1980s and 1990s),^[83] rec=
ent
experiments at the Relativistic Heavy Ion Collider
have yielded evidence for liquid-like quark matter exhibiting "nearly
perfect" fluid motion.^[84]}

The quark–gluon pla= sma would be characterized by a great increase in the number of heavier quark p= airs 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<= /a> (the = quark epoch), the universe was filled with quark–gluon plasma, as the temperature was too high for hadrons to be stable.^[85]

Given sufficiently high b= aryon 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 Coo= per 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.^[86]

See also: = &nb= sp; | Home to Symsite > Quantum Physics | Home to Symsite |

Color–flavor locking

Qu= arkonium – Mesons made of a quark and antiquark of the same flavor <= /o:p>

Preons &= #8211; Hypothetical particles which were once postulated to be subcomponents = of quarks and leptons

Quark–lepton complementarity – Possible fundamental relation between quar= ks and leptons

= Quark star – A hypothetical degenerate neutron star with extreme density

Notes

^ As of August 2010^[update].

^ Several research groups claimed to have proven the existence of tetraquarks and pentaquarks in the early 2000s. While the status of tetraquarks is still under debate, all known pentaquark candidates have since been established as non-existent.

^ The main evidence is based on the resonance width of the Z⁰ boson, which constrains the= 4th generation neutrino to have a mass greater than ~45 GeV/c<= sup>2. This would be highly contrasting with the other three generations' neutrinos, whose masses cannot exceed 2 MeV/c².=

^ CP violation is a phenomenon which causes weak interactions to be= have differently when left and right are swapped (P symmetry) and particles are replaced with their corresponding antiparticles (C symmetry).

^ The actual probability of decay of one quark to another is a complicated function of (amongst other variables) the decaying quark's mass, the masses of the decay products, and the corresponding element of the CKM matrix. This probability is directly proportional (but not equal) to the magnitude squared of the corresponding CKM entry.

^ Despite its name, color charge is not related to the color spectr= um of visible light.

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Further reading

D.J. Griffiths (2008). Introduction to Elementary Particles<= /span> (2nd ed.). Wiley–VCH. ISBN 3527406018.=

I.S. Hughes (1985). Elementary particles (2nd ed.). Cambridge University Press. ISBN 0-521-26092-2.<= /span>

R. Oerter (2005). The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Pi Press. ISBN 0132366789.=

A. Pickering (1984). Constructing Quarks: A Sociological History of Particle Physics. The University of Chicago Pr= ess. ISBN 0-226-66799-5.<= /span>

B. Povh (1995). Particles and Nuclei: An Introduction to the Physical Concepts. Springer–Verlag. ISBN 0-387-59439-6.<= /span>

M. Riordan (1987). The Hunting of the Quark: A true story of modern physics. Simon & Schuster. ISBN 0-671-64884-5.<= /span>

B.A. Schumm (2004). Deep Down Things: The Breathtaking Beauty of Particle Physics. Johns Hopkins University Pres= s. ISBN 0-8018-7971-X.<= /span>

External links

1969 Physics Nobel Prize lecture by Murray Gell-Mann =

1976 Physics Nobel Prize lecture by Burton Richter
1976 Physics Nobel Prize lecture by Samuel C.C. Ting =

2008 Physics Nobel Prize lecture by Makoto Kobayashi =

2008 Physics Nobel Prize lecture by Toshihide Maskawa

The Top Quark And The Higgs Particle by T.A. Heppenheimer –= A description of CERN's experiment to count the families of quarks.

Retrieved from "http://en.wikipedia.org/wiki/Qu= ark"

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