This article is about the elementary particle and its antiparticle. For other uses, seeQuark (disambiguation).
Quark
Aproton is composed of twoup quarks, onedown quark, and thegluons that mediate the forces "binding" them together. Thecolor assignment of individual quarks is arbitrary, but all three colors must be present; red, blue and green are used as an analogy to the primary colors that together produce a white color.
There are six types, known asflavors, of quarks:up,down,charm,strange,top, andbottom.[4] Up and down quarks have the lowestmasses of all quarks. The heavier quarks rapidly change into up and down quarks through a process ofparticle decay: the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in theuniverse, whereas strange, charm, bottom, and top quarks can only be produced inhigh energy collisions (such as those involvingcosmic rays and inparticle accelerators). For every quark flavor there is a corresponding type ofantiparticle, known as anantiquark, that differs from the quark only in that some of its properties (such as the electric charge) haveequal magnitude but opposite sign.
Thequark model was independently proposed by physicistsMurray Gell-Mann andGeorge Zweig in 1964.[5] Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence untildeep inelastic scattering experiments at theStanford Linear Accelerator Center in 1968.[6][7] Accelerator program experiments have provided evidence for all six flavors. The top quark, first observed atFermilab in 1995, was the last to be discovered.[5]
Six of the particles in theStandard Model are quarks (shown in purple). Each of the first three columns forms ageneration of matter.
TheStandard Model is the theoretical framework describing all the knownelementary particles. This model contains sixflavors of quarks ( q ), namedup ( u ),down ( d ),strange ( s ),charm ( c ),bottom ( b ), andtop ( t ).[4]Antiparticles of quarks are calledantiquarks, and are denoted by a bar over the symbol for the corresponding quark, such as u for an up antiquark. As withantimatter in general, antiquarks have the same mass,mean lifetime, and spin as their respective quarks, but the electric charge and othercharges have the opposite sign.[8]
The quarks that determine thequantum numbers of hadrons are calledvalence quarks; apart from these, any hadron may contain an indefinite number ofvirtual "sea" quarks, antiquarks, andgluons, which do not influence its quantum numbers.[10] There are two families of hadrons:baryons, with three valence quarks, andmesons, with a valence quark and an antiquark.[11] The most common baryons are the proton and the neutron, the building blocks of theatomic nucleus.[12] A great number of hadrons are known (seelist of baryons andlist of mesons), most of them differentiated by their quark content and the properties these constituent quarks confer. The existence of"exotic" hadrons with more valence quarks, such astetraquarks ( q q q q ) andpentaquarks ( q q q q q ), was conjectured from the beginnings of the quark model[13] but not discovered until the early 21st century.[14][15][16][17]
Elementary fermions are grouped into threegenerations, each comprising two leptons and two quarks. The first generation includes up and down quarks, the second strange and charm quarks, and the third bottom and top quarks. All searches for a fourth generation of quarks and other elementary fermions have failed,[18][19] and there is strong indirect evidence that no more than three generations exist.[nb 2][20][21][22] Particles in higher generations generally have greater mass and less stability, causing them todecay into lower-generation particles by means ofweak 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 involvingcosmic rays), and decay quickly; however, they are thought to have been present during the first fractions of a second after theBig Bang, when the universe was in an extremely hot and dense phase (thequark epoch). Studies of heavier quarks are conducted in artificially created conditions, such as inparticle accelerators.[23]
Having electric charge, mass, color charge, and flavor, quarks are the only known elementary particles that engage in all fourfundamental interactions of contemporary physics: electromagnetism, gravitation, strong interaction, and weak interaction.[12] 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 successfulquantum theory of gravity exists, gravitation is not described by the Standard Model.
See thetable of properties below for a more complete overview of the six quark flavors' properties.
Thequark model was independently proposed by physicistsMurray Gell-Mann[24] andGeorge Zweig[25][26] in 1964.[5] The proposal came shortly after Gell-Mann's 1961 formulation of a particle classification system known as theEightfold Way – or, in more technical terms,SU(3)flavor symmetry, streamlining its structure.[27] PhysicistYuval Ne'eman had independently developed a scheme similar to the Eightfold Way in the same year.[28][29] An early attempt at constituent organization was available in theSakata model.
At the time of the quark theory's inception, the "particle zoo" included a multitude ofhadrons, among other particles. Gell-Mann and Zweig posited that they were not elementary particles, but were instead composed of combinations of quarks and antiquarks. Their model involved three flavors of quarks,up,down, andstrange, to which they ascribed properties such as spin and electric charge.[24][25][26] The initial reaction of the physics community to the proposal was mixed. There was particular contention about whether the quark was a physical entity or a mere abstraction used to explain concepts that were not fully understood at the time.[30]
In less than a year, extensions to the Gell-Mann–Zweig model were proposed.Sheldon Glashow andJames Bjorken predicted the existence of a fourth flavor of quark, which they calledcharm. The addition was proposed because it allowed for a better description of theweak interaction (the mechanism that allows quarks to decay), equalized the number of known quarks with the number of knownleptons, and implied a mass formula that correctly reproduced the masses of the knownmesons.[31]
Deep inelastic scattering experiments conducted in 1968 at theStanford Linear Accelerator Center (SLAC) and published on October 20, 1969, showed that the proton contained much smaller,point-like objects and was therefore not an elementary particle.[6][7][32] Physicists were reluctant to firmly identify these objects with quarks at the time, instead calling them "partons" – a term coined byRichard Feynman.[33][34][35] The objects that were observed at SLAC would later be identified as up and down quarks as the other flavors were discovered.[36] Nevertheless, "parton" remains in use as a collective term for the constituents of hadrons (quarks, antiquarks, andgluons).Richard Taylor,Henry Kendall andJerome Friedman received the 1990 Nobel Prize in physics for their work at SLAC.
The strange quark's existence was indirectly validated by SLAC's scattering experiments: not only was it a necessary component of Gell-Mann and Zweig's three-quark model, but it provided an explanation for thekaon ( K ) andpion ( π ) hadrons discovered in cosmic rays in 1947.[37]
Charm quarks were produced almost simultaneously by two teams in November 1974 (seeNovember Revolution) – one at SLAC underBurton Richter, and one atBrookhaven National Laboratory underSamuel Ting. The charm quarks were observedbound with charm antiquarks in mesons. The two parties had assigned the discovered meson two different symbols,J andψ; thus, it became formally known as the J/ψ meson. The discovery finally convinced the physics community of the quark model's validity.[35]
In the following years a number of suggestions appeared for extending the quark model to six quarks. Of these, the 1975 paper byHaim Harari[41] was the first to coin the termstop andbottom for the additional quarks.[42]
In 1977, the bottom quark was observed by a team atFermilab led byLeon Lederman.[43][44] This was a strong indicator of the top quark's existence: without the top quark, the bottom quark would have been without a partner. It was not until 1995 that the top quark was finally observed, also by theCDF[45] andDØ[46] teams at Fermilab.[5] It had a mass much larger than expected,[47] almost as large as that of agold atom.[48]
For some time, Gell-Mann was undecided on an actual spelling for the term he intended to coin, until he found the wordquark inJames Joyce's 1939 bookFinnegans Wake:[49]
– Three quarks for Muster Mark! Sure he hasn't got much of a bark And sure any he has it's all beside the mark.
The wordquark is an outdated English word meaningto croak[50] and the above-quoted lines are about a bird choir mocking kingMark of Cornwall in the legend ofTristan and Iseult.[51] Especially in the German-speaking parts of the world there is a widespread legend, however, that Joyce had taken it from the wordQuark,[52] aGerman word ofSlavic origin which denotesa curd cheese,[53] but is also a colloquial term for "trivial nonsense".[54] In the legend it is said that he had heard it on a journey to Germany at afarmers' market inFreiburg.[55][56]Some authors, however, defend a possible German origin of Joyce's wordquark.[57] Gell-Mann went into further detail regarding the name of the quark in his 1994 bookThe Quark and the Jaguar:[58]
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 ofFinnegans 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 inThrough the Looking-Glass. From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar. I argued, therefore, that perhaps one of the multiple sources of the cry "Three quarks for Muster Mark" might be "Three quarts for Mister Mark", in which case the pronunciation "kwork" would not be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.
Zweig preferred the nameace for the particle he had theorized, but Gell-Mann's terminology came to prominence once the quark model had been commonly accepted.[59]
The quark flavors were given their names for several reasons. The up and down quarks are named after the up and down components ofisospin, which they carry.[60] Strange quarks were given their name because they were discovered to be components of thestrange particles discovered in cosmic rays years before the quark model was proposed; these particles were deemed "strange" because they had unusually long lifetimes.[61] Glashow, who co-proposed the charm quark with Bjorken, is quoted as saying, "We called our construct the 'charmed quark', for we were fascinated and pleased by the symmetry it brought to the subnuclear world."[62] The names "bottom" and "top", coined by Harari, were chosen because they are "logical partners for up and down quarks".[41][42][61] Alternative names for bottom and top quarks are "beauty" and "truth" respectively,[nb 4] but these names have somewhat fallen out of use.[66] While "truth" never did catch on, accelerator complexes devoted to massive production of bottom quarks are sometimes called "beauty factories".[67]
Quarks havefractional electric charge values – either (−1/3) or (+2/3) times theelementary charge (e), depending on flavor. Up, charm, and top quarks (collectively referred to asup-type quarks) have a charge of +2/3 e; down, strange, and bottom quarks (down-type quarks) have a charge of −1/3 e. Antiquarks have the opposite charge to their corresponding quarks; up-type antiquarks have charges of −2/3 e and down-type antiquarks have charges of +1/3 e. Since the electric charge of ahadron is the sum of the charges of the constituent quarks, all hadrons have integer charges: the combination of three quarks (baryons), three antiquarks (antibaryons), or a quark and an antiquark (mesons) always results in integer charges.[68] For example, the hadron constituents of atomic nuclei, neutrons and protons, have charges of 0 e and +1 e respectively; the neutron is composed of two down quarks and one up quark, and the proton of two up quarks and one down quark.[12]
Spin is an intrinsic property of elementary particles, and its direction is an importantdegree of freedom. It is sometimes visualized as the rotation of an object around its own axis (hence the name "spin"), though this notion is somewhat misguided at subatomic scales because elementary particles are believed to bepoint-like.[69]
Spin can be represented by avector whose length is measured in units of thereduced Planck constantħ (pronounced "h bar"). For quarks, a measurement of the spin vectorcomponent along any axis can only yield the values +ħ/2 or −ħ/2; for this reason quarks are classified asspin-1/2 particles.[70] The component of spin along a given axis – by convention thez 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 thez axis is denoted by u↑.[71]
Feynman diagram ofbeta 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 fourfundamental interactions in particle physics. By absorbing or emitting aW boson, any up-type quark (up, charm, and top quarks) can change into any down-type quark (down, strange, and bottom quarks) and vice versa. This flavor transformation mechanism causes theradioactive process ofbeta decay, in which a neutron ( n ) "splits" into a proton ( p ), anelectron ( e− ) and anelectron antineutrino ( ν e) (see picture). This occurs when one of the down quarks in the neutron ( u d d ) decays into an up quark by emitting avirtual W− boson, transforming the neutron into a proton ( u u d ). The W− boson then decays into an electron and an electron antineutrino.[72]
Thestrengths of the weak interactions between the six quarks. The "intensities" of the lines are determined by the elements of theCKM matrix.
While the process of flavor transformation is the same for all quarks, each quark has a preference to transform into the quark of its own generation. The relative tendencies of all flavor transformations are described by amathematical table, called theCabibbo–Kobayashi–Maskawa matrix (CKM matrix). Enforcingunitarity, the approximatemagnitudes of the entries of the CKM matrix are:[73]
whereVij represents the tendency of a quark of flavori to change into a quark of flavorj (or vice versa).[nb 5]
There exists an equivalent weak interaction matrix for leptons (right side of the W boson on the above beta decay diagram), called thePontecorvo–Maki–Nakagawa–Sakata matrix (PMNS matrix).[74] Together, the CKM and PMNS matrices describe all flavor transformations, but the links between the two are not yet clear.[75]
All types of hadrons have zero total color charge.The pattern of strong charges for the three colors of quark, three antiquarks, and eight gluons (with two of zero charge overlapping).
According toquantum chromodynamics (QCD), quarks possess a property calledcolor charge. There are three types of color charge, arbitrarily labeledblue,green, andred.[nb 6] Each of them is complemented by an anticolor –antiblue,antigreen, andantired. Every quark carries a color, while every antiquark carries an anticolor.[76]
The system of attraction and repulsion between quarks charged with different combinations of the three colors is calledstrong interaction, which is mediated byforce carrying particles known asgluons; this is discussed at length below. The theory that describes strong interactions is calledquantum chromodynamics (QCD). A quark, which will have a single color value, can form abound system with an antiquark carrying the corresponding anticolor. The result of two attracting quarks will be color neutrality: a quark with color chargeξ plus an antiquark with color charge −ξ will result in a color charge of 0 (or "white" color) and the formation of ameson. This is analogous to theadditive color model in basicoptics. Similarly, the combination of three quarks, each with different color charges, or three antiquarks, each with different anticolor charges, will result in the same "white" color charge and the formation of abaryon orantibaryon.[77]
In modern particle physics,gauge symmetries – a kind ofsymmetry group – relate interactions between particles (seegauge theories). ColorSU(3) (commonly abbreviated to SU(3)c) is the gauge symmetry that relates the color charge in quarks and is the defining symmetry for quantum chromodynamics.[78] Just as the laws of physics are independent of which directions in space are designatedx,y, andz, 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 acomplex space). Every quark flavorf, each with subtypesfB,fG,fR corresponding to the quark colors,[79] forms a triplet: a three-componentquantum field that transforms under the fundamentalrepresentation of SU(3)c.[80] The requirement that SU(3)c should belocal – that is, that its transformations be allowed to vary with space and time – determines the properties of the strong interaction. In particular, it implies the existence ofeight gluon types to act as its force carriers.[78][81]
Current quark masses for all six flavors in comparison, asballs of proportional volumes.Proton (gray) andelectron (red) are shown in bottom left corner for scale.
Two terms are used in referring to a quark's mass:current quark mass refers to the mass of a quark by itself, whileconstituent quark mass refers to the current quark mass plus the mass of thegluonparticle field surrounding the quark.[82] These masses typically have very different values. Most of a hadron's mass comes from the gluons that bind the constituent quarks together, rather than from the quarks themselves. While gluons are inherently massless, they possess energy – more specifically,quantum chromodynamics binding energy (QCBE) – and it is this that contributes so greatly to the overall mass of the hadron (seemass in special relativity). For example, a proton has a mass of approximately938 MeV/c2, of which the rest mass of its three valence quarks only contributes about9 MeV/c2; much of the remainder can be attributed to the field energy of the gluons[83][84] (seechiral symmetry breaking). The Standard Model posits that elementary particles derive their masses from theHiggs mechanism, which is associated to theHiggs boson. It is hoped that further research into the reasons for the top quark's large mass of ~173 GeV/c2, almost the mass of a gold atom,[83][85] might reveal more about the origin of the mass of quarks and other elementary particles.[86]
In QCD, quarks are considered to be point-like entities, with zero size. As of 2014, experimental evidence indicates they are no bigger than 10−4 times the size of a proton, i.e. less than 10−19 metres.[87]
The following table summarizes the key properties of the six quarks.Flavor quantum numbers (isospin (I3),charm (C),strangeness (S, not to be confused with spin),topness (T), andbottomness (B′)) are assigned to certain quark flavors, and denote qualities of quark-based systems and hadrons. Thebaryon number (B) is +1/3 for all quarks, as baryons are made of three quarks. For antiquarks, the electric charge (Q) and all flavor quantum numbers (B,I3,C,S,T, andB′) are of opposite sign. Mass andtotal angular momentum (J; equal to spin for point particles) do not change sign for the antiquarks.
* Notation such as173210±510 ± 710, in the case of the top quark, denotes two types ofmeasurement uncertainty: The first uncertainty isstatistical in nature, and the second issystematic.
As described byquantum chromodynamics, thestrong interaction between quarks is mediated by gluons, masslessvectorgauge 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 asperturbation theory), gluons are constantly exchanged between quarks through avirtual emission and absorption process. When a gluon is transferred between quarks, a color change occurs in both; for example, if a red quark emits a red–antigreen gluon, it becomes green, and if a green quark absorbs a red–antigreen gluon, it becomes red. Therefore, while each quark's color constantly changes, their strong interaction is preserved.[88][89][90]
Since gluons carry color charge, they themselves are able to emit and absorb other gluons. This causesasymptotic freedom: as quarks come closer to each other, the chromodynamic binding force between them weakens.[91] Conversely, as the distance between quarks increases, the binding force strengthens. The color field becomes stressed, much as an elastic band is stressed when stretched, and more gluons of appropriate color are spontaneously created to strengthen the field. Above a certain energy threshold, pairs of quarks and antiquarksare created. These pairs bind with the quarks being separated, causing new hadrons to form. This phenomenon is known ascolor confinement: quarks never appear in isolation.[92][93] This process ofhadronization occurs before quarks formed in a high energy collision are able to interact in any other way. The only exception is the top quark, which may decay before it hadronizes.[94]
Hadrons contain, along with thevalence quarks ( q v) that contribute to theirquantum numbers,virtual quark–antiquark ( q q ) pairs known assea quarks ( q s). Sea quarks form when a gluon of the hadron's color field splits; this process also works in reverse in that theannihilation of two sea quarks produces a gluon. The result is a constant flux of gluon splits and creations colloquially known as "the sea".[95] Sea quarks are much less stable than their valence counterparts, and they typically annihilate each other within the interior of the hadron. Despite this, sea quarks can hadronize into baryonic or mesonic particles under certain circumstances.[96]
A qualitative rendering of thephase diagram of quark matter. The precise details of the diagram are the subject of ongoing research.[97][98]
Under sufficiently extreme conditions, quarks may become "deconfined" out of bound states and propagate as thermalized "free" excitations in the larger medium. In the course ofasymptotic freedom, the strong interaction becomes weaker at increasing temperatures. Eventually, color confinement would be effectively lost in an extremely hotplasma of freely moving quarks and gluons. This theoretical phase of matter is calledquark–gluon plasma.[99]
The exact conditions needed to give rise to this state are unknown and have been the subject of a great deal of speculation and experimentation. An estimate puts the needed temperature at(1.90±0.02)×1012kelvin.[100] While a state of entirely free quarks and gluons has never been achieved (despite numerous attempts byCERN in the 1980s and 1990s),[101] recent experiments at theRelativistic Heavy Ion Collider have yielded evidence for liquid-like quark matter exhibiting "nearly perfect"fluid motion.[102]
The quark–gluon plasma would be characterized by a great increase in the number of heavier quark pairs in relation to the number of up and down quark pairs. It is believed that in the period prior to 10−6 seconds after theBig Bang (thequark epoch), the universe was filled with quark–gluon plasma, as the temperature was too high for hadrons to be stable.[103]
Given sufficiently high baryon densities and relatively low temperatures – possibly comparable to those found inneutron stars – quark matter is expected to degenerate into aFermi liquid of weakly interacting quarks. This liquid would be characterized by acondensation of colored quarkCooper pairs, therebybreaking the local SU(3)c symmetry. Because quark Cooper pairs harbor color charge, such a phase of quark matter would becolor superconductive; that is, color charge would be able to pass through it with no resistance.[104]
^The main evidence is based on theresonance width of the Z0 boson, which constrains the 4th generation neutrino to have a mass greater than ~45 GeV/c2. This would be highly contrasting with the other three generations' neutrinos, whose masses cannot exceed2 MeV/c2.
^CP violation is a phenomenon that causes weak interactions to behave differently when left and right are swapped (P symmetry) and particles are replaced with their corresponding antiparticles (C symmetry).
^"Beauty" and "truth" are contrasted in the last lines ofKeats' 1819 poem "Ode on a Grecian Urn" and may have been the origin of those names.[63][64][65]
^The actual probability of decay of one quark to another is a complicated function of (among other variables) the decaying quark's mass, the masses of thedecay products, and the corresponding element of the CKM matrix. This probability is directly proportional (but not equal) to the magnitude squared (|Vij|2) of the corresponding CKM entry.
^Despite its name, color charge is not related to the color spectrum of visible light.
^U. Heinz; M. Jacob (2000). "Evidence for a New State of Matter: An Assessment of the Results from the CERN Lead Beam Programme".arXiv:nucl-th/0002042.
![A tree diagram consisting mostly of straight arrows. A down quark forks into an up quark and a wavy-arrow W[superscript minus] boson, the latter forking into an electron and reversed-arrow electron antineutrino.](/mt/?noimg=&dark=on&url=http%3a%2f%2fwww.wikipedia.org%2fwiki%2fFile%3aBeta_Negative_Decay.svg)
