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128-625: A quark ( / k w ɔːr k , k w ɑːr k / ) is a type of elementary particle and a fundamental constituent of matter . Quarks combine to form composite particles called hadrons , the most stable of which are protons and neutrons , the components of atomic nuclei . All commonly observable matter is composed of up quarks, down quarks and electrons . Owing to a phenomenon known as color confinement , quarks are never found in isolation; they can be found only within hadrons, which include baryons (such as protons and neutrons) and mesons , or in quark–gluon plasmas . For this reason, much of what

256-592: A farmers' market in Freiburg . Some authors, however, defend a possible German origin of Joyce's word quark . Gell-Mann went into further detail regarding the name of the quark in his 1994 book The Quark and the Jaguar : 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

384-472: A gold atom. 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 1939 book Finnegans Wake : – 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 The word quark is an outdated English word meaning to croak and the above-quoted lines are about

512-553: A jet of particles is emitted. This inelastic scattering suggests that the charge in the proton is not uniform but split among smaller charged particles: quarks. In the Standard Model, vector ( spin -1) bosons ( gluons , photons , and the W and Z bosons ) mediate forces, whereas the Higgs boson (spin-0) is responsible for the intrinsic mass of particles. Bosons differ from fermions in the fact that multiple bosons can occupy

640-476: A mass of 172.76 ± 0.3  GeV/ c , which is close to the rhenium atom mass. The antiparticle of the top quark is the top antiquark (symbol: t , sometimes called antitop quark or simply antitop ), which differs from it only in that some of its properties have equal magnitude but opposite sign . The top quark interacts with gluons of the strong interaction and is typically produced in hadron colliders via this interaction. However, once produced,

768-414: A vector whose length is measured in units of the reduced Planck constant ħ (pronounced "h bar"). For quarks, a measurement of the spin vector component along any axis can only yield the values + ⁠ ħ / 2 ⁠ or − ⁠ ħ / 2 ⁠ ; for this reason quarks are classified as spin- ⁠ 1 / 2 ⁠ particles. The component of spin along a given axis – by convention

896-442: A bar over the symbol for the corresponding quark, such as u for an up antiquark. As with antimatter 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. Quarks are spin- ⁠ 1 / 2 ⁠ particles, which means they are fermions according to the spin–statistics theorem . They are subject to

1024-499: A better 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 masses of the known mesons . Deep inelastic scattering experiments conducted in 1968 at the Stanford Linear Accelerator Center (SLAC) and published on October 20, 1969, showed that

1152-483: A bird choir mocking king Mark of Cornwall in the legend of Tristan and Iseult . Especially in the German-speaking parts of the world there is a widespread legend, however, that Joyce had taken it from the word Quark , a German word of Slavic origin which denotes a curd cheese , but is also a colloquial term for "trivial nonsense". In the legend it is said that he had heard it on a journey to Germany at

1280-414: A collective term for the constituents of hadrons (quarks, antiquarks, and gluons ). Richard Taylor , Henry Kendall and Jerome 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

1408-427: A collision, a highly energetic gluon is created, which subsequently decays into a top and antitop. This process was responsible for the majority of the top events at Tevatron and was the process observed when the top was first discovered in 1995. It is also possible to produce pairs of top–antitop through the decay of an intermediate photon or Z-boson . However, these processes are predicted to be much rarer and have

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1536-421: A color charge of 0 (or "white" color) and the formation of a meson . This is analogous to the additive color model in basic optics . 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 a baryon or antibaryon . In modern particle physics, gauge symmetries –

1664-636: A color-neutral baryon . Symmetrically, three antiquarks with the colors "antired", "antiblue" and "antigreen" can form a color-neutral antibaryon . Quarks also carry fractional electric charges , but, since they are confined within hadrons whose charges are all integral, fractional charges have never been isolated. Note that quarks have electric charges of either ⁠+ + 2 / 3 ⁠   e or ⁠− + 1 / 3 ⁠   e , whereas antiquarks have corresponding electric charges of either ⁠− + 2 / 3 ⁠   e or  ⁠+ + 1 / 3 ⁠   e . Evidence for

1792-420: A fact explained by confinement . Every quark carries one of three color charges of the strong interaction ; antiquarks similarly carry anticolor. Color-charged particles interact via gluon exchange in the same way that charged particles interact via photon exchange. Gluons are themselves color-charged, however, resulting in an amplification of the strong force as color-charged particles are separated. Unlike

1920-549: A great deal of speculation and experimentation. An estimate puts the needed temperature at (1.90 ± 0.02) × 10 kelvin . While a state of entirely free quarks and gluons has never been achieved (despite numerous attempts by CERN in the 1980s and 1990s), recent experiments at the Relativistic Heavy Ion Collider have yielded evidence for liquid-like quark matter exhibiting "nearly perfect" fluid motion . The quark–gluon plasma would be characterized by

2048-511: 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 seconds after the Big Bang (the quark epoch ), the universe was filled with quark–gluon plasma, as the temperature was too high for hadrons to be stable. Given sufficiently high baryon densities and relatively low temperatures – possibly comparable to those found in neutron stars – quark matter

2176-556: A hadron surrounding the top quark provides physicists with the unique opportunity to study the behavior of a "bare" quark. In particular, it is possible to directly determine the branching ratio : The best current determination of this ratio is 0.957 ± 0.034 . Since this ratio is equal to | V tb | according to the Standard Model , this gives another way of determining the CKM element  | V tb | , or in combination with

2304-426: 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 relates the color charge in quarks and is the defining symmetry for quantum chromodynamics. 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

2432-440: A loop (a one-dimensional sphere, that is, a circle). As a string moves through space it sweeps out something called a world sheet . String theory predicts 1- to 10-branes (a 1- brane being a string and a 10-brane being a 10-dimensional object) that prevent tears in the "fabric" of space using the uncertainty principle (e.g., the electron orbiting a hydrogen atom has the probability, albeit small, that it could be anywhere else in

2560-453: A lower mass state. Because of this, up and down quarks are generally stable and the most common in the universe , whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators ). For every quark flavor there is a corresponding type of antiparticle , known as an antiquark , that differs from the quark only in that some of its properties (such as

2688-468: A means to discriminate between competing theories of new physics beyond the Standard Model. The top quark is the only quark that has been directly observed due to its decay time being shorter than the hadronization time. In 1973, Makoto Kobayashi and Toshihide Maskawa predicted the existence of a third generation of quarks to explain observed CP violations in kaon decay . The names top and bottom were introduced by Haim Harari in 1975, to match

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2816-407: A multitude of hadrons , 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 , and strange , to which they ascribed properties such as spin and electric charge. The initial reaction of the physics community to the proposal was mixed. There

2944-423: A neutron into a proton then decays into an electron and electron-antineutrino pair. The Z does not convert particle flavor or charges, but rather changes momentum; it is the only mechanism for elastically scattering neutrinos. The weak gauge bosons were discovered due to momentum change in electrons from neutrino-Z exchange. The massless photon mediates the electromagnetic interaction . These four gauge bosons form

3072-407: 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 , f G , f R corresponding to the quark colors, forms

3200-516: A new theory of so-called Techniquarks, interacting via so called Technigluons. The main idea is that the Higgs boson is not an elementary particle but a bound state of these objects. According to preon theory there are one or more orders of particles more fundamental than those (or most of those) found in the Standard Model. The most fundamental of these are normally called preons, which is derived from "pre-quarks". In essence, preon theory tries to do for

3328-594: A particle classification system known as the Eightfold Way – or, in more technical terms, SU(3) flavor symmetry , streamlining its structure. Physicist Yuval Ne'eman had independently developed a scheme similar to the Eightfold Way in the same year. An early attempt at constituent organization was available in the Sakata model . At the time of the quark theory's inception, the " particle zoo " included

3456-432: A property called color charge . There are three types of color charge, arbitrarily labeled blue , green , and red . Each of them is complemented by an anticolor – antiblue , antigreen , and antired . Every quark carries a color, while every antiquark carries an anticolor. The system of attraction and repulsion between quarks charged with different combinations of the three colors is called strong interaction , which

3584-529: A quark's mass: current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark. These 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

3712-723: A top quark by exchanging a W boson with an up or down quark ("t-channel"). A single top quark can also be produced in association with a W boson, requiring an initial-state bottom quark ("tW-channel"). The first evidence for these processes was published by the DØ collaboration in December ;2006, and in March ;2009 the CDF and DØ collaborations released twin articles with the definitive observation of these processes. The main significance of measuring these production processes

3840-424: A triplet: a three-component quantum field that transforms under the fundamental representation of SU(3) c . 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, it implies the existence of eight gluon types to act as its force carriers. Two terms are used in referring to

3968-414: A unique opportunity to study a "bare" quark (all other quarks hadronize , meaning that they combine with other quarks to form hadrons and can only be observed as such). Because the top quark is so massive, its properties allowed indirect determination of the mass of the Higgs boson (see § Mass and coupling to the Higgs boson below). As such, the top quark's properties are extensively studied as

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4096-596: A valence quark and an antiquark. The most common baryons are the proton and the neutron, the building blocks of the atomic nucleus . 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 quarks, such as tetraquarks ( q q q q ) and pentaquarks ( q q q q q ),

4224-406: A virtually identical experimental signature in a hadron collider like Tevatron. The production of single top quarks via weak interaction is a distinctly different process. This can happen in several ways (called channels): Either an intermediate W-boson decays into a top and antibottom quarks ("s-channel") or a bottom quark (probably created in a pair through the decay of a gluon) transforms to

4352-488: Is a consequence of the high masses of the W and Z bosons, which in turn are a consequence of the Higgs mechanism . Through the process of spontaneous symmetry breaking , the Higgs selects a special direction in electroweak space that causes three electroweak particles to become very heavy (the weak bosons) and one to remain with an undefined rest mass as it is always in motion (the photon). On 4 July 2012, after many years of experimentally searching for evidence of its existence,

4480-695: Is composed of atoms , themselves once thought to be indivisible elementary particles. The name atom comes from the Ancient Greek word ἄτομος ( atomos ) which means indivisible or uncuttable . Despite the theories about atoms that had existed for thousands of years the factual existence of atoms remained controversial until 1905. In that year Albert Einstein published his paper on Brownian motion , putting to rest theories that had regarded molecules as mathematical illusions. Einstein subsequently identified matter as ultimately composed of various concentrations of energy . Subatomic constituents of

4608-467: Is composed of two down quarks and one up quark, and the proton of two up quarks and one down quark. Spin is an intrinsic property 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 elementary particles are believed to be point-like . Spin can be represented by

4736-512: Is differentiated via the spin–statistics theorem : it is half-integer for fermions, and integer for bosons. Notes : [†] An anti-electron ( e ) is conventionally called a " positron ". [‡] The known force carrier bosons all have spin = 1. The hypothetical graviton has spin = 2; it is unknown whether it is a gauge boson as well. In the Standard Model , elementary particles are represented for predictive utility as point particles . Though extremely successful,

4864-449: Is expected to degenerate into a Fermi liquid of weakly interacting quarks. This liquid would be characterized by a condensation of colored quark Cooper 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. Elementary particle Ordinary matter

4992-444: Is known about quarks has been drawn from observations of hadrons. Quarks have various intrinsic properties , including electric charge , mass , color charge , and spin . They are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions , also known as fundamental forces ( electromagnetism , gravitation , strong interaction , and weak interaction ), as well as

5120-442: Is massless, although some models containing massive Kaluza–Klein gravitons exist. Although experimental evidence overwhelmingly confirms the predictions derived from the Standard Model , some of its parameters were added arbitrarily, not determined by a particular explanation, which remain mysterious, for instance the hierarchy problem . Theories beyond the Standard Model attempt to resolve these shortcomings. One extension of

5248-462: 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, which will have a single color value, can form a bound system with an antiquark carrying the corresponding anticolor. 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

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5376-481: Is preserved. Since gluons carry color charge, they themselves are able to emit and absorb other gluons. This causes asymptotic freedom : as quarks come closer to each other, the chromodynamic binding force between them weakens. 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

5504-486: Is strong indirect evidence that no more than three generations exist. Particles in higher generations generally have greater mass and less 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

5632-522: Is that their frequency is directly proportional to the | V tb |  component of the CKM matrix . The only known way the top quark can decay is through the weak interaction , producing a W boson and a bottom quark . Because of its enormous mass , the top quark is extremely short-lived, with a predicted lifetime of only 5 × 10  s . As a result, top quarks do not have time before they decay to form hadrons as other quarks do. The absence of

5760-534: Is the existence of X and Y bosons , which cause proton decay . The non-observation of proton decay at the Super-Kamiokande neutrino observatory rules out the simplest GUTs, however, including SU(5) and SO(10). Supersymmetry extends the Standard Model by adding another class of symmetries to the Lagrangian . These symmetries exchange fermionic particles with bosonic ones. Such a symmetry predicts

5888-437: Is the level of significance required to officially label experimental observations as a discovery . Research into the properties of the newly discovered particle continues. The graviton is a hypothetical elementary spin-2 particle proposed to mediate gravitation. While it remains undiscovered due to the difficulty inherent in its detection , it is sometimes included in tables of elementary particles. The conventional graviton

6016-400: Is the top quark, which may decay before it hadronizes. Hadrons contain, along with the valence quarks ( q v ) that contribute to their quantum numbers , virtual quark–antiquark ( q q ) pairs known as sea quarks ( q s ). Sea quarks form when a gluon of the hadron's color field splits; this process also works in reverse in that

6144-454: 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/ c , of which the rest mass of its three valence quarks only contributes about 9 MeV/ c ; much of the remainder can be attributed to the field energy of the gluons (see chiral symmetry breaking ). The Standard Model posits that elementary particles derive their masses from

6272-544: 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. The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. The proposal came shortly after Gell-Mann's 1961 formulation of

6400-420: The Higgs boson was announced to have been observed at CERN's Large Hadron Collider. Peter Higgs who first posited the existence of the Higgs boson was present at the announcement. The Higgs boson is believed to have a mass of approximately 125 GeV/ c . The statistical significance of this discovery was reported as 5 sigma, which implies a certainty of roughly 99.99994%. In particle physics, this

6528-631: The Higgs field . This coupling y t is very close to unity; in the Standard Model of particle physics , it is the largest (strongest) coupling at the scale of the weak interactions and above. The top quark was discovered in 1995 by the CDF and DØ experiments at Fermilab . Like all other quarks , the top quark is a fermion with spin-1/2 and participates in all four fundamental interactions : gravitation , electromagnetism , weak interactions , and strong interactions . It has an electric charge of + ⁠ 2  / 3 ⁠   e . It has

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6656-443: The Higgs mechanism , which is associated to the Higgs boson . It is hoped that further research into the reasons for the top quark's large mass of ~ 173 GeV/ c , almost the mass of a gold atom, might reveal more about the origin of the mass of quarks and other elementary particles. 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 times

6784-528: The Large Hadron Collider ( ATLAS and CMS ). The Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, however, since it is not known if it is compatible with Einstein 's general relativity . There may be hypothetical elementary particles not described by the Standard Model, such as the graviton , the particle that would carry the gravitational force , and sparticles , supersymmetric partners of

6912-443: The Large Hadron Collider at CERN became the only accelerator that generates a beam of sufficient energy to produce top quarks, with a center-of-mass energy of 7 TeV. There are multiple processes that can lead to the production of top quarks, but they can be conceptually divided in two categories: top-pair production, and single-top production. The most common is production of a top–antitop pair via strong interactions . In

7040-520: The Large Hadron Collider at CERN . String theory is a model of physics whereby all "particles" that make up matter are composed of strings (measuring at the Planck length) that exist in an 11-dimensional (according to M-theory , the leading version) or 12-dimensional (according to F-theory ) universe. These strings vibrate at different frequencies that determine mass, electric charge, color charge, and spin. A "string" can be open (a line) or closed in

7168-574: The Nobel Prize in physics in 1999. Because top quarks are very massive, large amounts of energy are needed to create one. The only way to achieve such high energies is through high-energy collisions. These occur naturally in the Earth's upper atmosphere as cosmic rays collide with particles in the air, or can be created in a particle accelerator . In 2011, after the Tevatron ceased operations,

7296-451: The Pauli exclusion principle , which states 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. Unlike leptons , quarks possess color charge , which causes them to engage in the strong interaction . The resulting attraction between different quarks causes

7424-534: The annihilation of two sea quarks produces a gluon. The result is a constant flux of gluon splits and creations colloquially known as "the sea". 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. Under sufficiently extreme conditions, quarks may become "deconfined" out of bound states and propagate as thermalized "free" excitations in

7552-403: The atomic nucleus . Like quarks, gluons exhibit color and anticolor – unrelated to the concept of visual color and rather the particles' strong interactions – sometimes in combinations, altogether eight variations of gluons. There are three weak gauge bosons : W , W , and Z ; these mediate the weak interaction . The W bosons are known for their mediation in nuclear decay: The W converts

7680-562: The electromagnetic force , which diminishes as charged particles separate, color-charged particles feel increasing force. Nonetheless, color-charged particles may combine to form color neutral composite particles called hadrons . A quark may pair up with an antiquark: the quark has a color and the antiquark has the corresponding anticolor. The color and anticolor cancel out, forming a color neutral meson . Alternatively, three quarks can exist together, one quark being "red", another "blue", another "green". These three colored quarks together form

7808-462: The electron has a minuscule coupling y electron = 2 × 10 , while the top quark has the largest coupling to the Higgs, y t ≈ 1 . In the Standard Model, all of the quark and lepton Higgs–Yukawa couplings are small compared to the top-quark Yukawa coupling. This hierarchy in the fermion masses remains a profound and open problem in theoretical physics. Higgs–Yukawa couplings are not fixed constants of nature, as their values vary slowly as

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7936-510: The elementary charge (e), depending on flavor. Up, charm, and top quarks (collectively referred to as up-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

8064-500: The kaon ( K ) and pion ( π ) hadrons discovered in cosmic rays in 1947. In a 1970 paper, Glashow, John Iliopoulos and Luciano Maiani presented the GIM mechanism (named from their initials) to explain the experimental non-observation of flavor-changing neutral currents . This theoretical model required the existence of the as-yet undiscovered charm quark . The number of supposed quark flavors grew to

8192-409: The on-shell scheme . Estimates of the values of quark masses depend on the version of quantum chromodynamics used to describe quark interactions. Quarks are always confined in an envelope of gluons that confer vastly greater mass to the mesons and baryons where quarks occur, so values for quark masses cannot be measured directly. Since their masses are so small compared to the effective mass of

8320-473: The running of the large Higgs–Yukawa coupling of the top quark. If a quark Higgs–Yukawa coupling has a large value at very high energies, its Yukawa corrections will evolve downward in mass scale and cancel against the QCD corrections. This is known as a (quasi-) infrared fixed point , which was first predicted by B. Pendleton and G.G. Ross, and by Christopher T. Hill , No matter what the initial starting value of

8448-919: 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. 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." The names "bottom" and "top", coined by Harari, were chosen because they are "logical partners for up and down quarks". Alternative names for bottom and top quarks are "beauty" and "truth" respectively, but these names have somewhat fallen out of use. While "truth" never did catch on, accelerator complexes devoted to massive production of bottom quarks are sometimes called " beauty factories ". Quarks have fractional electric charge values – either (− ⁠ 1 / 3 ⁠ ) or (+ ⁠ 2 / 3 ⁠ ) times

8576-408: 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↑. A quark of one flavor can transform into a quark of another flavor only through the weak interaction, one of

8704-428: The " multiverse " outside our known universe). Some predictions of the string theory include existence of extremely massive counterparts of ordinary particles due to vibrational excitations of the fundamental string and existence of a massless spin-2 particle behaving like the graviton . Technicolor theories try to modify the Standard Model in a minimal way by introducing a new QCD-like interaction. This means one adds

8832-548: 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

8960-428: The DØ data (which had been searched for a much lighter top), the two groups jointly reported the discovery of the top at a mass of 176 ± 18 GeV/ c . In the years leading up to the top-quark discovery, it was realized that certain precision measurements of the electroweak vector boson masses and couplings are very sensitive to the value of the top-quark mass. These effects become much larger for higher values of

9088-517: The GIM mechanism to become part of the Standard Model. With the acceptance of the GIM mechanism, Kobayashi and Maskawa's prediction also gained in credibility. Their case was further strengthened by the discovery of the tau by Martin Lewis Perl 's team at SLAC between 1974 and 1978. The tau announced a third generation of leptons , breaking the new symmetry between leptons and quarks introduced by

9216-517: The GIM mechanism. Restoration of the symmetry implied the existence of a fifth and sixth quark. It was in fact not long until a fifth quark, the bottom, was discovered by the E288 experiment team, led by Leon Lederman at Fermilab in 1977. This strongly suggested that there must also be a sixth quark, the top, to complete the pair. It was known that this quark would be heavier than the bottom, requiring more energy to create in particle collisions, but

9344-403: The Standard Model attempts to combine the electroweak interaction with the strong interaction into a single 'grand unified theory' (GUT). Such a force would be spontaneously broken into the three forces by a Higgs-like mechanism . This breakdown is theorized to occur at high energies, making it difficult to observe unification in a laboratory. The most dramatic prediction of grand unification

9472-606: The Standard Model have been made since its codification in the 1970s. These include notions of supersymmetry , which double the number of elementary particles by hypothesizing that each known particle associates with a "shadow" partner far more massive. However, like an additional elementary boson mediating gravitation, such superpartners remain undiscovered as of 2024. All elementary particles are either bosons or fermions . These classes are distinguished by their quantum statistics : fermions obey Fermi–Dirac statistics and bosons obey Bose–Einstein statistics . Their spin

9600-416: The Standard Model is limited by its omission of gravitation and has some parameters arbitrarily added but unexplained. According to the current models of Big Bang nucleosynthesis , the primordial composition of visible matter of the universe should be about 75% hydrogen and 25% helium-4 (in mass). Neutrons are made up of one up and two down quarks, while protons are made of two up and one down quark. Since

9728-447: The Standard Model what the Standard Model did for the particle zoo that came before it. Most models assume that almost everything in the Standard Model can be explained in terms of three to six more fundamental particles and the rules that govern their interactions. Interest in preons has waned since the simplest models were experimentally ruled out in the 1980s. Accelerons are the hypothetical subatomic particles that integrally link

9856-492: The Standard Model. The branching ratios for these decays have been determined to be less than 1.8 in 10000 for photonic decay and less than 5 in 10000 for Z boson decay at 95% confidence . The Standard Model generates fermion masses through their couplings to the Higgs boson . This Higgs boson acts as a field that fills space. Fermions interact with this field in proportion to their individual coupling constants y i , which generates mass. A low-mass particle, such as

9984-454: The Yukawa coupling changes with energy scale  μ . Solutions to this equation for large initial values y t cause the right-hand side of the equation to quickly approach zero, locking y t to the QCD coupling g 3 . The value of the top quark fixed point is fairly precisely determined in the Standard Model, leading to a top-quark mass of 220 GeV. This is about 25% larger than

10112-421: The additional quarks. In 1977, the bottom quark was observed by a team at Fermilab led by Leon Lederman . 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 the CDF and DØ teams at Fermilab. It had a mass much larger than expected, almost as large as that of

10240-450: The atom were first identified toward the end of the 19th century , beginning with the electron , followed by the proton in 1919, the photon in the 1920s, and the neutron in 1932. By that time the advent of quantum mechanics had radically altered the definition of a "particle" by putting forward an understanding in which they carried out a simultaneous existence as matter waves . Many theoretical elaborations upon, and beyond ,

10368-433: The coupling is, if sufficiently large, it will reach this fixed-point value. The corresponding quark mass is then predicted. The top-quark Yukawa coupling lies very near the infrared fixed point of the Standard Model. The renormalization group equation is: where g 3 is the color gauge coupling, g 2 is the weak isospin gauge coupling, and g 1 is the weak hypercharge gauge coupling. This equation describes how

10496-551: The current experimental and theoretical knowledge about elementary particle physics is the Particle Data Group , where different international institutions collect all experimental data and give short reviews over the contemporary theoretical understanding. other pages are: Top quark The top quark , sometimes also referred to as the truth quark , (symbol: t) is the most massive of all observed elementary particles . It derives its mass from its coupling to

10624-556: 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. Charm quarks were produced almost simultaneously by two teams in November 1974 (see November Revolution ) – one at SLAC under Burton Richter , and one at Brookhaven National Laboratory under Samuel Ting . The charm quarks were observed bound with charm antiquarks in mesons. The two parties had assigned

10752-516: The determination of | V tb | from single top production provides tests for the assumption that the CKM matrix is unitary. The Standard Model also allows more exotic decays, but only at one loop level, meaning that they are extremely rare. In particular, it is conceivable that a top quark might decay into another up-type quark (an up or a charm) by emitting a photon or a Z-boson. However, searches for these exotic decay modes have produced no evidence that they occur, in accordance with expectations of

10880-464: 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 quark model's validity. In the following years a number of suggestions appeared for extending the quark model to six quarks. Of these, the 1975 paper by Haim Harari was the first to coin the terms top and bottom for

11008-507: The discovery of the top was imminent. As the SPS gained competition from the Tevatron at Fermilab there was still no sign of the missing particle, and it was announced by the group at CERN that the top mass must be at least 41 GeV/ c . After a race between CERN and Fermilab to discover the top, the accelerator at CERN reached its limits without creating a single top, pushing the lower bound on its mass up to 77 GeV/ c . The Tevatron

11136-561: The down quarks in the neutron ( u d d ) decays into an up quark by emitting a virtual W boson, transforming the neutron into a proton ( u u d ). The W boson then decays into an electron and an electron antineutrino. Both beta decay and the inverse process of inverse beta decay are routinely used in medical applications such as positron emission tomography (PET) and in experiments involving neutrino detection . While

11264-439: The electric charge ( Q ) and all flavor quantum numbers ( B , I 3 , C , S , T , and B ′) are of opposite sign. Mass and total angular momentum ( J ; equal to spin for point particles) do not change sign for the antiquarks. As described by quantum chromodynamics , the strong interaction between quarks is mediated by gluons, massless vector gauge bosons . Each gluon carries one color charge and one anticolor charge. In

11392-409: The electric charge of a hadron 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. For example, the hadron constituents of atomic nuclei, neutrons and protons, have charges of 0 e and +1 e respectively; the neutron

11520-524: The electric charge) have equal magnitude but opposite sign . The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until deep inelastic scattering experiments at the Stanford Linear Accelerator Center in 1968. Accelerator program experiments have provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995,

11648-472: The electroweak interaction among elementary particles. Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces are theorized to unify as a single electroweak force at high energies. This prediction was clearly confirmed by measurements of cross-sections for high-energy electron-proton scattering at the HERA collider at DESY . The differences at low energies

11776-450: The energy scale (distance scale) at which they are measured. These dynamics of Higgs–Yukawa couplings, called "running coupling constants", are due to a quantum effect called the renormalization group . The Higgs–Yukawa couplings of the up, down, charm, strange and bottom quarks are hypothesized to have small values at the extremely high energy scale of grand unification, 10  GeV . They increase in value at lower energy scales, at which

11904-543: The existence of supersymmetric particles , abbreviated as sparticles , which include the sleptons , squarks , neutralinos , and charginos . Each particle in the Standard Model would have a superpartner whose spin differs by 1 ⁄ 2 from the ordinary particle. Due to the breaking of supersymmetry , the sparticles are much heavier than their ordinary counterparts; they are so heavy that existing particle colliders would not be powerful enough to produce them. Some physicists believe that sparticles will be detected by

12032-483: The existence of quarks comes from deep inelastic scattering : firing electrons at nuclei to determine the distribution of charge within nucleons (which are baryons). If the charge is uniform, the electric field around the proton should be uniform and the electron should scatter elastically. Low-energy electrons do scatter in this way, but, above a particular energy, the protons deflect some electrons through large angles. The recoiling electron has much less energy and

12160-563: The existence of quarks, including the other second generation quark, the strange quark , was obtained in 1968; strange particles were discovered back in 1947.) When in November 1974 teams at Brookhaven National Laboratory (BNL) and the Stanford Linear Accelerator Center (SLAC) simultaneously announced the discovery of the J/ψ meson , it was soon after identified as a bound state of the missing charm quark with its antiquark. This discovery allowed

12288-399: The field. Above a certain energy threshold, pairs of quarks and antiquarks are created . These pairs bind with the quarks being separated, causing new hadrons to form. This phenomenon is known as color confinement : quarks never appear in isolation. This process of hadronization occurs before quarks formed in a high energy collision are able to interact in any other way. The only exception

12416-510: The first fractions of a second after the Big Bang , when the universe was in an extremely hot and dense phase (the quark epoch ). Studies of heavier quarks are conducted in artificially created conditions, such as in particle accelerators . Having electric charge, mass, color charge, and flavor, quarks are the only known elementary particles that engage in all four fundamental interactions of contemporary physics: electromagnetism, gravitation, strong interaction, and weak interaction. Gravitation

12544-460: The formation of composite particles known as hadrons (see § Strong interaction and color charge below). The quarks that determine the quantum numbers of hadrons are called valence quarks ; apart from these, any hadron may contain an indefinite number of virtual " sea " quarks, antiquarks, and gluons , which do not influence its quantum numbers. There are two families of hadrons: baryons , with three valence quarks, and mesons , with

12672-533: 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 quark (down, strange, and bottom quarks) and vice versa. This flavor transformation 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

12800-633: The general expectation was that the sixth quark would soon be found. However, it took another 18 years before the existence of the top was confirmed. Early searches for the top quark at SLAC and DESY (in Hamburg ) came up empty-handed. When, in the early 1980s, the Super Proton Synchrotron (SPS) at CERN discovered the W ;boson and the Z ;boson , it was again felt that

12928-407: The larger medium. In the course of asymptotic freedom , the strong interaction becomes weaker at increasing temperatures. Eventually, color confinement would be effectively lost in an extremely hot plasma of freely moving quarks and gluons. This theoretical phase of matter is called quark–gluon plasma . The exact conditions needed to give rise to this state are unknown and have been the subject of

13056-410: The names of the first generation of quarks ( up and down ) reflecting the fact that the two were the "up" and "down" component of a weak isospin doublet . The proposal of Kobayashi and Maskawa heavily relied on the GIM mechanism put forward by Sheldon Glashow , John Iliopoulos and Luciano Maiani , which predicted the existence of the then still unobserved charm quark . (Direct evidence for

13184-452: The newfound mass of the neutrino to the dark energy conjectured to be accelerating the expansion of the universe . In this theory, neutrinos are influenced by a new force resulting from their interactions with accelerons, leading to dark energy. Dark energy results as the universe tries to pull neutrinos apart. Accelerons are thought to interact with matter more infrequently than they do with neutrinos. The most important address about

13312-482: The observable universe. The number of protons in the observable universe is called the Eddington number . In terms of number of particles, some estimates imply that nearly all the matter, excluding dark matter , occurs in neutrinos, which constitute the majority of the roughly 10 elementary particles of matter that exist in the visible universe. Other estimates imply that roughly 10 elementary particles exist in

13440-428: The observed top mass and may be hinting at new physics at higher energy scales. The quasi-infrared fixed point subsequently became the basis of top quark condensation and topcolor theories of electroweak symmetry breaking, in which the Higgs boson is composed of a pair of top and antitop quarks. The predicted top-quark mass comes into improved agreement with the fixed point if there are additional Higgs scalars beyond

13568-466: The only elementary fermions with neither electric nor color charge . The remaining six particles are quarks (discussed below). The following table lists current measured masses and mass estimates for all the fermions, using the same scale of measure: millions of electron-volts relative to square of light speed (MeV/ c ). For example, the most accurately known quark mass is of the top quark ( t ) at 172.7  GeV/ c , estimated using

13696-417: The only known particles whose electric charges are not integer multiples of the elementary charge . There are six types, known as flavors , of quarks: up , down , charm , strange , top , and bottom . Up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay : the transformation from a higher mass state to

13824-475: The ordinary particles. The 12 fundamental fermions are divided into 3  generations of 4 particles each. Half of the fermions are leptons , three of which have an electric charge of −1  e , called the electron ( e ), the muon ( μ ), and the tau ( τ ); the other three leptons are neutrinos ( ν e , ν μ , ν τ ), which are

13952-501: The other common elementary particles (such as electrons, neutrinos, or weak bosons) are so light or so rare when compared to atomic nuclei, we can neglect their mass contribution to the observable universe's total mass. Therefore, one can conclude that most of the visible mass of the universe consists of protons and neutrons, which, like all baryons , in turn consist of up quarks and down quarks. Some estimates imply that there are roughly 10 baryons (almost entirely protons and neutrons) in

14080-486: 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 a mathematical table , called the Cabibbo–Kobayashi–Maskawa matrix (CKM matrix). Enforcing unitarity , the approximate magnitudes of the entries of the CKM matrix are: where V ij represents

14208-422: The proton contained much smaller, point-like objects and was therefore not an elementary particle. Physicists were reluctant to firmly identify these objects with quarks at the time, instead calling them " partons " – a term coined by Richard Feynman . The objects that were observed at SLAC would later be identified as up and down quarks as the other flavors were discovered. Nevertheless, "parton" remains in use as

14336-447: The quark masses are generated by the Higgs. The slight growth is due to corrections from the QCD coupling. The corrections from the Yukawa couplings are negligible for the lower-mass quarks. One of the prevailing views in particle physics is that the size of the top-quark Higgs–Yukawa coupling is determined by a unique nonlinear property of the renormalization group equation that describes

14464-430: The same quantum state ( Pauli exclusion principle ). Also, bosons can be either elementary, like photons, or a combination, like mesons . The spin of bosons are integers instead of half integers. Gluons mediate the strong interaction , which join quarks and thereby form hadrons , which are either baryons (three quarks) or mesons (one quark and one antiquark). Protons and neutrons are baryons, joined by gluons to form

14592-498: The size of a proton, i.e. less than 10 metres. The following table summarizes the key properties of the six quarks. Flavor quantum numbers ( isospin ( I 3 ), charm ( C ), strangeness ( S , not to be confused with spin), topness ( T ), and bottomness ( B ′)) are assigned to certain quark flavors, and denote qualities of quark-based systems and hadrons. The baryon number ( B ) is + ⁠ 1 / 3 ⁠ for all quarks, as baryons are made of three quarks. For antiquarks,

14720-515: 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 color constantly changes, their strong interaction

14848-399: The surrounding gluons, slight differences in the calculation make large differences in the masses. There are also 12 fundamental fermionic antiparticles that correspond to these 12 particles. For example, the antielectron (positron) e is the electron's antiparticle and has an electric charge of +1  e . Isolated quarks and antiquarks have never been detected,

14976-541: The tendency of a quark of flavor i to change into a quark of flavor j (or vice versa). There exists an equivalent weak interaction matrix for leptons (right side of the W boson on the above beta decay diagram), called the Pontecorvo–Maki–Nakagawa–Sakata matrix (PMNS matrix). Together, the CKM and PMNS matrices describe all flavor transformations, but the links between the two are not yet clear. According to quantum chromodynamics (QCD), quarks possess

15104-427: The top (or antitop) can decay only through the weak force . It decays to a W boson and either a bottom quark (most frequently), a strange quark , or, on the rarest of occasions, a down quark . The Standard Model determines the top quark's mean lifetime to be roughly 5 × 10  s . This is about a twentieth of the timescale for strong interactions, and therefore it does not form hadrons , giving physicists

15232-574: The top mass and therefore could indirectly see the top quark even if it could not be directly detected in any experiment at the time. The largest effect from the top-quark mass was on the T ;parameter , and by 1994 the precision of these indirect measurements had led to a prediction of the top-quark mass to be between 145 GeV/ c and 185 GeV/ c . It is the development of techniques that ultimately allowed such precision calculations that led to Gerardus 't Hooft and Martinus Veltman winning

15360-399: The top. In the following years, more evidence was collected and on 22 April 1994, the CDF group submitted their article presenting tentative evidence for the existence of a top quark with a mass of about 175 GeV/ c . In the meantime, DØ had found no more evidence than the suggestive event in 1992. A year later, on 2 March 1995, after having gathered more evidence and reanalyzed

15488-409: The universe at any given moment). String theory proposes that our universe is merely a 4-brane, inside which exist the three space dimensions and the one time dimension that we observe. The remaining 7 theoretical dimensions either are very tiny and curled up (and too small to be macroscopically accessible) or simply do not/cannot exist in our universe (because they exist in a grander scheme called

15616-405: The visible universe (not including dark matter ), mostly photons and other massless force carriers. The Standard Model of particle physics contains 12 flavors of elementary fermions , plus their corresponding antiparticles , as well as elementary bosons that mediate the forces and the Higgs boson , which was reported on July 4, 2012, as having been likely detected by the two main experiments at

15744-442: The way quarks occur in nature. Zweig preferred the name ace for the particle he had theorized, but Gell-Mann's terminology came to prominence once the quark model had been commonly accepted. The quark flavors were given their names for several reasons. The up and down quarks are named after the up and down components of isospin , which they carry. Strange quarks were given their name because they were discovered to be components of

15872-413: 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

16000-551: Was (until the start of LHC operation at CERN in 2009) the only hadron collider powerful enough to produce top quarks. In order to be able to confirm a future discovery, a second detector, the DØ detector , was added to the complex (in addition to the Collider Detector at Fermilab (CDF) already present). In October 1992, the two groups found their first hint of the top, with a single creation event that appeared to contain

16128-431: Was conjectured from the beginnings of the quark model but not discovered until the early 21st century. Elementary fermions are grouped into three generations , 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, and there

16256-465: 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. In less than a year, extensions to the Gell-Mann–Zweig model were proposed. Sheldon 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

16384-454: Was the last to be discovered. The Standard Model is the theoretical framework describing all the known elementary particles . This model contains six flavors of quarks ( q ), named up ( u ), down ( d ), strange ( s ), charm ( c ), bottom ( b ), and top ( t ). Antiparticles of quarks are called antiquarks , and are denoted by

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