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Quarks ( or ) are a type of elementary particle and major constituents of matter. They are the only particles in the Standard Model to experience all four fundamental forces, which are also known as fundamental interactions. Due to a phenomenon known as color confinement, single quarks are not normally found on their own; they can only be found in composite particles called hadrons, such as protons and neutrons.
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Quarks ( or ) are a type of elementary particle and major constituents of matter. They are the only particles in the Standard Model to experience all four fundamental forces, which are also known as fundamental interactions. Due to a phenomenon known as color confinement, single quarks are not normally found on their own; they can only be found in composite particles called hadrons, such as protons and neutrons. For this reason, much of what is known about quarks has been inferred from observations on the hadrons themselves. However as of March 6, 2009 statistical evidence collected by Fermilab points towards the creation of singly produced top quarks.
There are six different types of quarks, known as flavors: up (symbol: ), down , charm , strange , top and bottom . The up and down quarks have the lowest masses of all quarks, and thus are generally stable and very common in the universe. The other quarks are much more massive, and will rapidly decay into the lighter up and down quarks. Because of this, the heavier charm, strange, top and bottom quarks can only be produced in high energy collisions, such as in particle accelerators and cosmic rays.
Quarks have various properties, such as electric charge, color charge, spin and mass. For every quark flavor there is a corresponding antiparticle, called antiquark, that differs from the quark only in that some of its properties have the opposite sign. Quarks are the only known particles whose electric charge comes in fractions of the elementary charge.
The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. There was little evidence for the quark model until 1968, when electron–proton scattering experiments indicated that the electrons were scattering off three point-like constituents inside the proton. By 1995, when the top quark was observed at Fermilab, all six flavors had been observed.
Classification
The Standard Model is the theoretical framework describing all the currently known elementary particles, plus the Higgs boson (unobserved ). This model comprises six flavors of quarks, named up, down, charm, strange, top and bottom. The top and bottom flavors are sometimes known as truth and beauty, respectively. In this context, flavor is an arbitrarily chosen term referring to different kinds of particles, and has nothing to do with the everyday experience of flavor.
In the Standard Model, particles of matter, comprising quarks and leptons, are elementary fermions, meaning that they have a half-integer quantum number (a property related to their intrinsic angular momentum); as a consequence, they are subject to the Pauli exclusion principle, stating that no two fermions of the same flavor can ever simultaneously occupy the same state. This contrasts with particles that mediate forces—the other particles of the standard model. Such particles are bosons, meaning that they have integer spin; as a consequence, the Pauli exclusion principle does not apply to them. Quarks, unlike leptons, have a color charge, a property causing them to engage in the strong interaction. This interaction is the reason quarks attract each other to form hadrons. In the same way that the electric force is responsible for atoms attracting each other to form molecules, the strong interaction is responsible for protons and neutrons attracting each other to form atomic nuclei. But unlike the electric force which has infinite range, the strong interaction effectively only acts at close distances, comparable to or less. See nuclear force for more details.
Elementary fermions are grouped into three generations, each comprising two leptons and two quarks. The first generation includes up and down quarks, the second charm and strange quarks, and the third top and bottom quarks. All searches for a fourth generation of quarks and other elementary fermions have failed, and there is strong indirect evidence that more than three generations cannot exist: each generation comprises only one flavor of neutrino, and the existence of a fourth generation would imply values of the lifetime of the Z boson and the abundance of helium-4 in the universe that are at odds with experimental results. Particles in higher generations generally have greater mass and are less stable, tending to decay into lower-generation, less massive particles by means of weak interactions. Only the first-generation up and down quarks occur commonly in nature; heavier quarks can only be created in high-energy collisions, such as in cosmic rays, and quickly decay. As a result, these particles play little part in the universe of today, but likely were much more prominent in an earlier, hotter universe. Most studies conducted on heavier quarks have been performed in artificially created conditions, such as in particle accelerators.
Antiparticles of quarks are called antiquarks, and are denoted by a bar over the letter for the quark, such as for an up antiquark. As with antimatter in general, antiquarks have the same mass, lifetime and spin as their respective quarks, but the electric charge and other charges have the opposite sign.
Having electric charge, flavor, color charge and mass, quarks are the only known elementary particles that engage in all four fundamental interactions of contemporary physics: electromagnetism, weak interaction, strong interaction and gravitation. Gravitation, however, is usually irrelevant at subatomic scales, and is not described by the Standard Model.
See the table of properties below for a more complete analysis of the six quark flavors' properties.
History
The quark model was first postulated independently by physicists Murray Gell-Mann and George Zweig in 1964. At the time of the theory's initial proposal, the "particle zoo" consisted of a few leptons and a multitude of hadrons. Gell-Mann and Zweig posited that hadrons were not elementary particles, but instead composed of various combinations of quarks and antiquarks. They postulated 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, many having reservations regarding the actual physicality of the quark concept. Some believed the quark was merely an abstract concept that could be temporarily used to help explain certain concepts that were not well understood, while others believed that the quark was a physical entity.
In less than a year, extensions to the Gell-Mann–Zweig model were proposed when another duo of physicists, Sheldon Lee Glashow and James Bjorken, predicted the existence of a fourth flavor of quark, which they referred to as charm. The addition was proposed because it expanded the power and self-consistency of the theory: it allowed a better description of the weak interaction (the mechanism that allows quarks to decay), equalized the number of quarks with the number of known leptons, and implied a mass formula that correctly reproduced the masses of the known mesons (hadrons with integer spin).
In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center showed that the proton was not an elementary particle, but instead contained much smaller, point-like objects. While this showed that hadrons indeed had a substructure, as predicted by the quark model, physicists remained reluctant to identify these smaller objects with quarks. Instead, they became known as partons (a term proposed by Richard Feynman, and supported by some experimental project reports). These partons were later identified as up and down quarks when the other flavors were beginning to surface. Their discovery also validated the existence of the strange quark, because it was necessary to the model Gell-Mann and Zweig had proposed.
In a 1970 paper, Glashow, John Iliopoulos and Luciano Maiani gave more compelling theoretical arguments for the as-yet undiscovered charm quark. The number of supposed quark flavors grew to the current six in 1973, following proposition by Makoto Kobayashi and Toshihide Maskawa; the two had noted that the experimental observation of CP violation (a phenomenon that has been observed to cause changes in the way particles weakly interact when particle and antiparticle are swapped) could be explained if there were another pair of quarks.
It was the observation of the charm quark that finally convinced the physics community of the quark model's correctness. Following a decade without empirical evidence supporting their existence, charm quarks were finally produced and observed almost simultaneously by two teams in November 1974 (see November Revolution): one at the Stanford Linear Accelerator Center under Burton Richter and one at Brookhaven National Laboratory under Samuel Ting. The two parties had assigned the discovered particle two different names, J and ?. The particle hence became formally known as the meson and it was considered a charm quark–charm antiquark pair that Glashow and Bjorken had predicted.
In 1977, the bottom quark was observed by Leon Lederman and a team at Fermilab. This indicated that a top quark probably existed, because the bottom quark would have been without a partner if it had not. However, it was not until eighteen years later, in 1995, that the top quark was finally observed. The top quark's discovery was quite important, because it proved to be significantly more massive than expected; almost as heavy as a gold atom. Reasons for the top quark's extremely large mass remain unclear.
Etymology
Gell-Mann originally named the quark after the sound made by ducks. For some time, he was undecided on an actual spelling for the term he intended to coin, until he found the word quark in James Joyce's book Finnegans Wake:
Gell-Mann went into further detail regarding the name of the quark in his book, The Quark and the Jaguar:
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.
Properties
Hadronization
Various quark flavor combinations result in the formation of composite particles known as hadrons through the process of hadronization. There are two types of hadrons: baryons, formed of three quarks, and mesons, formed of a quark and an antiquark. The quarks (and antiquarks) which determine the quantum numbers of hadrons are called valence quarks. Apart from these, any hadron may contain an indefinite number of virtual quarks, antiquarks and gluons which do not influence their quantum numbers. Such virtual quarks are called sea quarks (see below).
The building blocks of the atomic nucleus—the proton and the neutron—are baryons. There are a great number of known hadrons, and most of them are differentiated by their quark content and the properties that these constituent quarks confer upon them. The existence of hadrons with more valence quarks, called exotic hadrons, such as the tetraquarks and pentaquarks has been postulated. and several experiments have been claimed to reveal the existence of tetraquarks and pentaquarks in the early 2000s, but all the reported pentaquarks candidates have been established as being non-existent since. The status of tetraquarks is still a matter of debate.
Electric charge
A quark has a fractional electric charge value, either - or + times the elementary charge (e). Specifically, up, charm and top quarks (collectively referred to as up-type quarks) have a charge of + each, while down, strange and bottom quarks (down-type quarks) have -. The antiquark have the opposite charge of their corresponding quark—up-type antiquarks have charges of - and down-type antiquarks have charges of +. Since the electric charge of a hadron is the sum of the charges of the constituent quarks, the combinations of three quarks, or three anti-quarks, or a quark with an anti-quark, always result in integer charge.
The electric charge of quarks is important in the construction of nuclei. The hadron constituents of the atom, the neutron and proton, have charges of 0 and +1 respectively; the neutron is composed of two down quarks and one up quark, and the proton of two up quarks and one down quark. The total electric charge of a nucleus, that is, the number of protons in it, is known as the atomic number, and it is the defining difference between atoms of different chemical elements.
Spin
Spin is an intrinsic property of quantum particles, and its direction is an important degree of freedom. Roughly speaking, the spin of a particle is a contribution to its angular momentum that is not due to its motion. It is sometimes visualized as the rotation of an object around its own axis (hence the name spin), but this description is somewhat misguided at subatomic scales, as elementary particles are believed to be point-like and so they cannot rotate around themselves.
It can be represented by a vector whose length is measured in units of h/(2π), where h is the Planck constant. This unit is often denoted by h ("h bar"), and called the "reduced Planck constant". The result of a measurement of the component of the spin of a quark along any axis is always either h/2 or -h/2; for this reason quarks are classified as spin- particles, which means they are fermions. The component of spin along any given axis—by convention the z axis—is often denoted by an up arrow ? for the value + and down arrow ? for the value -, placed after the symbol for flavor. For example, an up quark with a spin of + along the z axis is denoted by u?.
The quark's spin value contributes to the overall spin of the parent hadron, much as quark's electrical charge does to the overall charge of the hadron. Varying combinations of quark spins result in the total spin value that can be assigned to the hadron.
Weak interaction
A quark of one flavor can transform into a quark of a different flavor through the weak interaction, one of the four fundamental interactions through which particles interact with each other. A quark can decay into a lighter quark by emitting a W boson, or can absorb a W boson to turn into a heavier quark. This mechanism causes the radioactive process of beta decay, in which a neutron "splits" into a proton, an electron and an electron antineutrino. This occurs when one of the down quarks in the neutron (composition ) decays into an up quark by emitting a virtual boson, transforming the neutron into a proton (composition ). The boson then decays into an electron and an electron antineutrino . A quark can also emit or absorb Z bosons.
| ? | | + | | + | | | (Beta decay, hadron notation) | | ? | | + | | + | | | (Beta decay, quark notation) | |
Weak interactions can also allow quarks or hadrons to decay into completely different elementary particles through a process of annihiliation. For example, for the pi meson (composition ), a decay into a corresponding quark–antiquark flavor pair such as or would result in an annihilation of the quark–antiquark pair. The release of energy therein could effect the creation of the new leptons, such as muons or neutrinos.
As well as being the only interaction capable of causing flavor changes, the weak interaction is the only interaction violating parity symmetry. It affects only left-handed quarks and leptons, and right-handed antiquarks and antileptons, so its effects would be different if left and right were swapped.
Cabibbo angle and CKM matrix
In 1963, Nicola Cabibbo introduced the Cabibbo angle (?C) to keep track of how often certain weak interaction decays occurred in nature. In light of current knowledge (quarks were not yet theorized when the angle was introduced), the Cabibbo angle is related to the probability that down and strange quarks decay into up quarks. In particle physics parlance, the down and strange quarks were said to form a weak interaction eigenstate, a quantum superposition of down and strange quarks quantum states (|d> and |s> respectively). Mathematically this can be represented in bra-ket notation as:
where |d′> is the weak interaction eigenstate and both Vud and Vus are complex coefficients. The square of the magnitudes of Vud and Vus (|Vud|2 and |Vus|2) represent the probability that down and strange quarks decay into up quarks. Using the currently accepted values for |Vud| and |Vus| (see below), the Cabbibo angle can be calculated using
The modern equivalent of the Cabibbo angle is a mathematical table called the Cabibbo–Kobayashi–Maskawa matrix (or CKM matrix), developed by Makoto Kobayashi and Toshihide Maskawa in 1972. The CKM matrix keeps track of the weak decays of all six quarks.
The CKM matrix has additional features beyond the description of how often quarks of a flavor decay into quarks of other flavors. Kobayashi and Maskawa first built the CKM matrix to explain the violation of CP symmetry in weak interactions—weak interactions do not behave the same way if particles are replaced by their antiparticles (C symmetry) and if left is swapped with right (P symmetry). CP violation cannot be explained with one or two generations of quarks, but is possible with three or more generations (see CKM matrix for more details).
Currently, the best determination of the magnitude of the entries of the CKM matrix is approximately:
Strong interaction and color charge
Quarks possess a property called color charge. Despite its name, this is not related to colors of visible light, just as the property flavor is not related to taste. There are three types of color charge, named blue, green and red. Each of them is complemented by an anti-color; antiblue, antigreen and antired. Each quark carries a color, while each antiquark carries an anticolor.
The system of attraction and repulsion between quarks charged with any of the three colors is called strong interaction. The area of physics that studies strong interactions is called quantum chromodynamics (QCD). A quark charged with one color value can be bound with an antiquark carrying the corresponding anticolor, while three quarks all charged with different colors will similarly be bound together. In any other case, the resulting system will be unstable. Quarks obtain their color and interact in this way via force mediating particles known as gluons, a concept which is further discussed below.
The three color types play a role in the process of hadronization, which is the process of hadron formation out of quarks and gluons. The result of two attracting quarks that form a stable quark–antiquark pair will be color neutrality: a quark with ? color charge plus an antiquark of -? color charge will result in a color charge of 0—or "white" color—and in the formation of a meson. Analogous to the additive color model, the combination of all three color charges will similarly result in a "white" color charge. This is what happens when quarks combine to form a baryon.
The properties of the color charge in quarks are explained by a gauge symmetry (a type of symmetry group) known as SU(3), or a special unitary group in three-dimensions. This concept of a symmetry group can be compared to the group consisting of the various states of a two-dimensional shape as it is rotated about an axis; in the same way, Gell-Mann and Yuval Ne'eman realised in 1961 that hadrons could be grouped together into subgroups of 8 or 10 based on their symmetrical transformations and their similarities in quantum numbers. For example, the spin- baryons formed one octet, as did the spin-1 and spin-0 mesons, while the spin- baryons were in a decuplet. It was realised that these octets and decuplets were all derivatives of a basic triplet subgroup, and it was this realisation that lead to the proposal of the quark theory and the idea that hadrons had substructure. The triplet is now understood to correspond to the up, down and strange quark and the colors they can adopt.
This close connection between symmetries and interactions is present in all forces described by the standard model. The general description of these connections are called Yang-Mills theories.
Mass
There are two different terms used when describing a quark's mass; current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark. These two values are typically very different in their relative size, for several reasons.
In a hadron most of the mass comes from the gluons that bind the constituent quarks together, rather than from the individual quarks. While gluons are inherently massless, they possess energy, and it is this energy that contributes so greatly to the overall mass of the hadron (see mass in special relativity). For example, a proton is composed of one and two quarks and has an overall mass of approximately 938 MeV/c2, of which the mass of three valence quarks contributes around 11 MeV/c2, with the remainder coming from the quantum chromodynamics binding energy (QCBE) provided by sea quarks and gluons.
Often, mass values can be derived after calculating the difference in mass between two related hadrons that have opposing or complementary quark components. For example, in comparing the proton to the neutron, where the difference between the two particles is one down quark to one up quark, the relative masses and the mass differences can be measured by the difference in the overall mass of the two hadrons.
The masses of most quarks were within predicted ranges at the time of their discovery, with the notable exception of the top quark, which was found to have a mass approximately equal to that of a gold nucleus, significantly heavier than expected. Various theories have been offered to explain this very large mass. The Standard Model posits that elementary particles derive their masses from the Higgs mechanism, related to the unobserved Higgs boson. Physicists hope that, in the next years, the detection of the Higgs boson in particle accelerators—such as the Large Hadron Collider—and the study of the top quark's interaction with the Higgs field might help answer the question.
Table of properties
The following table summarizes the key properties of the six quarks. Flavour quantum numbers (isospin (Iz), strangeness (S, not to be confused with spin), charmness (C), bottomness (B′) and topness (T)) are assigned to certain quark flavours, and is useful in the context of hadrons. For exemple a sigma baryon has a strangeness of -1, therefore contains 1 strange quark. The baryon number (B) is + for all quarks, as baryons are made of three quarks. For antiquarks, the electric charge (Q) and all flavour quantum numbers (B, Iz, C, S, T, and B′) are of opposite sign. Mass and total angular momentum (J) do not change sign for the antiquarks.
Quark flavor properties| Name | Symbol | Mass (MeV/c2) | J | B | Q | Iz | C | S | T | B′ | Antiparticle | Antiparticle symbol | | First generation | | Up | | 1.5 to 3.3 | | | | | 0 | 0 | 0 | 0 | Antiup | | | Down | | 3.5 to 6.0 | | | | | 0 | 0 | 0 | 0 | Antidown | | | Second generation | | Charm | | | | | | 0 | +1 | 0 | 0 | 0 | Anticharm | | | Strange | | | | | | 0 | 0 | -1 | 0 | 0 | Antistrange | | | Third generation | | Top | | | | | | 0 | 0 | 0 | +1 | 0 | Antitop | | | Bottom | | | | | | 0 | 0 | 0 | 0 | -1 | Antibottom | |
Color confinement and gluons
A key phenomenon called color confinement is thought to keep quarks within a hadron and prevent them from appearing in isolation. Color confinement is primarily caused by interactions with the gluon color field and the gluon exchange between quarks.
Color confinement applies to all quarks, with the possible exception of the top quark, whose behaviour with regard to this mechanism remains uncertain. Therefore, because all quarks are always confined, most of what is known experimentally about quarks has been inferred indirectly from the effects they have on their parent hadrons.
The top quark is an exception because its lifetime is so short that it does not have a chance to hadronize. One method used is to compare two hadrons that have all but one quark in common. The properties of the differing quarks are then inferred from the difference in values between the two.
Quarks have an inherent relationship with the gluon, which is technically a massless vector gauge boson. Gluons are responsible for the color field, or the strong interaction, that ensures that quarks remain bound in hadrons and causes color confinement.
Gluons are constantly exchanged between quarks through a virtual emission and
absorption process. When a gluon is transferred between one quark and another, a color change occurs in the receiving and emitting quark; for example, if a red quark emits a red–antigreen gluon, it becomes green, and if a green quark absorbs it, it becomes red. Therefore, while the quark colors continuously change, the forces of attraction are preserved.
The color field carried by the gluon contributes most significantly to a hadron's indivisibility into single quarks, or color confinement. This is demonstrated by the varying strength of the chromodynamic binding force between the constituent quarks of a hadron; as quarks come closer to each other, the chromodynamic binding force actually weakens in a process called asymptotic freedom. However, when they drift further apart, the strength of the bind dramatically increases. The color field becomes stressed by the drifting away of a quark, much as an elastic band is stressed when pulled apart, and a proportionate and necessary multitude of gluons of appropriate color property are created to strengthen the stretched field. In this way, an infinite amount of energy would be required to wrench a quark from its hadronized state. In practice, as soon as enough energy has been spent to distance the quarks, a quark–antiquark pair would be produced so that two hadrons would exist at the end.
These strong interactions are highly non-linear, because gluons can emit gluons and exchange gluons with other gluons. This property has led to a postulate regarding the possible existence of a glueball—an object that is purely made of gluons—despite previous observations indicating that gluons cannot exist without the 'attached' quarks.
Sea quarks
The quarks that contribute to the quantum numbers of the hadrons are called valence quarks . Hadrons also contain virtual quark–antiquark pairs, known as sea quarks , originating from the gluons' strong interaction field. Such sea quarks are much less stable, and they annihilate each other very quickly within the interior of the hadron. When a gluon is split, sea quarks are formed, and this process also works in reverse in that the annihilation of two sea quarks will reproduce a gluon. In addition to this, sea quarks can hadronize into baryonic or mesonic particles under the right circumstances. There is a constant quantum flux of sea quarks that are born from the vacuum, and this allows for a steady cycle of gluon splits and rebirths. This flux is colloquially known as "the sea".
Virtual quark-antiquark pairs have a tendency to form a kind of colored "cloud" or "shield" around the valence quarks in hadrons. This cloud is complimented by another layer of virtual gluons that lies beyond it. These two clouds have the property of adopting color charges based on that of the valence quark. The intrinsic qualities of quarks and the way they respond to color cause the virtual antiquarks in the cloud, which exhibit the anticolor of the valence quark, to be closer to the inside of the field. For example, in a red quark, the antired virtual antiquarks would tend to be closer to the red valence quark than their red virtual quark partners. This disparity in proximity and the anticolor barrier it creates has the effect of "de-amplifying" the color charge of the valence quark. However, this anticolor tilt is neutralised by the virtual gluon field beyond, which carries the original color charge and "re-amplifies" the intensity of the valence quark's color. The cancelling of these two influences on the valence quark results in the balanced color charge that valence quarks have been observed to possess.
QCD matter and free quarks
A notion that has recently come to prominence is that of quark matter, or QCD matter, a number of theorized phases of matter containing free quarks and gluons. One of these is the quark-gluon plasma. This model posits that, at sufficiently high temperatures and densities, quarks and gluons could potentially become deconfined and degenerate into a fluid-like plasma consisting of a non-uniform mix of gluons and quarks. The precise extremity of the conditions needed to give rise to this state are unknown and have been the subject of a great deal of speculation; CERN made many attempts to produce such conditions in the 1980s and 1990s. The symptoms of the state would variously include a great increase in the number of heavier quark pairs compared to the volume of up and down quark pairs. It is believed that, in the period prior to 10-6 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. It is also hypothesized that strange matter, that is non-nuclear matter containing relatively equal numbers of up, down and strange quarks, might also be stable at "ordinary" temperatures and pressures, in atomic nucleus-sized strangelets and kilometer-sized quark stars.
See also
- Eightfold way (physics) – Gell-Mann's original classification of hadrons into octets.
- Fundamental interactions – Processes by which elementary particles interact with each other
- Gravitational force – Attraction between bodies with mass
- Electromagnetic force – Force that the electromagnetic field exerts on electrically charged particles
- Strong interaction – The interaction responsible for the formation of hadrons
- Weak interaction – The interaction by which quarks decay into other quarks
- Electroweak theory – The unified theory of the weak interaction and of electromagnetism
- Gauge boson – Force mediating particles
- Gluons – Particles mediating the strong interaction
- Gravitons – The hypothetical particles mediating the gravitational force
- Photons – Particles mediating the electromagnetic force
- W and Z bosons – Particles mediating the weak interaction
- Hadrons – Particles made of quarks
- Mesons – Particles made of a quark and an antiquark
- Quarkonium – Mesons made of a quark and antiquark of the same flavour
- Baryons – Particles made of three quarks
- Diquarks – A hypothetical state of two quarks grouped inside a baryon, treated as a single particle with which the third quark interacts
- Tetraquarks – Hypothetical exotic mesons, made of two quarks and two anti quarks
- Pentaquarks – Hypothetical exotic baryons, made of four quarks and two anti quarks
- Leptons – The other fundamental particles of matter
- Partons – Model used to analyze high-energy hadron collisions
- Preons – Hypothetical particles which were once postulated to be subcomponents of quarks and leptons
- Quark–lepton complementarity – Possible fundamental relation between quarks and leptons
- Quark star – A hypothetical degenerate neutron star that is so dense it is considered a single giant hadron
Further reading
External links
- – A description of CERN’s experiment to count the families of quarks.
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