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Quark
In physics, a '''quark''' ( or ) is a type of subatomic particle.[
] In technical terms, quarks are elementary particle|elementary fermions which engage in the strong interaction due to their color charge.[
] Because of a phenomenon known as color confinement, quarks are never found on their own in nature; they are always bound together in composite particles named hadrons.[
]
The lightest flavors, the up quark and the down quark, are generally stable and are very common in the universe, as they are found in protons and neutrons and are one of the primary building blocks of matter. The more massive charm, strange, top and bottom quarks are unstable and rapidly particle decay|decay; these can only be produced under high energy physics|high energy conditions, such as in particle accelerators and in cosmic rays. For every quark flavor there is a corresponding antiparticle, or ''antiquark'', that differs from the quark only in that some of its properties have the opposite sign.
The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964.[
] There was little evidence for the theory until 1968, when electron–proton scattering experiments indicated the existence of substructure within the proton resembling three "sphere-like" regions.[
][
] By 1995, when the top quark was observed at Fermilab, all the six flavors had been observed.
Since quarks are not found in isolation, their properties can only be deduced from experiments on hadrons.[ Furthermore, it has been theorized in some of the Big Bang theories, that in the very beginning, the extremely hot early universe may have contained single quarks, including "free" top quarks, in a quark-gluon plasma.
] Classification
The Standard Model is the theoretical framework describing all the currently known elementary particles, plus the as-yet-unobserved Higgs boson. This model comprises six flavor (particle physics)|flavors of quarks,[
] named ''up'', ''down'', ''charm'', ''strange'', ''top'' and ''bottom''. The top and bottom flavors are also sometimes known as ''truth'' and ''beauty'', respectively.[
] The term "flavor" has nothing to do with the everyday experience of flavor, and is simply an arbitrarily named property that is easy to remember. Any two quarks of the same flavor are identical particles, meaning that all of their properties are the same.
In the Standard Model, particles of matter, including quarks and leptons, are fermions, meaning that their spin (physics)|spin quantum number (a property related to their intrinsic angular momentum) is half-integer; as a consequence, they are subject to the Pauli exclusion principle, stating that no two fermions of the same flavor can ever simultaneously occupy the same quantum state|state. This contrasts with force carrier|particles mediating forces, which are bosons, and have integer spin; as a consequence, the Pauli exclusion principle does not apply to them.[
] Quarks, unlike leptons (the best-known flavor of which is the electron), have a color charge, a property causing them to engage in the strong interaction, which binds them together in hadrons.
Elementary fermions are grouped into three generation (particle physics)|generations, each one comprising two leptons and two quarks. The first generation includes up and down quarks, the second includes charm and strange quarks, and the third includes top and bottom quarks. All searches for a fourth generation of quarks and other elementary fermions have failed, and there is strong indirect evidence that there cannot exist more than three generations.[Each generation comprises exactly one flavor of neutrinos, and if there were more than three neutrino flavors, the abundance of helium-4 produced in Big Bang nucleosynthesis would be greater, and the lifetime of the Z boson would be shorter, than what is observed. See Barrow (1994).][
] Particles in higher generations generally have greater mass and are less stable, tending to particle decay|decay into lower-generation, less massive particles by means of weak interaction. Typically, only the first-generation up and down quarks are in common natural occurrence; heavier quarks can only be created in high-energy conditions, such as in cosmic rays, and quickly decay. Most studies conducted on heavier quarks have been performed in artificially created conditions, such as in particle accelerators.
Antiparticles of quarks are called ''antiquark''s, and denoted by a bar over the letter for the quark, such as for an up antiquark. Like antimatter in general, antiquarks have the same mass and spin of their respective quarks, but the electric charge and other charge (physics)|charges have the opposite sign.[
]
Having electric charge, flavor, color charge and mass, quarks are the only known elementary particles engaging in all the four fundamental interactions of contemporary physics: respectively, electromagnetism, weak interaction, strong interaction and gravitation. The last is usually irrelevant at subatomic scales, and is not described by the Standard Model.
See the #Table of properties|table of properties below for a more complete analysis of the six quark flavors' properties.
History
The quark theory was first postulated by physicists Murray Gell-Mann and George Zweig in 1964.[
]
The Gell-Mann–Zweig model predicted three quarks, which they named ''up'', ''down'' and ''strange''.[
] At the time, the pair of physicists ascribed various properties and values to the three new proposed particles, such as electric charge and spin (physics)|spin.[
] The initial reaction of the physics community to the proposal was mixed, many having reservations regarding the actual physicality of the quark concept. They believed the quark was merely an abstract concept that could be temporarily used to help explain certain concepts that were not well understood, rather than an actual entity that existed in the way that Gell-Mann and Zweig had envisioned.[
]
In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center showed that the proton had substructure.[
][
][
] but it "was unfashionable to identify them explicitly with quarks".[
] 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 proposed quark flavors grew to the current six in 1973, when Makoto Kobayashi (physicist)|Makoto Kobayashi and Toshihide Maskawa noted that the experimental observation of CP violation could be explained if there were another pair of quarks. They named the two additional quarks ''top'' and ''bottom''.[
]
Etymology
Gell-Mann originally named the quark after the sound ducks make.[
] 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|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: Adventures in the Simple and the Complex'':
George Zweig, the co-proposer of the theory, preferred the name ''ace'' for the particle he had theorized, but Gell-Mann's terminology came to prominence once the quark model had been commonly accepted.[
]
Properties
Hadronization
Various quark flavor combinations result in the formation of composite particles known as ''hadrons''. There are two types of hadrons: baryons, which get their quantum numbers from three quarks, and mesons, which get their quantum numbers from a quark and an antiquark. The quarks (and antiquarks) which impart quantum numbers to hadrons are called ''valence quarks''. Apart from these, any hadron may contain an indefinite number of virtual particle|virtual quarks, antiquarks and gluons which together contribute nothing to their quantum numbers. Such virtual quarks are called ''sea quarks'' (#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.[
] Several experiments have been claimed to reveal the existence of tetraquarks and pentaquarks,[
] in the early 2000s, but all the reported candidates have been established as being non-existent since.[
]
Weak interaction
A quark of one flavor can transform into a quark of a different flavor by the weak interaction. A quark can particle decay|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 decay|radioactive process of beta decay, in which a neutron "splits" into a proton, an electron and an antineutrino. This occurs when one of the down quarks in the neutron (composed by ) decays into an up quark by emitting a boson, transforming the neutron into a proton (). The boson then decays into an electron () and an electron antineutrino ().[
] A quark can also emit or absorb a Z boson.
Weak interaction can also allow quarks or hadrons to decay into completely different elementary particles through a process of annihiliation. Taking the , a mesonic particle consisting of , a decay into a corresponding quark-antiquark flavor pair such as or would result in an annihiliation of the quark-antiquark pair. The release of energy therein could effect the creation of the new leptons, such as muons or neutrinos.[
]
As well as being the only interaction capable of causing flavor changes, the weak interaction is the only interaction parity violation|violating parity symmetry, that is, the only one which would not stay unchanged if left and right were swapped. It exclusively acts on chirality (physics)|left-handed quarks and leptons, and on right-handed antiquarks and antileptons.
Electric charge
A quark has a fractional electric charge value, either −1/3 or +2/3 (measured in elementary charges). Specifically, up, charm and top quarks (collectively referred to as ''up-type quarks'') have a charge of +2/3 each, while down, strange and bottom quarks (''down-type quarks'') have −1/3; the charge of an antiquark is the negative of the charge of the corresponding quark. The electric charge of a hadron is the sum of the charges of the constituent quarks;[
] the total is always an integer.
The electric charge of quarks is important in the construction of nuclei. The hadron constituents of the atom, the neutron and proton, have charges of 0 and +1 respectively; the neutron is composed of two down quarks and one up quark, and the proton of two up quarks and one down quark. The total electric charge of a nucleus, that is, the number of protons in it, is known as the atomic number, and it is the main difference between atoms of different chemical elements.[
]
Spin
The term ''spin'' denotes a intrinsic property of quantum particles, whose direction is an important degree of freedom. Roughly speaking, the spin of a particle is a contribution to its angular momentum that is not due to its motion. It is sometimes visualized as the rotation of an object around its own axis; hence the name ''Wikt:spin|spin''. However, this description is incorrect, as elementary particles are believed to be point particle|point-like and so they cannot rotate around themselves.
Spin is measured in units of ''h''/(2π), where ''h'' is the Planck constant. This unit is often denoted by ''ħ'', and called the "reduced Planck constant". The result of a measurement of the component of the spin of a quark along any axis is always either ''ħ''/2 or −''ħ''/2; for this reason quarks are classified as spin-½|spin-1/2 particles, which means they are fermions.[
] The component of spin along any given axis—by convention the ''z'' axis—is denoted by an up arrow ↑ for the value +1/2 and down arrow ↓ for −1/2, respectively, which follows the symbol for the flavor. For example, an up quark with a positive spin of 1/2 along the ''z'' axis is denoted by u↑.[
]
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.[
] For example, the proton and the Delta baryon| baryon are both composed of two up quarks and one down quarks: in the their spins are all aligned in the same direction, yielding a total spin of 3/2, whereas in the proton one of them has the opposite direction, giving a total spin of 1/2. However, this view has been recently challenged in quantum chromodynamics by theories that include vacuum polarization and the coupling of quark hadrons to strange quarks in the vacuum.[
]
Color charge
In addition to the electric charge, quarks possess a property called ''color charge''. Despite its name, this is not related to colors of visible light.[
] There are three types of color charge a quark can carry, named ''blue'', ''green'' and ''red''; each of them is complemented by an anti-color: ''antiblue'', ''antigreen'' and ''antired'', respectively. While a quark can have red, green or blue charge, an antiquark can have antired, antigreen, or antiblue charge.
In quantum chromodynamics, the system of attraction and repulsion between quarks charged with any of the three colors is called strong interaction. A quark charged with one color value will be attracted to an antiquark carrying the corresponding anticolor, while three quarks all charged with different colors will similarly be forced together. In any other case, a force of repulsion will come into effect.[
] Quarks obtain their color and interact in this way via the exchange of quantum field carrier particles known as ''gluons'', a concept which is further #Color confinement and gluons|discussed below.
The three color types play a role in the process of hadronization, which is 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 ''n'' color charge plus an antiquark of −''n'' color charge will result in a color charge of 0, or "white". The combination of all three color charges will similarly result in the cancelling out of color charge, yielding the same "white" color charge.
These two methods of color neutral hadronization are the same as the two ways in which all hadrons are formed. Hence, all hadrons will be color neutral. A meson, comprised of two particles, is the result of the binding of a quark and antiquark that have opposite color charges, whereas a baryon, containing three particles, arises from the hadronization of three quarks, all charged with different colors.[
]
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; the mass of the quarks is almost negligible compared to the mass derived from the gluons' energy. While gluons are inherently massless, they possess energy, and it is this energy that contributes so greatly to the overall mass of the hadron: see "Mass in special relativity". This is demonstrated by a common hadron—the proton. Composed of one and two quarks, the proton has an overall mass of approximately 938 Electron volt#As a unit of mass|MeV/c2, of which the three valence quarks contribute around 10 MeV/c2, with the remainder coming from the quantum chromodynamics binding energy (QCBE) provided by sea quarks and gluons.[
] This makes direct calculations of quark masses based on quantum chromodynamics quite difficult, and often unreliable, as Perturbation theory (quantum mechanics)|quantum perturbation methods, that were very successful in quantum electrodynamics, now fail most of the time.
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.[
] Various theories have been offered to explain this very large mass. Since in the Standard Model it is theorized that elementary particles get their masses from the Higgs mechanism, it is hoped that, in the next years, the detection of the Higgs boson in particle accelerators—such as the Large Hadron Collider—and the study of the top quark's interaction with the Higgs field might help answer the question. Table of properties
The following table summarizes the key properties of the six known quarks:
*Notation like denotes measurement uncertainty: the value is between and , with 104 being the most likely value.
Color confinement and gluons
A key phenomenon called ''color confinement'' is thought to keep quarks within a hadron. This refers to a quark's inability to escape as a single particle from its parent hadron, thereby rendering impossible the actual observation of a single quark. Color confinement is primarily caused by interactions with the gluon color field and the gluon exchange between quarks.
Color confinement applies to all quarks, except for the case of the top quark where the actual escape mechanism at extremely high energies is still uncertain. Therefore, most of what is known experimentally about quarks has been inferred indirectly from the effects they have on their parent hadron's properties.[
][
] 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 hadrons.
Quarks have an inherent relationship with the gluon, which is technically a massless vector field|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, and are the subjects of the quantum chromodynamics research area.[
] Gluons, roughly speaking, carry both a color charge and an anti-color charge, for example red–antiblue.[
][
]
Gluons are constantly exchanged between quarks through a "virtual particle|virtual" emission and
re-absorption process. These gluon exchange events between quarks are extremely frequent, occurring approximately 1024 times every second.[
] 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.[
] These constant switches in color within quarks are mediated by the gluons in such a way that a bound hadron will constantly retain a dynamic and ever-changing set of color types that will preserve the force of attraction, therefore forever disallowing quarks to exist in isolation.[
]
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 pair production|produced so that two hadrons would exist at the end.
These strong interactions are highly Nonlinear system|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—a particle 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 particle|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 via a certain fragmentation function (mathematics)|function; for instance, a sea quark hadronizing into a pion () does so through the fragmentation function .[
] 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".[
]
Notes
See Also
- Hadron
- Lepton
- Strong interaction
- Weak interaction
References
Further reading
*
*
*
External links
- A Positron Named Priscilla – A description of CERN|CERN’s experiment to count the families of quarks.
- An elementary popular introduction
- Quark dance
- The original English word ''quark'' and its adaptation to particle physics
Category:Quarks
Category:Fundamental physics concepts
simple:Quark
Related Images- A proton, composed by two up quarks and one down quark.
- A pictorial representation of the six quarks' most likely decay modes, with mass increasing from left to right.
- All types of hadrons always have zero total color charge.
Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL
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