In theoretical physics, quantum chromodynamics ( QCD ) is a strong interaction theory between quarks and gluons, fundamental particles that form composite hadrons such as protons, neutrons and pions. QCD is a type of quantum field theory called the non-abelian measuring theory, with the SU symmetry group (3). The QCD analog electric charge is a property called color . Gluon is the carrier of theoretical powers, such as photons for electromagnetic forces in quantum electrodynamics. This theory is an important part of the Standard Model of particle physics. A large amount of experimental evidence for QCD has been collected over the years.
QCD exhibits two main properties:
- Colors of confinement, plasma. This is a consequence of the constant force between two color charges when they are separated: To increase the separation between two quarks in a hadron, an increase in the amount of energy is required. Eventually this energy produces a pair of quark-antiquarks, converting the early hadron into a pair of hadrons rather than producing an isolated color charge. Although not analytically proven, color confinement is well known from lattice QCD calculations and several decades of experimentation.
- Asymptotic freedom, a steady reduction in the strength of the interaction between quarks and gluons as the energy scale of the interaction increases (and the corresponding scale diminishes). The asymptotic freedom of QCD was discovered in 1973 by David Gross and Frank Wilczek, and independently by David Politzer in the same year. For this work all three shared the 2004 Nobel Prize in Physics.
Video Quantum chromodynamics
Terminology
Physicist Murray Gell-Mann (born 1929) creates the word quark in his current sense. Originally derived from the phrase "Three quarks for Muster Mark" in Finnegans Wake by James Joyce. On June 27, 1978, Gell-Mann wrote a personal letter to the editor of the Oxford English Dictionary, where he recounted that he had been influenced by Joyce's words: "The three quarks look perfect." (Initially , only three quarks have been found.) Gell-Mann, however, wants to say a word to rhyme with "fork" rather than with "park", as Joyce looks with the words of the surrounding poem like Mark . Gell-Mann found that "assuming that one of the items of the 'Three quark for Muster Mark' line is a cry of 'Three liters for Master...' sounded in the HC Earwicker pub", a sensible suggestion given the complex blow in Novel Joyce.
The three types of charge in QCD (compared to one in quantum electrodynamics or QED) are usually referred to as "color charge" with a loose analogy to the three types of colors (red, green and blue) felt by humans. In addition to this nomenclature, the "color" of quantum parameters really has nothing to do with the everyday and familiar color phenomenon.
The force between quarks is known as color strength (or color strength ) or strong interaction, and is responsible for strong nuclear forces.
Because the theory of electric charges dubbed "electrodynamics", the Greek word ????? chroma "color" is applied to the color charge theory, "chromodynamics".
Maps Quantum chromodynamics
History
With the discovery of bubble spaces and spark spaces in the 1950s, experimental particle physics discovered a large number of particles called hadrons. It seems like the number of particles that are so many is not all fundamentals. First, the particles are classified by charge and isospin by Eugene Wigner and Werner Heisenberg; then, in 1953-56, according to peculiarities by Murray Gell-Mann and Kazuhiko Nishijima (see Gell-Mann-Nishijima formula). To gain greater insight, the hadrons were sorted into groups of the same nature and mass using the eightfold way, discovered in 1961 by Gell-Mann and Yuval Ne'eman. Gell-Mann and George Zweig, corrected the previous approach of Shoichi Sakata, went on to propose in 1963 that the group structure can be explained by the presence of three smaller particle flavors in the hadron: quark.
Perhaps the first statement that a quark should have an additional quantum number made as a short foot note in a Boris Struminsky printout with respect to? - hyperon consisting of three strange quarks with parallel spins (this situation is strange, because since quarks are fermions, such combinations are prohibited by the Pauli exclusion principle):
Three identical quarks can not form S-state antisymmetric. To realize S-state antisymmetric orbitals, it is necessary to quark to have additional quantum numbers.
Boris Struminsky is a PhD student from Nikolay Bogolyubov. The issues considered in this precast are suggested by Nikolay Bogolyubov, who suggested Boris Struminsky in the study. At the beginning of 1965, Nikolay Bogolyubov, Boris Struminsky and Albert Tavkhelidze wrote a precast with a more detailed discussion of additional quantum quantum level of freedom. This work was also presented by Albert Tavchelidze without the consent of his colleagues to do so at an international conference in Trieste (Italy), in May 1965.
The mysterious situation is similar to baryon ; in the quark model, it consists of three quarks with parallel spins. In 1964-65, Greenberg and Han-Nambu independently solved the problem by proposing that the quark has an SU (3) additional rate of freedom, which is then called the color charge. Han and Nambu note that the quark may interact through the vector boson octet octet: gluon.
Since the free quark quest consistently fails to find any evidence for new particles, and since the current particles are defined as isolated and isolated particles, Gell-Mann often says that quarks are math constructs only convenient, not a real particle. The meaning of this statement is usually clear in context: He means a limited quark, but he also implies that strong interactions may not be fully explained by quantum field theory.
Richard Feynman argues that high energy experiments show quarks are real particles: it calls them partons (because they are part of hadrons). With particles, Feynman means objects that run along paths, elementary particles in field theory.
The difference between the Feynman and Gell-Mann approaches reflects the deep divisions within the theoretical physics community. Feynman thinks the quark has a position or momentum distribution, like other particles, and he (correctly) believes that the diffusion of the parton momentum explains the diffractive scattering. Although Gell-Mann believes that a particular quark charge can be localized, it is open to the possibility that the quark itself can not be localized because space and time are disconnected. This is a more radical approach than the S-matrix theory.
James Bjorken proposed that such particular points would imply certain relationships in electron and proton scatterings in inelastic, which were verified in experiments at SLAC in 1969. This led physicists to abandon the S-matrix approach to strong interactions.
In 1973 the concept of color as a source of "strong field" was developed into QCD theory by physicists Harald Fritzsch and Heinrich Leutwyler, along with physicist Murray Gell-Mann. In particular, they used a general field theory developed in 1954 by Chen Ning Yang and Robert Mills (see Yang-Mills theory), in which the carrier particles of a force can radiate the carrier particles further. (This is different from QED, where photons carrying electromagnetic forces do not emit more photons.)
The discovery of asymptotic freedom in strong interactions by David Gross, David Politzer and Frank Wilczek enabled physicists to make precise predictions from the results of many high energy experiments using quantum field theory techniques of perturbation theory. The evidence of gluon was found in a three-jet event at PETRA in 1979. This experiment became more and more precise, culminating in disturbing QCD verification at a rate of a few percent at the LEP at CERN.
The other side of asymptotic freedom is confinement. Since the force between the color charge does not decrease with distance, it is believed that quarks and gluons can not be liberated from hadrons. This aspect of the theory is verified in lattice QCD calculations, but not mathematically proven. One of the Millennium Gift Issues announced by the Clay Mathematics Institute requires plaintiffs to produce such evidence. Another aspect of non-perturbative QCD is the exploration of the quark matter phase, including the quark-gluon plasma.
The relationship between short-range particle boundaries and limiting long-distance limits is one of the recently explored topics using string theory, the modern form of the S-matrix theory.
Theory
Multiple definitions
Each particle of particle physics theory is based on a particular symmetry of nature whose existence is inferred from observation. This can happen
- local symmetry, that symmetry acts independently at any point in the spacetime. Any such symmetry is the basis of the measurement theory and requires the introduction of its own measuring boson.
- global symmetry, which is a symmetry whose operations must be applied simultaneously to all spacetime points.
QCD is the measuring theory of the measuring group SU (3) obtained by taking the color charge to define the local symmetry.
Because strong interactions do not distinguish between different quark flavors, QCD has an approximate taste symmetry , which is broken down by different quark masses.
There are additional global symmetries whose definitions require a sense of chirality, discrimination between the left and the right hand. If the spin of a particle has a positive projection in the direction of its movement then it is called left-handed; if not, it's not right-handed. Truth and handedness are not the same, but being less equal to high energy.
- Chiral symmetry involves the independent transformation of these two types of particles.
- Vector symmetry (also called diagonal symmetry) means the same transformation is applied to two chiralities.
- Axial symmetry is a symmetry in which one transformation is applied to the left hand and inverse particles on the right hand particle.
Additional notes: duality
As mentioned, asymptotic freedom means that at enormous energy - it is also related to short distance - practically no interaction between particles. This is different - more precisely people will say double - with what is used, because usually one connects no interaction with large distances . However, as mentioned in the original paper of Franz Wegner, a solid state theorist who introduced the simple invariant grid model of 1971, the high temperature behavior of the original model, such as the decay of strong correlations over long distances, according to the low temperature behavior of the double model (usually ordered!) , ie asymptotic decay of non-trivial correlations, eg. short distance deviation from the almost perfect setting, for short distances. Here, unlike Wegner, we have only a dual model, which is described in this article. Group symmetry
The SU (3) color group corresponds to the local symmetry whose measurements give rise to QCD. The electrical charge signifies the representation of the local symmetry group U (1) measured to provide QED: this is an abelian group. If one considers the QCD version with N f a sense of quark without mass, then there is a group of global flavor symmetry (chiral) SU L ( N f ) ÃÆ'â € "SU R ( N f ) ÃÆ'â €" U B (1) ÃÆ'â € "U A (1). The chiral symmetry is spontaneously broken by the vacuum QCD into the vector (L R) SU V ( N f ) with chiral condensate formation. The vector symmetry, U B (1) corresponds to the number of baryonic quarks and is an appropriate symmetry. Axial symmetry U A (1) is appropriate in classical theory, but is broken in quantum theory, an event called anomaly. Configuring a gluon field called instantons is closely related to this anomaly.
There are two different types of symmetry SU (3): there are symmetries that work on different colors of quarks, and this is the right gluon-mediated measuring symmetry, and there are also flavor symmetries that rotate different quark flavors. to each other, or taste SU (3) . Flavor SU (3) is the approximate symmetry of the QCD vacuum, and not the fundamental symmetry at all. This is the accidental consequence of the small mass of the lightest three quarks.
In a QCD vacuum there is a vacuum condensation of all quarks whose mass is smaller than the QCD scale. This includes the top and bottom quarks, and on the lower levels quarks are weird, but nothing else. Vacuum is symmetrical under SU (2) upper and lower isospin rotation, and to a lower level below the upper, lower and peculiar rotation, or the full flavored group SU (3), and the observed particles make isospin and SU (3) multiples.
The symmetry sense sensation has a corresponding measuring bosson, the observed particles are like rho and omega, but these particles are not like gluons and they are not mass. They are the bosons that appear in the QCD approximate string description.
Lagrangian
The quarks and gluons dynamics are controlled by the Lagrangian quantum chromodynamics. QCD Lagrangian invariant size is
di mana adalah bidang quark, fungsi dinamis ruangwaktu, dalam representasi fundamental dari kelompok pengukur SU (3), diindeks oleh ; adalah turunan kovarian turunan; "? adalah matriks Dirac menghubungkan representasi spinor ke representasi vektor dari grup Lorentz.
Simbol merepresentasikan pengukur kekuatan medan gluon invarian gonon, analog dengan tensor kekuatan medan elektromagnetik, F ?? , dalam elektrodinamika kuantum. Ini diberikan oleh:
di mana adalah bidang gluon, fungsi dinamik ruangwaktu, dalam representasi adjoint dari kelompok pengukur SU (3), yang diindeks oleh a , b ,...; dan f abc adalah konstanta struktur dari SU (3). Perhatikan bahwa aturan untuk menaikkan atau mencabut indeks a , b , atau c adalah sepele , ( ,..., ), sehingga f abc = f abc = f a bc sedangkan untuk ? atau ? indeks seseorang memiliki aturan relativistic yang tidak sepele yang berhubungan dengan tanda tangan metrik ( - - -).
The variables m and g correspond to mass quarks and couples of theories, respectively, which are subject to renormalization.
The important theoretical concept is the Wilson loop (named after Kenneth G. Wilson). In the lattice QCD, the final term of the above Lagrangian is discredited through the Wilson loop, and more generally the Wilson loop behavior can differentiate the limited and decompressed phases.
Columns
Quark is a massive spin- / 2 that carries a color charge that measures it is QCD content. Quark is represented by the Dirac field in the fundamental representation 3 of the measuring group SU (3). They also carry an electrical charge (either - // 3 or 2 / 3 ) and participate in weak interactions as part of a weak dual isospin. They carry global quantum numbers including baryon numbers, which are 1 / 3 for each quark, hypercharge and one of spice quantum numbers.
Gluon is a spin-1 boson that also carries a color charge, since they lie in the adjoint representation 8 of SU (3). They have no electric charge, do not participate in weak interactions, and have no taste. They lie in the singlet representation 1 of all these symmetry groups.
Each quark has its own antiquark. The charge of each antiquark is the opposite of the corresponding quark.
Dynamics
According to the rules of quantum field theory, and related Feynman diagrams, the above theory raises three basic interactions: the quark can emit (or absorb) the gluon, the gluon can excrete (or absorb) the gluon, and the two gluons can interact directly. This is different from QED, where only the first type of interaction occurs, because the photon has no charge. Diagrams involving the Faddeev-Popov ghost must be considered as well (except in the size of unitarity).
Legal and confinement area
Detailed computations with the Lagrangian mentioned above show that the effective potential between quarks and anti-quarks in a meson contains terms that increase in proportion to the distance between quarks and anti-quarks ( ), which represents some kind of "rigidity" of the interaction between particles and anti-particles at large distances, similar to the entropic elasticity of the rubber bands (See below ). This leads to the confinement of quarks to the interior of hadrons, ie mesons and nucleons, with the typical radius of R c , corresponding to the previous one The "bag model" of the hadron The order of magnitude "bag radius" is 1 fm (= Ã, 10 -15 m). In addition, the above-mentioned rigidity is quantitatively related to the so-called "jurisdictional" behavior of the Wilson loop product's expected value P W of the coupled coupled constant. around the closed loop W ; e.g. proportional to the area is flanked by the loop. For this behavior the non-abelian behavior of the measuring group is very important.
Method
Further analysis of the contents of the theory is complicated. Various techniques have been developed to work with QCD. Some of them are discussed briefly below.
Perturbative QCD
This approach is based on asymptotic freedom, which allows the interference theory to be used accurately in experiments conducted at very high energies. Although limited in scope, this approach has resulted in the most appropriate QCD tests to date.
QCD Lattice
Among the non-perturbative approaches to QCD, the most established is the QCD lattice. This approach uses a series of discrete spacetime points (called grids) to reduce the integral analytic path of the continuum theory to very difficult numerical calculations which are then performed on supercomputers like QCDOC built for this purpose. Although this is a slow and resource-intensive approach, this approach has broad applicability, providing insight into parts of the theory that are inaccessible in other ways, particularly into the explicit forces acting between quarks and antiquarks in a meson. However, the problem of numerical marking makes it difficult to use lattice methods to study QCD at high density and low temperatures (eg nuclear matter or interior neutron stars).
1 / N expansion
The famous approximation scheme, the extension of 1 / N , starts from the idea that the number of colors is unlimited, and makes a series of corrections to explain the fact that it is not. To date, this has been a source of qualitative insight rather than a method for quantitative predictions. Modern variants include the AdS/CFT approach.
Effective theory
For specific problems, effective theory can be written that gives qualitatively correct results within certain limits. In the best case, this can then be obtained as a systematic expansion in some Qagraph Lagrangian parameters. One of the most effective field theories is chiral contrast theory or ChiPT, which is the effective theory of QCD at low energy. More precisely, it is a low energy expansion based on spontaneous chiral symmetry breaking QCD, which is the exact symmetry when the quark mass is equal to zero, but for u, d and s quarks, which have a small mass, it is still a good approximation symmetry. Depending on the number of quarks treated as light, a person uses SU (2) ChiPT or SU (3) ChiPT. Another effective theory is the theory of effective heavy quarks (which expand around the mass of the heavy quarks near infinity), and the soft-collinear effective theory (which develops around large energy-scale ratios). In addition to effective theory, models such as the Nambu-Jona-Lasinio model and chiral models are often used when discussing common features.
QCD sum rule
Based on the operator's product expansion, one can obtain sets of relations that connect different observations to one another.
Nambu-Jona-Lasinio
In one of his recent works, Kei-Ichi Kondo is revealed as a low-energy QCD, a theory related to the Nambu-Jona-Lasinio model because it is essentially a non-local version of Polyakov-Nambu-Jona-Lasinio Model. Later in its local version, there is nothing besides the Nambu-Jona-Lasinio model in which one has incorporated the Polyakov circle effect, to describe a 'certain confinement'.
The Nambu-Jona-Lasinio model itself, among many other things, is used because it is a relatively simple chiral symmetry model, a phenomenon present to certain conditions (the chiral boundary ie the fermion without mass) in the QCD itself. In this model, however, there is no limit. In particular, the energy of isolated quarks in the physical vacuum is well-defined and limited.
Experimental test
The idea of ​​quark flavor is driven by the need to explain the properties of hadrons during the development of the quark model. Color ideas needed by the
puzzle? . This has been discussed in the history section of QCD.
The first evidence for quarks as a real constituent element of hadrons is obtained in deep inelastic scattering experiments in SLAC. The first evidence for gluon comes in three jet events at PETRA.
Some good quantitative tests of perennial QCD exist:
- Runs a concluded QCD coupling from many observations
- Internal scale violations are polarized in unpolarized and unpolarized scattering
- Production of vector bosons in colliders (this includes the Drell-Yan process)
- Direct photons generated in hadronic collisions
- Jet cross section in colliders
- Event forms can be observed in LEP
- Production of heavy-quarks in colliders
Quaditative QCD tests are non-perturbative less, as predictions are more difficult to make. The best is probably the run of the QCD coupling as probed through the calculation of the lattice of the heavy quarkonium spectrum. There have been recent claims about the heavy mass of B mesons c [3]. Other non-perturbative tests are currently at the best 5% level. Continue to work on the masses and form the hadron factors and their weak matrix elements are promising candidates for future quantitative tests. All quark matter matter and quark-gluon plasma are non-perturbative bed test for QCD that still needs to be exploited properly.
One qualitative prediction of QCD is that there are composite particles made of gluons called glueballs that have not been definitively observed experimentally. The definitive observation of glueballs with properties predicted by QCD would strongly confirm the theory. In principle, if glueballs can be sidelined definitively, this would be a serious experimental blow to QCD. However, by 2013, scientists can not confirm or deny the existence of glueballs definitively, despite the fact that particle accelerators have enough energy to produce them.
Cross-links to solid state physics
Namun, di sini penggabungan derajat kebebasan , yang dalam QCD sesuai dengan gluon , "dibekukan" menjadi nilai tetap (quenching). Sebaliknya, di QCD mereka "berfluktuasi" (annealing), dan melalui sejumlah besar tingkat derajat kebebasan, entropi memainkan peran penting (lihat di bawah).
Untuk positif J 0 termodinamika kaca spin Mattis sesuai sebenarnya hanya untuk "ferromagnet in disguise", hanya karena sistem ini tidak memiliki "frustrasi" sama sekali. Istilah ini adalah ukuran dasar dalam teori kaca spin. Secara kuantitatif itu identik dengan produk loop sepanjang loop tertutup W . Namun, untuk kaca spin Mattis - berbeda dengan gelas spin "asli" - kuantitas P W tidak pernah menjadi negatif.
The basic idea of ​​"frustration" of spin-glass is actually similar to the quantity of Wilson's loop from QCD. The only difference is again that in QCD one deals with SU (3) matrices, and this one deals with a "fluctuating" quantity. Energetically, the absence of perfect frustration must be unfavorable and unusual for the spin glass, which means that one must add a loop product to the Hamiltonian, with some kind of term representing "punishment". In QCD, Wilson's loop is very important for the Lagrangian.
The relationship between QCD and the "irregular magnetic system" (their spin glass) was also emphasized in a paper by Fradkin, Huberman and Shenker, who also emphasized the idea of ​​duality.
A further analogy consists of the similarities mentioned in polymer physics, in which, analogously with the Wilson Loops, the so-called "mesh webs" arise, which are essential for the formation of elasticity of entropy (strength proportional to the length) of rubber. tape. The non-abelian character of SU (3) corresponds thereby to the non-trivial "chemical linkage", which attaches different segments of the loop together, and "asymptotic freedom" means in the polymer analogy only the fact that in the shortwave, ie for (where R c is the characteristic correlation length for the glued loop, as mentioned above "bag radius", while w is the excitation wavelength) any non-trivial correlation disappears altogether, as if the system has been crystallized.
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