Standard model of particle physics  
Standard
Model


In theoretical physics, Quantum chromodynamics (QCD) is a theory of the strong interaction (color force), a fundamental force describing the interactions of the quarks and gluons making up hadrons (such as the proton, neutron or pion). It is the study of the SU(3) Yang–Mills theory of colorcharged fermions (the quarks). QCD is a quantum field theory of a special kind called a nonabelian gauge theory. It is an important part of the Standard Model of particle physics. A huge body of experimental evidence for QCD has been gathered over the years.
QCD enjoys two peculiar properties:
Moreover: the abovementioned two properties are continuous all the way, i.e. there is no phasetransition line separating them.
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The word quark was coined by American physicist Murray GellMann (b. 1929) in its present sense, the word having been taken from the phrase "Three quarks for Muster Mark" in Finnegans Wake by James Joyce. GellMann wrote in a private letter of June 27, 1978, to the editor of the Oxford English Dictionary that he had been influenced by Joyce's words: "The allusion to three quarks seemed perfect" (originally there were only three subatomic quarks.) GellMann, however, wanted to pronounce the word with (ô) not (ä), as Joyce seemed to indicate by rhyming words in the vicinity such as Mark. GellMann got around that "by supposing that one ingredient of the line 'Three quarks for Muster Mark' was a cry of 'Three quarts for Mister . . . ' heard in H.C. Earwicker's pub," a plausible suggestion given the complex punning in Joyce's novel.^{[1]}
The three kinds of charge in QCD (as opposed to one in quantum electrodynamics or QED) are usually referred to as "color charge" by loose analogy to the three kinds of color (red, green and blue) perceived by humans. Other than this "clever" nomenclature, the quantum parameter "color" is completely unrelated to the everyday, familiar phenomenon of color.
Since the theory of electric charge is dubbed "electrodynamics", the Greek word "chroma" Χρώμα (meaning color) is applied to the theory of color charge, "chromodynamics".
With the invention of bubble chambers and spark chambers in the 1950s, experimental particle physics discovered a large and evergrowing number of particles called hadrons. It seemed that such a large number of particles could not all be fundamental. First, the particles were classified by charge and isospin by Eugene Wigner and Werner Heisenberg; then, in 1953, according to strangeness by Murray GellMann and Kazuhiko Nishijima. To gain greater insight, the hadrons were sorted into groups having similar properties and masses using the eightfold way, invented in 1961 by GellMann and Yuval Ne'eman. GellMann and George Zweig, correcting an earlier approach of Shoichi Sakata, went on to propose in 1963 that the structure of the groups could be explained by the existence of three flavours of smaller particles inside the hadrons: the quarks.
Perhaps the first remark that quarks should possess an additional quantum number was made^{[2]} as a short footnote in the preprint of Boris Struminsky^{[3]} in connection with Ω ^{−} hyperon composed of three strange quarks with parallel spins (this situation was peculiar, because since quarks are fermions, such combination is forbidden by the Pauli exclusion principle):
Three identical quarks cannot form an antisymmetric Sstate. In order to realize an antisymmetric orbital Sstate, it is necessary for the quark to have an additional quantum number.
– B. V. Struminsky, Magnetic moments of barions in the quark model, JINRPreprint P1939, Dubna, Submitted on January 7, 1965
Boris Struminsky was a PhD student of Nikolay Bogolyubov. The problem considered in this preprint was suggested by Nikolay Bogolyubov, who advised Boris Struminsky in this research^{[3]}. In the beginning of 1965, Nikolay Bogolyubov, Boris Struminsky and Albert Tavchelidze wrote a preprint with a more detailed discussion of the additional quark quantum degree of freedom^{[4]}. This work was also presented by Albert Tavchelidze without obtaining consent of his collaborators for doing so at an international conference in Trieste (Italy), in May 1965^{[5]}^{[6]}.
A similar mysterious situation was with the Δ^{++} baryon; in the quark model, it is composed of three up quarks with parallel spins. However, since quarks are fermions, this combination is forbidden by the Pauli exclusion principle. In 1965, MooYoung Han with Yoichiro Nambu and Oscar W. Greenberg independently resolved the problem by proposing that quarks possess an additional SU(3) gauge degree of freedom, later called color charge. Han and Nambu noted that quarks might interact via an octet of vector gauge bosons: the gluons.
Since free quark searches consistently failed to turn up any evidence for the new particles, and because an elementary particle back then was defined as a particle which could be separated and isolated, GellMann often said that quarks were merely convenient mathematical constructs, not real particles. The meaning of this statement was usually clear in context: He meant quarks are confined, but he also was implying that the strong interactions could probably not be fully described by quantum field theory.
Richard Feynman argued that high energy experiments showed quarks are real particles: he called them partons (since they were parts of hadrons). By particles, Feynman meant objects which travel along paths, elementary particles in a field theory.
The difference between Feynman's and GellMann's approaches reflected a deep split in the theoretical physics community. Feynman thought the quarks have a distribution of position or momentum, like any other particle, and he (correctly) believed that the diffusion of parton momentum explained diffractive scattering. Although GellMann believed that certain quark charges could be localized, he was open to the possibility that the quarks themselves could not be localized because space and time break down. This was the more radical approach of Smatrix theory.
James Bjorken proposed that pointlike partons would imply certain relations should hold in deep inelastic scattering of electrons and protons, which were spectacularly verified in experiments at SLAC in 1969. This led physicists to abandon the Smatrix approach for the strong interactions.
The discovery of asymptotic freedom in the strong interactions by David Gross, David Politzer and Frank Wilczek allowed physicists to make precise predictions of the results of many high energy experiments using the quantum field theory technique of perturbation theory. Evidence of gluons was discovered in three jet events at PETRA in 1979. These experiments became more and more precise, culminating in the verification of perturbative QCD at the level of a few percent at the LEP in CERN.
The other side of asymptotic freedom is confinement. Since the force between color charges does not decrease with distance, it is believed that quarks and gluons can never be liberated from hadrons. This aspect of the theory is verified within lattice QCD computations, but is not mathematically proven. One of the Millennium Prize Problems announced by the Clay Mathematics Institute requires a claimant to produce such a proof. Other aspects of nonperturbative QCD are the exploration of phases of quark matter, including the quarkgluon plasma.
The relation between the shortdistance particle limit and the confining longdistance limit is one of the topics recently explored using string theory, the modern form of Smatrix theory.^{[7]}^{[8]}
Every field theory of particle physics is based on certain symmetries of nature whose existence is deduced from observations. These can be
QCD is a gauge theory of the SU(3) gauge group obtained by taking the color charge to define a local symmetry.
Since the strong interaction does not discriminate between different flavors of quark, QCD has approximate flavor symmetry, which is broken by the differing masses of the quarks.
There are additional global symmetries whose definitions require the notion of chirality, discrimination between left and righthanded. If the spin of a particle has a positive projection on its direction of motion then it is called lefthanded; otherwise, it is righthanded. Chirality and handedness are not the same, but become approximately equivalent at high energies.
The color group SU(3) corresponds to the local symmetry whose gauging gives rise to QCD. The electric charge labels a representation of the local symmetry group U(1) which is gauged to give QED: this is an abelian group. If one considers a version of QCD with N_{f} flavors of massless quarks, then there is a global (chiral) flavor symmetry group . The chiral symmetry is spontaneously broken by the QCD vacuum to the vector (L+R) SU_{V}(N_{ f}) with the formation of a chiral condensate. The vector symmetry, U_{B}(1) corresponds to the baryon number of quarks and is an exact symmetry. The axial symmetry U_{A}(1) is exact in the classical theory, but broken in the quantum theory, an occurrence called an anomaly. Gluon field configurations called instantons are closely related to this anomaly.
There are two different types of SU(3) symmetry: there is the symmetry that acts on the different colors of quarks, and this is an exact gauge symmetry mediated by the gluons, and there is also a flavor symmetry which rotates different flavors of quarks to each other, or flavor SU(3). Flavor SU(3) is an approximate symmetry of the vacuum of QCD, and is not a fundamental symmetry at all. It is an accidental consequence of the small mass of the three lightest quarks.
In the QCD vacuum there are vacuum condensates of all the quarks whose mass is less than the QCD scale. This includes the up and down quarks, and to a lesser extent the strange quark, but not any of the others. The vacuum is symmetric under SU(2) isospin rotations of up and down, and to a lesser extent under rotations of up, down and strange, or full flavor group SU(3), and the observed particles make isospin and SU(3) multiplets.
The approximate flavor symmetries do have associated gauge bosons, observed particles like the rho and the omega, but these particles are nothing like the gluons and they are not massless. They are emergent gauge bosons in an approximate string description of QCD.
The dynamics of the quarks and gluons are controlled by the quantum chromodynamics Lagrangian. The gauge invariant QCD Lagrangian is
where is the quark field, a dynamical function of spacetime, in the fundamental representation of the SU(3) gauge group, indexed by ; are the gluon fields, also a dynamical function of spacetime, in the adjoint representation of the SU(3) gauge group, indexed by ; are the Dirac matrices, connecting the spinor representation to the vector representation of the Lorentz group; and are the generators, connecting the fundamental, antifundamental and adjoint representations of the SU(3) gauge group. The GellMann matrices provide one such representation for the generators.
The symbol represents the gauge invariant gluonic field strength tensor, analogous to the electromagnetic field strength tensor, , in Electrodynamics. It is given by
where are the structure constants of SU(3). Note that the rules to moveup or pulldown the a, b, or c indexes are trivial, (+......+), so that whereas for the μ or ν indexes one has the nontrivial relativistic rules, corresponding e.g. to the signature (+). Furthermore, for mathematicians, according to this formula the gluon colour field can be represented by a SU(3)Lie algebravalued "curvature"2form where is a "vector potential"1form corresponding to and is the (antisymmetric) "wedge product" of this algebra, producing the "structure constants" f^{abc}.
The constants m and g control the quark mass and coupling constants of the theory, subject to renormalization in the full quantum theory.
An important theoretical notion concerning the final term of the above Lagrangian is the Wilson loop variable. This loop variable plays a mostimportant role in discretized forms of the QCD (see lattice QCD), and more generally, it distinguishes confined and deconfined states of a gauge theory. It was introduced by the Nobel prize winner Kenneth G. Wilson and is treated in a separate article.
Quarks are massive spin1/2 fermions which carry a color charge whose gauging is the content of QCD. Quarks are represented by Dirac fields in the fundamental representation 3 of the gauge group SU(3). They also carry electric charge (either 1/3 or 2/3) and participate in weak interactions as part of weak isospin doublets. They carry global quantum numbers including the baryon number, which is 1/3 for each quark, hypercharge and one of the flavor quantum numbers.
Gluons are spin1 bosons which also carry color charges, since they lie in the adjoint representation 8 of SU(3). They have no electric charge, do not participate in the weak interactions, and have no flavor. They lie in the singlet representation 1 of all these symmetry groups.
Every quark has its own antiquark. The charge of each antiquark is exactly the opposite of the corresponding quark.
According to the rules of quantum field theory, and the associated Feynman diagrams, the above theory gives rise to three basic interactions: a quark may emit (or absorb) a gluon, a gluon may emit (or absorb) a gluon, and two gluons may directly interact. This contrasts with QED, in which only the first kind of interaction occurs, since photons have no charge. Diagrams involving FaddeevPopov ghosts must be considered too.
Further analysis of the content of the theory is complicated. Various techniques have been developed to work with QCD. Some of them are discussed briefly below.
This approach is based on asymptotic freedom, which allows perturbation theory to be used accurately in experiments performed at very high energies. Although limited in scope, this approach has resulted in the most precise tests of QCD to date.
Among nonperturbative approaches to QCD, the most well established one is lattice QCD. This approach uses a discrete set of spacetime points (called the lattice) to reduce the analytically intractable path integrals of the continuum theory to a very difficult numerical computation which is then carried out on supercomputers like the QCDOC which was constructed for precisely this purpose. While it is a slow and resourceintensive approach, it has wide applicability, giving insight into parts of the theory inaccessible by other means. However, the numerical sign problem makes it difficult to use lattice methods to study QCD at high density and low temperature (e.g. nuclear matter or the interior of neutron stars).
A wellknown approximation scheme, the 1/N expansion, starts from the premise that the number of colors is infinite, and makes a series of corrections to account for the fact that it is not. Until now it has been the source of qualitative insight rather than a method for quantitative predictions. Modern variants include the AdS/CFT approach.
For specific problems effective theories may be written down which give qualitatively correct results in certain limits. In the best of cases, these may then be obtained as systematic expansions in some parameter of the QCD Lagrangian. One such effective field theory is chiral perturbation theory or ChiPT, which is the QCD effective theory at low energies. More precisely, it is a low energy expansion based on the spontaneus chiral symmetry breaking of QCD, which is an exact symmetry when quark masses are equal to zero, but for the u,d and s quark, which have small mass, it is still a good approximate symmetry. Depending on the number of qarks which are treated as light, one uses either SU(2) ChiPT or SU(3) ChiPT . Other effective theories are heavy quark effective theory (which expands around heavy quark mass near infinity), and softcollinear effective theory (which expands around large ratios of energy scales). In addition to effective theories, models like the NambuJonaLasinio model and the chiral model are often used when discussing general features.
The notion of quark flavours was prompted by the necessity of explaining the properties of hadrons during the development of the quark model. The notion of colour was necessitated by the puzzle of the Δ^{++}. This has been dealt with in the section on the history of QCD.
The first evidence for quarks as real constituent elements of hadrons was obtained in deep inelastic scattering experiments at SLAC. The first evidence for gluons came in three jet events at PETRA.
Good quantitative tests of perturbative QCD are
Quantitative tests of nonperturbative QCD are fewer, because the predictions are harder to make. The best is probably the running of the QCD coupling as probed through lattice computations of heavyquarkonium spectra. There is a recent claim about the mass of the heavy meson B_{c} [1]. Other nonperturbative tests are currently at the level of 5% at best. Continuing work on masses and form factors of hadrons and their weak matrix elements are promising candidates for future quantitative tests. The whole subject of quark matter and the quarkgluon plasma is a nonperturbative test bed for QCD which still remains to be properly exploited.

