Cromodinamica Quantistica Nel corso del XX secolo, il panorama delle forze conosciute era notevolmente mutato rispetto al secolo precedente, quando le quattro forze fondamentali erano ritenute. Quando due atomi neutri sono molto lontani, tra essi non si manifestano forze elettromagnetiche. In tal modo le forze fondamentali diventarono di nuovo quattro. Queste tre particelle sono oggi conosciute complessivamente con il nome di astenoni dal greco "asthenos", "debole". La loro teoria costituisce attualmente uno dei pilastri del Modello Standard.
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Terminology[ edit ] Physicist Murray Gell-Mann coined the word quark in its present sense. Other than this nomenclature, the quantum parameter "color" is completely unrelated to the everyday, familiar phenomenon of color.
The force between quarks is known as the colour force  or color force  or strong interaction , and is responsible for the strong nuclear force. Main articles: History of quantum mechanics and History of quantum field theory With the invention of bubble chambers and spark chambers in the s, experimental particle physics discovered a large and ever-growing 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 —56,    according to strangeness by Murray Gell-Mann and Kazuhiko Nishijima see Gell-Mann—Nishijima formula.
Gell-Mann and George Zweig , correcting an earlier approach of Shoichi Sakata , went on to propose in that the structure of the groups could be explained by the existence of three flavors of smaller particles inside the hadrons: the quarks. In order to realize an antisymmetric orbital S-state, it is necessary for the quark to have an additional quantum number.
The problem considered in this preprint was suggested by Nikolay Bogolyubov, who advised Boris Struminsky in this research. In —65, Greenberg  and Han — Nambu  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, Gell-Mann 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.
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 Gell-Mann 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 S-matrix theory. James Bjorken proposed that pointlike partons would imply certain relations in deep inelastic scattering of electrons and protons, which were verified in experiments at SLAC in This led physicists to abandon the S-matrix approach for the strong interactions.
In the concept of color as the source of a "strong field" was developed into the theory of QCD by physicists Harald Fritzsch and Heinrich Leutwyler , together with physicist Murray Gell-Mann. This is different from QED, where the photons that carry the electromagnetic force do not radiate further photons.
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. 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 non-perturbative QCD are the exploration of phases of quark matter , including the quark—gluon plasma. The relation between the short-distance particle limit and the confining long-distance limit is one of the topics recently explored using string theory , the modern form of S-matrix theory.