Strong Interaction

The strong interaction , or strong force (called sometimes force of color ), is, with the force of gravitation, the electromagnetic Force, and the weak Interaction, one of the four interactions fundamental of the Physique. Only the Quark S and the Antiquark S are affected by this force which is carried by Boson S called Gluon S (in the same way that the electromagnetic force is carried by the Photon S). This strong force maintains the Quark S to form the Baryon S together, such as the Proton S or the Neutron S and to form the Méson S, the such Pion S or the Kaon S. All the sets of quarks (i.e let us baryons them and the mesons) are named Hadron S.

An effect derived from the strong force is responsible for the cohesion of the Nucléon S (protons and neutrons) within the core of the Atome (even Nuclear force).

History

Until in the the Seventies the protons and neutrons were regarded as the elementary particles and the expression strong interaction what is called indicated today the Nuclear force or residual strong interaction. One observed a force responsible for the cohesion of the atomic nucleus, while being able to exceed the electric repulsion between protons. It draws its name from this strong effect with short distance.
After the discovery of the quarks, the scientists realized that this force between nucleons with average distance was actually only the reflection of the interaction between the quarks (which constitute the protons) and gluons them, acting within the protons themselves. The old concept was thus replaced by that of residual strong interaction, and the “  nouvelle  ” interaction called force of color or quite simply strong interaction .

Basic principles

The theory which describes this strong interaction is the quantum Chromodynamique, so called by its English acronym QCD (ChromoDynamics Quantum). According to this theory, each quark carries a Charge of color which can be of three kinds: “  bleue  ”, “  verte  ” or “  rouge  ”. These “  couleurs  ” are only names and have nothing to do with the colors with the usual direction. The antiquarks on their side carry a load “  antibleue  ” (named also yellow, and equivalent to vert+rouge), “  antiverte  ” (named also magenta = bleu+rouge) or “  antirouge  ” (also named cyan = bleu+vert). A Hadron can exist only if its total color is neutral or “  blanche  ” (what one calls color also a singulet). Thus a Méson is composed of a pair quark-antiquark which can be only one symmetrical combination of “  bleue  ”   –   “  antibleue  ”, “  verte  ”   –   “  antiverte  ” and “  rouge  ”   –   “  antirouge  ”. In the same way a Baryon is made of three quarks (or three antiquarks) which will have to carry each one a different color “  bleue  ”, “  verte  ” and “  rouge  ” (or “  antibleu  ”, “  antiverte  ” and “  antirouge  ”), the sum of the three colors being neutral.

The Gluon S, intermediaries of the strong interaction, carry for their part at the same time a color and a anti-color (for example, blue-antirouge, or green-antibleu). There are 9 possibilities of associations of color-anticouleur but only 8 let us gluons, for mathematical reasons related to the Symétrie of gauge KNOWN (3) at the base of chromodynamic quantum (very briefly, the linear combination blue-antibleu + green-antivert + red-antirouge is completely neutral and does not correspond to a gluon). The interaction of a gluon with a quark can modify the color of this dernier : a gluon blue-antirouge absorptive by a red quark will transform it into quark bleu  ; or a green quark will be able to emit a gluon green-antirouge while becoming red. A consequence of this mechanism is that the load of color of a given quark will change in a continual way by exchange of gluons with its neighbors, but the total load of a system isolated from particles will be preserved during time. Thus the pair quark-antiquark of a Méson constantly passes from red-antirouge to green-antivert (by exchange of a gluon red-antivert), and blue-antibleu, etc, only the sum of the colors remain neutral.

A particular characteristic of the strong interaction is that it acts also on its own particles vectors, i.e. the Gluon S, because of their load of color. For example, a gluon green-antirouge can absorb a gluon blue-antivert to transform itself into blue-antirouge. This phenomenon is marginal in the case of the others fundamental interactions  : the photon, for example, is not electrically charged (makes of it the weak interaction shows a similar characteristic, from the load of W⁺ and W^– , but the effects on this interaction are negligible). For the strong interaction, this characteristic has as a consequence a very reduced range of this force, about the diameter of a high-energy particle (~  1  Fm). Another consequence is that the force between two quarks is about constant, unlike the other interactions where the force is proportional contrary to the square of the distance. If one seeks to separate two quarks, one will have to thus spend an increasingly large energy as the distance increases. To one moment, one will have provided enough energy to create new quarks or antiquarks which will join the initial quarks to create new high-energy particles.

This explains the fact that one cannot observe a quark alone, any attempt to isolate a quark (or a gluon) pleasing with creation from new quarks which will form a high-energy particle with the first. This phenomenon is called containment. Parallel to this, two very close quarks almost will not interact and will be free for between-them (as the two ends of a slackened spring), it is what one calls the asymptotic Liberté.

See too

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