Strong interaction

The strong interaction (also strong force Gluonenkraft, color strength, for historical reasons called nuclear or strong nuclear force ) is one of the four fundamental forces of physics. With it, the bond between the quarks is explained in the hadrons.

Before the introduction of the quark model was referred to as the strong interaction, the attractive force between the nucleons (protons and neutrons) the atomic nucleus. Even today, with the strong interaction often only these residual interaction meant.

Binding between quarks

According to quantum chromodynamics (hereinafter QCD ), the strong interaction - such as the electromagnetic and the weak interaction - described by the exchange of gauge bosons. The exchange particle of the strong interaction are called gluons, of which there are eight varieties (different color charge states). The gluons carry a color charge between quarks. A gluon can interact with other gluons and exchange color charges.

The attractive force between quarks remains constant even with increasing distance, unlike eg in the Coulomb force at which it becomes easier with increasing distance to separate two mutually attracting particles. This makes it roughly comparable to a rubber cord or a tension spring. If the distance is too great, " rips " the rope in this analogy and it is a meson formed by producing a quark-antiquark pair from the vacuum. At a small distance the quarks can therefore be considered as free particles ( asymptotic freedom ), creating a containment ( confinement) is established. With greater distance causes the increasing interaction energy that the quarks the character of independent particles lose, so they can not be observed as free particles.

Binding between nucleons

Although nucleons always the color charge have zero, there is between them a residual interaction or nuclear power ( distance comparable to the van der Waals forces, which may be regarded as a residual electromagnetic interactions between electrically neutral atoms and / or molecules).

The range of the attraction by the residual interaction is about 2.5 femtometers (fm). At this distance it is comparable strong as the electrical repulsion ( Coulomb force ) between the protons and at shorter intervals outweighs them the Coulomb force. Distance above this, however, increases the attraction from steeper than the Coulomb force which decreases proportionally to square of 1 / r. This interplay of the two fundamental forces explains the cohesion and the magnitude of the atomic nuclei, but for example, the fission of heavy nuclei.

In the very short distances, the nuclear force is repulsive, according to a hard core (hard core) of 0.4 to 0.5 fm.

Before the introduction of the quark model, the residual interaction were and their low range explained by an effective theory by the exchange of pions between nucleons ( Yukawa potential) and the mass of the pions. Furthermore, the exchange of other mesons was in the nucleon -nucleon potential models considered ( such as the rho meson ). Since calculations of nuclear power with the QCD so far are not possible, one in the description of the nucleon - nucleon scattering used as various phenomenological potentials adjusted based on Mesonenaustauschmodellen (such as the Bonn- potential).

Explanation of the residual interaction

Interstitial atoms, the repulsive potential is a consequence of the Pauli principle for the electron states at small distances. At the approach of two nucleons with six quarks but each quark has considerably more degrees of freedom in the lowest state ( orbital angular momentum l = 0): in addition to Spin ( 2 states ) nor a color charge (3 states) and isospin (2 states), so together 12 on the the six quarks can distribute according to the Pauli principle. The Pauli principle here is not directly responsible for the rejection, which is noticeable below about 0.8 fm. The reason lies rather in the strong spin -spin interaction of the quarks, the obvious fact is expressed that the Delta - resonance ( with parallel spins of the three quarks ) is higher by about a third mass as the proton. Thus the spins are parallel to each other of the curd so increases the potential energy of the system. This also applies to overlapping nucleons, namely the stronger, the smaller the distance between the nucleons is another. Try to minimize the quarks by reversing the spin of their chromomagnetische energy, this can only be achieved by transition to a higher energy orbital angular momentum state (l = 1).

With even greater distance from each other, the nucleons move in the attractive part of the strong interaction. This plays less of the quark- quark- exchange ( two quarks are simultaneously two participating nucleons assigned ), one would expect by analogy with the covalent bond, a role, but rather that of color-neutral quark-antiquark pairs ( mesons ) from the Seequark share the Nukleonwellenfunktion in QCD.

However, a complete description of the nuclear power from the quantum chromodynamics is not yet possible.