How do nuclear forces work between nucleons?

strong nuclear power

Since the same electrical charges repel each other strongly, it is initially astonishing that atomic nuclei are stable. After all, there are many positively charged protons gathered in a small space. So why don't the protons just fly apart with great force?

The answer to this is that there is another, much stronger force. This force is called strong nuclear power or strong interaction. It apparently only works within the atomic nuclei, i.e. among the nucleons. The strong nuclear force has no influence on leptons, such as the electron. From the fact that the atomic nuclei of neighboring atoms do not attract each other, one can also conclude that the strong nuclear force is a short range Has. Only directly neighboring protons and neutrons attract each other.

For quarks only

More precisely, the strong nuclear force only affects quarks and particles made up of quarks. These particles can be divided into mesons, which each consist of a quark and an antiquark, and baryons and antibaryons, which consist of three quarks or antiquarks. All of these particles are also grouped under the term hadrons.

Strong color charges

The strong interaction does not know, like the electromagnetic force, only one charge, there are three charges. These three charges are often named "red", "green" and "blue" after the basic colors of a monitor. In addition, there are also the negative charges for this, which are called "anti-red", "anti-green" and "anti-blue". Every quark carries a color charge, every antiquark carries an anti-color charge. One should however use the term Color charge not to be taken too literally, the color charges have nothing to do with real colors.

The strong force between the color-charged quarks works in such a way that there are very strong connections between a color charge and an anti-color charge. This explains the stability of mesons, opposite color charges attract each other, just as positively and negatively electrically charged particles attract each other.

However, there is another important rule in strong nuclear force: three quarks, each of which has a different of the three color charges, form a bound state called a baryon. The proton and the neutron are the best-known examples of these "color-neutral" states.

Nuclear force as a residual interaction

Mesons and baryons are so strongly bound objects that hardly anything of their color charges can be noticed from the outside. In this sense, they resemble an atom, which also appears to be electrically neutral (i.e. uncharged) to the outside world. However, if you bring two baryons close together, a residual interaction can be felt that roughly corresponds to the Van der Waals force in the atom. This force is much weaker than the direct strong nuclear force but still stronger than the electrical repulsion between protons. This is how the strong nuclear force can explain the cohesion of protons and neutrons in the atomic nucleus.

The strong nuclear force between quarks has one infinite range. But since it is already saturated within the hadrons, the attraction between the hadrons is only noticeable for small distances and decreases very quickly with a large distance. In the atomic nucleus, only directly neighboring nucleons attract each other.

Force transmission in the particle image

The strong interaction is often explained by the exchange of energy packets or quanta called gluons. Just as a short electrical force is described by an exchange of photons, the quarks exchange gluons with one another. In contrast to photons, however, gluons themselves carry color charges. One color charge and one anti-color charge. So there are different types of gluons with different charge states (for example a green-anti-red gluon, or a blue-anti-blue gluon). However, this means that gluons themselves interact with the strong force, i.e. also exchange gluons with one another. In addition, they are so energetic that they can spontaneously break down into a quark-antiquark pair. The gluon exchange thus forms a complicated web of gluons, quarks and antiquarks. This makes the strong nuclear force difficult to calculate.

Fortunately, the residual interaction between nucleons is much simpler. This interaction can be described by the exchange of pi mesons.

Pair creation

The strong interaction, like weak interaction and electric force, is able to create pairs of particles and antiparticles. If you try to split a proton into its components, the quarks, new quark-antiquark pairs are created as soon as enough energy is available. These Pair creation is one reason that quarks are never unbound.

In contrast to the weak interaction, the strong interaction can only generate quark-antilepton pairs but not lepton-antilepton pairs. In addition, under the strong nuclear force, no quark can change its group. For example, a K meson cannot decay through the strong nuclear force, since the strange quark it contains cannot simply become a lighter quark. Such a transformation is only possible for the weak nuclear force.

If protons are shot at each other in a particle accelerator, whole bundles of new particles and antiparticles (called jets) are created. A detailed examination of such jets provides information about the character of the strong interaction and the particles involved.

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Last change: 10.03.2014