# Strong interaction - Yukawa theory

Experimentally, the strong interaction manifests itself as the interaction between nucleons. The nucleon-nucleon interaction is characterized by high strength and short range. It is caused by the virtual emission and absorption of field quanta, in which process energy conservation is temporarily violated. However, this is permissible within the bounds set by Heisenberg's uncertainty relation. A finite range of the force indicates a finite rest mass of the exchange particles. The quanta of the electromagnetic field, the photons, have rest mass zero. Therefore, the range of the electromagnetic interaction is infinite. After estimating the rest mass based on the range, the pions were identified as the quanta of the strong interactio

Figure: Feynman diagram for the pion exchange between two nucleons (left)

In 1938, Hideki YUKAWA (Japanese Nobel prize laureate, 1907 - 1981) suggested a differential equation for the field of the nuclear force, in analogy to the electromagnetic field. In contrast, the nucleon-nucleon-(Yukawa-)potential falls off more rapidly than the electromagnetic one, and acts repulsively at short distances, which explains the finite density of atomic nuclei.

Figure: Schematic drawing of the potential between two nucleons

The complex character of the nuclear force at separations below 1.2 fm gives a hint that the internal quark structure of the hadrons cannot be neglected anymore when the nucleons begin to overlap. A more fundamental force enters the scene, namely the color force between quarks, the actual origin of the strong interaction.