The pimesons as force carriers

The pimesons can be said to be responsible for the confining force in the nucleus. Since the distance between the nucleons is small, the electrostatic repulsion (black arrows in the figure to the right) between the protons in the nucleus is very strong. This repulsion is overcome by forces (coloured arrows in the figure) that has to do with the strong interaction, which primarily acts between the quarks inside the neutrons and protons.

Courtesy of The Particle Adventure.

 

 

The force between the nucleons is described as an exchange of pimesons. The three sequences describe the state before, during and after the exchange. (The process is virtual, since there is not enough energy to produce a pimeson.)

Right "outside" protons and neutrons, the strong interaction is noticeable as sort of a residual effect, which is strong enough to overcome the electrostatic repulsion. This effect can be explained as an exchange of pimesons between the hadrons of the nucleus. The exchange of pimesons can be seen as a left over of the more fundamental interaction that uses gluons. The exchange is important at distances larger than 1 fm, which is the limit at which the interaction using gluons stops making sense. All three kinds of pimesons can take part in this interaction. In the figure to the left we see how the attraction between a proton and a neutron can be illustrated through an exchange of a positive pimeson.