Introduction

We shall see later (chapter 10) that it is now clear that particles such as protons, neutrons and π-mesons (pions), once thought to be ‘elementary’ are, in fact, made from more basic constituents, the quarks. Thus the force between, say, two neutrons is basically the resultant of the forces between two groups, each of three quarks. In this respect it is approximately analogous to the Van der Waals force between two molecules, each of which consists of several atoms. As in the case of atomic or molecular problems, so for particles it is in many instances, however, very useful to treat the forces between particles such as nucleons rather than the forces between their constituent quarks. Although many of the ideas concerning particle interactions were developed before quarks were identified as the constituents of nucleons and mesons, these ideas are, in general, not invalidated by the discovery of the quark structure and, indeed, the subject is perhaps best understood by means of this semi-historical approach.

Prediction of the π-mesons by Yukawa

The prediction of the existence of mesons, by Yukawa in 1935, and their subsequent discovery in the cosmic radiation, present one of the most striking examples of the interaction of theory and experiment in modern physics.

We can understand Yukawa's argument in a qualitative way as follows. It had become well established that electromagnetic forces could be well understood in terms of a field of which the quanta were photons.