We have seen in earlier chapters that there seems to be a close parallelism between the sets of leptons and the sets of quarks, at least in so far as the unified weak and electromagnetic interaction is concerned. The leptons are essentially ‘point-like’ in their behaviour, and it is not inconceivable that the quarks too enjoy this property. In that case we might expect the hadrons to behave, in certain situations, in a less complicated fashion than usual. If we think of the hadrons as complicated ‘atoms’ or ‘molecules’ of quarks, then at high energies and momentum transfers, where we are probing the inner structure, we may discover a relatively simple situation, with the behaviour controlled by almost free, point-like constituents. The idea that hadrons possess a ‘granular’ structure and that the ‘granules’ behave as hard point-like, almost free (but nevertheless confined) objects, is the basis of Feynman's (1969) parton model.

We shall discuss the model in some detail in the following chapters, in particular the question as to whether the partons can be identified with the quarks. The introductory material of the chapter is largely based upon lecture notes of F. Close (1973) (see also Close, 1979). We shall also study more sophisticated versions of the picture, wherein the quarkpartons are not treated as free, but are allowed to interact with each other via the exchange of gluons, in the framework of QCD.