As with all theories of strong interactions things look simple to begin with, but as experimental accuracy improves models are forced to become increasingly complicated, with the continuous addition of new features much, it must be admitted, like the Ptolemaic cycles and epi-cycles of old. Thus the behaviour of the structure functions in deep inelastic lepton scattering will lead us initially to a simple picture of a hadron composed of granular constituents, partons. Soon thereafter we shall discover the need for antipartons and the beautiful association between partons and quarks. But the gross failure of the ‘momentum sum rules’ will indicate that some constituents are still missing; presumably the ‘gluons’. Although the gluons do not interact directly with the virtual photon used in our picture of deep inelastic scattering they are supposed to mediate the strong interactions and to give rise to the QCD corrections to the quark–parton model. These will be discussed in Chapters 20 and 21.

Finally we warn the reader that to meet the phenomenal accuracy of recent experiments it is necessary to give careful attention to kinematic details which to some extent detract from the elegant simplicity of the original picture. These effects are treated in the appendix to this chapter (Section 16.9) in which the parton model is reformulated as an impulse approximation which allows better control of the kinematic factors. The chapter can be perfectly well understood without reading the appendix.

The introduction of partons

We begin with a qualitative discussion of the quark-parton model. A more careful and quant it ive description is gradually developed thereafter.