Towards the parton model—deep inelastic scattering

Towards the parton model—deep inelastic scattering 

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 quark-partons are not treated as free, but are allowed to interact with each other via the exchange of gluons, in the framework of QCD. 

The essence of the parton model is the assumption that, when a sufficiently high momentum transfer reaction takes place, the projectile, be it a lepton or a parton inside a hadron, sees the target as made up of almost free constituents, and is scattered by a single, free, effectively massless constituent. Moreover the scattering from individual constituents is incoherent. The picture thus looks much like the impulse approximation of nuclear physics. 

A typical process, deep inelastic electron scattering on a nucleon, i.e. 

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Towards the parton model—deep inelastic scattering... 

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