Bjorken scaling

In analogy to the spin independent case [see eqn (15.5.4)) the functions Gi,2(t > Q ) multiplied by kinematic factors are expected to obey Bjorken scaling. One thus writes... 

In the following chapter we shall see how the quark-parton model explains the occurrence of Bjorken scaling and we shall obtain expressions for all the scaling functions in terms of number densities for the quark-partons in a hadron. 

We shall see later that diagrams of this type play a crucial role in the breaking of Bjorken scaling and in providing a slow dependence for the scaling functions. This behaviour will turn out to be one of the pieces of evidence in favour of QCD. 

The quark-paxton model was developed in great detail in the previous chapter. Here we discuss the experimental situation and confront the theory with the vast amoimt of data on deep inelastic scattering and related reactions. It must be borne in mind that we have not yet discussed the QCD corrections to the model. These are not small, but their dominant effect can be taken into account by allowing the parton number densities to depend upon in a way calculable in QCD, so that, fortuitously, the entire formalism is basically unchanged, except that each qj x) —> qj x,Q ). As mentioned earlier this implies a dynamical breaking of perfect Bjorken scaling. It also implies that if one is seeking accurate information about the qj x) from experiment then care must be taken to specify the involved. 

To end this brief survey we show how to derive the scaling version of the Bjorken sum rule (15.6.35) from the parton model (Hey, 1974). 

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