The parton model in polarized deep inelastic scattering

8 The parton model in polarized deep inelastic scattering 

In Section 15.6 we introduced the formalism necessary for studying deep inelastic scattering using polarized electron or muon beams on polarized nucleon targets. Here we briefly present the parton model predictions for the structure functions Gi,2 and consider what may be learnt from the experiments with polarized beams and targets. 

The original Bjorken analysis when applied to Gi,2 would suggest 

In the parton model we have a virtual photon undergoing a high energy collision with a parton. Since the parton mass is then irrelevant we expect that the parton helicity is unchanged in the collision (see Section 1.3) and as a consequence there is no mechanism to alter the helicity of the nucleon in the collision. From (15.6.11) we see that the combination m G +vG2 is proportional to an amplitude in which the nucleon helicity is flipped. Thus, in the parton model we expect 

Consider now a proton with helicity + ( ). We can introduce number densities 

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