Polarized Electroproduction and the Spin of the Quarks Inside the Proton

Abstract

Deep inelastic leptoproduction has played a fundamental rôle in the development of the QCD-improved parton model. This set of processes is important because of its simplicity. The processes are initiated by leptons (with no strong interactions) and are totally inclusive in the hadronic final state. As is also the case for the total hadronic e+e− cross-section at large centre-of-mass energy Q, these properties make a very clean theoretical approach possible for leptoproduction in the deep inelastic region. But leptoproduction has a much richer structure than the hadronic e+e− cross-section. First, there are a number of structure functions for each process and several processes are induced by different lepton beams. Then, the structure functions depend on two variables, the squared four-momentum transfer at the lepton vertex q2 = −Q2 < 0 and the Bjorken variable x = Q2/2(pq) with p μ being the nucleon target four-momentum (0 ≤ x ≤ 1). Thus while the hadronic e+e− cross-section is one single function of Q2 the leptoproduction structure functions are several functions of both x and Q2: a much wider theoretical laboratory. Over the years the experimental study of unpolarized deep inelastic scattering has led to the determination of the different parton densities in the nucleon and of their Q2 evolution in good agreement with the parton model and QCD.