The Spin Structure of the Nucleon

This paper gives an overview of the spin structure of the nucleon with emphasis on a pedagogical presentation of the tools used to study this fundamental property of protons and neutrons. While many of the data currently available are shown, this paper is not intended to be an exhaustive summary of the field. Rather, the data are used to illustrate the concepts presented. The most recent progress in the field can be found in the proceedings of the annual conference on deep inelastic scattering and QCD . The last ten years have seen a wealth of measurements of the polarisation asymmetry in the cross-section for deep inelastic lepton scattering (DIS). This has been made possible by impressive advances in polarised target and beam technologies. Precise data now exist on inclusive polarised DIS from experiments at SLAC, CERN, and DESY. These data can be interpreted as showing that surprisingly, only about 30% of the nucleon’s spin comes from the spins of the quarks. To make further progress on this problem, semi-inclusive measurements are needed where hadrons in coincidence with the scattered lepton are detected. These data allow us to determine the flavour of the quark that was struck and isolate the contribution to the nucleon’s spin by valence and sea quarks. Furthermore, semi-inclusive data on charm production should give information on the contribution of gluons to the nucleon’s spin. Current and future experiments are concentrating on semi-inclusive measurements. After a short discussion of magnetic moments, the formalism of polarised deep inelastic scattering is presented in Sec. 3.1-3.2. The data on the polarised structure function g1(x) are then shown in Sec. 3.3 which is followed by the interpretation of these data in terms of sum rules in Sec. 4. Section 5 shows how spin dependent structure functions can be used to determine the contribution of the quark spins to the spin of the nucleon. The Q evolution of g1(x) is discussed in Sec. 6, while other spin structure functions are presented in