Internal Spin Structure of the Nucleon

Quantum chromodynamics provides a theoretical explanation and a physical pic----ture of why the interaction between quarks and gluons would vanish asymptotically at very short distances. This perturbative QCD supports the assumptions of the quark ₗight cone algebra and the quark-parton model. Even-'more powerfully, as a theory should predict new things, it describes quantitatively the logarithmic rate at which quarks become asymptotically free at short distances. (g-lO) The logarithmic corrections to the scaling functions and sum rules are the most important new predictions of perturbative &CD. In particular, the scaling behavior of spin-dependent structure functions and the corrections to it are accurately predicted in &CD together with some important sum rules of the scaling functions. The calculation of the quark and gluon spin distributions inside the nucleon is a difficult nonperturbative calculation. As yet it has not been possible to calculate spindependent (or spin-independent) structure functions from the basic equations of &CD. However, models of nucleon structure have been developed consistent with the general picture of &CD, which make quantitative predictions of the structure functions. In atomic and nuclear physics it has often been found that spin-dependent observables provide particularly sensitive tests of a system's wave function, and we may anticipate that for the nucleon also spin-dependent quantities will be illuminating. We may remark that an enormous theoretical literature on the spin-dependent structure functions of the nucleon has developed since about 1970.