Theoretical Aspects of Lepton - Hadron Scattering *

I will emphasize two points in my lectures on Theoretical Aspects of LeptonHadron Scattering: (1) The crucial importance of testing the “exact” sum rules as tests of the local current algebra. Discrepancies, if found, between experiment and theory cannot be “interpreted away” in terms of more complex parton wave functions for the hadronic ground state. The three sum rules of interest are those of Adler, Bjorken, and Gross and LlewellynSmith. (2) An understanding of the corrections to scaling in QCD and what they teach us. To begin with, I will review the parton model, its intuitive physical basis, its predictions, and its limitations.’ The power and beauty of lepton-hadron scattering is that the electroweak field generated during the lepton scattering is as well understood as anything known in particle physics. This permits us to probe the unknown structure of the target hadron by means of a known current operator. Furthermore, at the same time, its strength is weak enough to allow a perturbative treatment in powers of the electroweak charge and strong enough to permit accurate measurements under physically interesting conditions of large energy and momentum transfers. The original round of high energy measurements of elastic electron scattering by Hofstadter and collaborators demonstrated that protons and neutrons, similar to the nuclei of which they are the constituents, have extended charge distributions. For nuclei, in which the nucleons are highly nonrelativistic-being bound by less than 1% of their rest energies -the charge distributions are measured by (to leading order in Zcu < 1, and neglecting center-of-mass corrections N l/A) d3 T,i’$? I > PZ,A =