What we know and what we don't know about the origin of the spin of the proton

Abstract

The origin of the spin of the proton is one of the most fundamental questions in modern hadron physics. Although tremendous progress has been made since the discovery of the "spin crisis" brought the issue to the fore, much remains to be understood. We carefully review what is known and, especially in the case of lattice QCD, what is not known. We also explain the importance of QCD inspired models in providing a physical picture of proton structure and the connection between those models and what is measured experimentally and on the lattice. We specifically apply these ideas to the issue of quark orbital angular momentum in the proton. We show that the Myhrer-Thomas resolution of the proton spin crisis is remarkably consistent with modern information from lattice QCD.