Spin physics is the study of how the spin of a complicated, many-body QCD bound state is generated. In 1989, Deep Inelastic Scattering experiments at CERN showed that a remarkably small amount to the spin of the proton was actually carried by the valence quarks from which it is comprised. While this was dubbed the "Spin Crisis", it is actually an extraordinary opportunity - studying spin-dependent interactions of the proton opens a window into the dynamics of the proton bound state.
Present experiments at the Relativistic Heavy Ion Collider (RHIC) with longitudinally polarized p+p collisions have focused tremendous effort to measure the contribution of the gluon (the gauge boson that carries the strong force) to the spin of the proton, as the original experiments were only sensitive to the spin carried by the quarks. These measurements are ongoing, but early indications are that the spin carried by the gluons is small.
Future experiments with transversely polarized p+p collisions at RHIC hope to be able to understand the fraction of the proton's spin that is generated by the orbital angular momentum of the partons that make up the proton. This exciting new field is developing rapidly, both experimentally and theoretically.
The Experimental Nuclear Physics group at Iowa State University is involved in a number of projects aimed at further understanding spin and QCD:
