Nucleons, as protons and neutrons, are the fundamental building blocks of nuclear matter and make up more than 99% of the visible mass in the universe. Despite their small size nucleons are highly complex objects with quarks and gluons as their constituents and the strong nuclear force binding them together.
Quantum Chromo Dynamics (QCD) continues to be widely accepted as the correct theory describing the strong nuclear force. However, despite enormous theoretical and experimental efforts an "ab initio" understanding of the nucleon as the fundamental bound state of QCD remains elusive. Important open questions include the understanding of mechanisms that explain the large nucleon masses and the origin of the intrinsic angular momentum of the nucleon, its spin.
For more then 30 years nucleon spin structure and its spin decomposition in spin and orbital angular momentum contributions from quarks and gluons has been studied in high energy lepton nucleon scattering experiments at SLAC, CERN, DESY and Jefferson Laboratory. These measurements have shown that about 30% of the proton spin result from quark spin contributions. However, the sensitivity of these experiments to gluon spin and orbital angular momentum contributions has been very limited.
The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is the first accelerator facility capable of accelerating and colliding polarized protons beams at high energies. This novel experimental technique opens a new window for the investigation of proton spin structure and provides sensitivity to the spin contributions from gluons and anti-quarks.
The colloquium will introduce the problem of nucleon spin structure, summarize the knowledge available from lepton nucleon scattering experiments and present results from polarized proton-proton collisions at RHIC.