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Understanding the
structure of the nucleon is of fundamental importance in sub-atomic physics. Already the experimental studies on the electro-magnetic form factors in the 1950s showed that the nucleon has a nontrivial internal structure, and the deep inelastic scattering experiments in the 1970s revealed the partonic substructure of the nucleon. Modern research focuses in particular on the spin and the gluonic structure of the nucleon. Experiments using deep inelastic scattering or polarized p-p collisions are carried out in the US at the CEBAF and RHIC facilities, respectively, and there are other experimental facilities around the world. A little over 20 years have passed now since the European Muon Collaboration published their first experimental results on the proton spin structure as revealed in polarized deep inelastic lepton-nucleon scattering. With additional experimental and theoretical investigations and progress in the following years, it is now established that, contrary to naive quark model expectations, quarks and anti-quarks carry only about 30\% of the total spin of the proton. To find out what carries the remaining spin is a key focus in current hadronic physics and also a major driving force for the new generation of spin experiments at RHIC and JLab. We believe it is very timely to organize a program to summarize the current status of proton spin physics and to lay out the future perspectives. On the frontiers of proton spin physics, there are currently three major research areas. First, Longitudinal Spin Physics focuses on the extraction of the quark and gluon helicity distribution in the nucleon and their spin contributions to the proton spin. Second, Generalized Parton Distributions offer the opportunity to pin down the orbital angular momenta of quarks and gluons, as well as their spatial distributions, and have generated much interest in the last decade. Third, recent developments have demonstrated that Transverse-spin Physics offers a new window into central aspects of QCD. Observables with transverse spin may for example also provide information on contributions by parton orbital angular momenta to the proton spin, as well as on the transverse momenta of partons in the nucleon. Accordingly, we will coordinate our program into three parts: longitudinal spin, transverse spin, and generalized parton distributions. We plan to have two presentations by participants each day. We anticipate active discussions and collaborations between the participants. |
