One of the biggest issues in the high-energy hadron physics is to understand the origin of the proton spin 1/2. In 1989, the EMC experiment at CERN reported their result which showed the quark-spin contribution to the nucleon spin is small. Naively we expected that the quark-spin contribution is dominant, but this is not true. It is called 'proton spin puzzle'. The origin of the proton spin 1/2 has to be understood as a sum of contributions from quark spin, gluon spin, and orbital-angular momenta of quarks and gluons inside the proton.
We are investigating a polarized structure of the proton with polarized- proton collisions at RHIC (Relativistic Heavy-Ion Collider) in BNL (Brookhaven National Laboratory). The gluon-spin contribution to the proton spin has been investigated as the first goal of spin physics at RHIC. Polarized proton collision experiments at a collision energy 200 GeV started in 2001. The PHENIX and STAR experiments have investigated the gluon-spin contribution by measuring a longitudinal-spin asymmetries of neutral pion and jet produced in longitudinally polarized proton collisions. Experimental results from data taken in 2006 showed that these asymmetries are very small and they strongly restrict the gluon- spin contribution in the theoretical calculations based on perturbative QCD. From 2009, polarized proton collisions at a collision energy 500 GeV started and we have investigated flavor-sorted contribution of the quark spin to the proton spin with weak-boson production.
The final remaining piece of information to understand the proton spin is orbital-angular momenta of quarks and gluons. Correlation between momentum-distribution of quarks and gluons and their spin direction in transversely-polarized proton collisions will be investigated to understand the polarized structure of the proton including the orbital- angular momentum.