2012 Volume 67 Issue 11 Pages 738-745
How the proton spin, which is 1/2, is composed of quarks and gluons is called 'proton spin problem'. It is one of the fundamental topics of physics today. EMC experiment in 1980's discovered that the quark spin contribution to the proton spin is small. It triggered a variety of experiments:charged lepton-nucleon polarized deep inelastic scattering and polarized proton-proton collision using many different accelerators in the world. The quark spin contribution to the proton spin was determined to be about 1/3 in these experiments. The gluon spin contribution to the proton spin is also being measured. There has been much progress in theory as well. The research on this topics up to now is reviewed.