Some could also come from quarks and gluons as they spin around each other the way orbiting planets contribute to the solar system's angular momentum. But the good news is that RHIC can measure the spin contribution of the gluons. Physicists will try to figure that out by colliding a beam of protons spinning in one direction with a second spinning the opposite way. RHIC is the first collider that can collide two oppositely spinning proton beams. Physicists will be delighted.
Nuclear physics is built on a theory known as quantum chromodynamics.
Ever since it was developed in the 1960s the theory has been predicting the outcomes of experiments with incredible precision. As proud as they are of it, physicists can't wait for the day when they find some hole, some special circumstance where their theory fails completely. The last time something like that happened, early in the 20th century, relativity and quantum dynamics came to the rescue. The new ideas revolutionized physics and made things like computers, lasers and nuclear weapons possible. So physicists actually welcome things they don't understand.