The Phys.org news item How did the proton get its spin? outlines various experiments that probe proton structure and are sensitive to spin, and the current news is that CEBAF's major energy upgrade will allow new kinds of investigations.
The news item identifies two experimentally measured sources of spin; the spin of the quarks and that of the gluons. Together these account for about half of the proton's total spin (25-30% and about 20-30%, respectively). It then says:
There's one major source that researchers haven't yet explored: orbital angular momentum. Orbital angular momentum comes from the movement of the quarks and gluons relative to each other. While theorists have developed simulations that model this contribution, scientists haven't had the equipment to test them.
The article says there's one more place to look for a contribution, and that "theorists have developed simulations that model this contribution".
While one might now expect orbital angular momentum to contribute the rest of the proton's spin, do predictions indicate a strong enough contribution to add up to all of the proton's spin?
above: In the 1980s, scientists discovered that a proton's three valance quarks (red, green, blue) account for only a fraction of the proton's overall spin. More recent measurements have revealed that gluons (yellow corkscrews) contribute as much as or possibly more than the quarks. Credit: Brookhaven National Laboratory, from here.