Orbital and field angular momentum in the nucleon
  The nucleon spin problem raises experimental and theoretical questions regarding the contribution of the orbital angular momentum of the quarks to the total spin of the nucleon. In this article we examine the commutation relationships of various operators that contribute to the total angular momentum of the nucleon. We find that the sum of the orbital plus gluon field angular momentum should satisfy the angular momentum commutators, at least up to the one-loop level. This requirement on the sum of these operators imposes a nontrivial restriction on the form of the color electric and magnetic fields. This is similar to the magnetic monopole/ electric charge system where it is only the sum of the orbital plus field angular momentum that satisfies the correct commutation relationships. 
D. Singleton and V. Dzhunushaliev