Spin and orbital angular momentum in gauge theories: Nucleon spin   structure and multipole radiation revisited
  We address and solve the long-standing gauge-invariance problem of the nucleon spin structure. Explicitly gauge-invariant spin and orbital angular momentum operators of quarks and gluons are obtained. This was previously thought to be an impossible task, and opens a more promising avenue towards the understanding of the nucleon spin. Our research also justifies the traditional use of the canonical, gauge-dependent angular momentum operators of photons and electrons in the multipole-radiation analysis and labeling of atomic states; and sheds much light on the related energy-momentum problem in gauge theories, especially in connection with the nucleon momentum. 
Xiang-Song Chen, Xiao-Fu L\"u, Wei-Min Sun, Fan Wang, and T. Goldman