Poincar\'e subalgebra and gauge invariance in nucleon structure
  By separating the gluon field into physical and pure-gauge components, the usual Poincar\'e subalgebra for an interacting system can be reconciled with gauge-invariance when decomposing the total rotation and translation generators of QCD into quark and gluon parts. The gauge-invariant quark/gluon parts act as the generators for the gauge-invariant physical component of the quark/gluon field, not the full quark/gluon field which also contains the gauge degrees of freedom. We clarify that the naive canonical decomposition of generators, while trivially respecting the Poincar\'e subalgebra, might not give a completely gauge-invariant quark-gluon structure of the nucleon momentum and spin, though limited invariance within a certain gauge class can be proven. 
Xiang-Song Chen
hep-ph