On gauge invariant nucleon spin decomposition
  A non-uniqueness problem of gauge invariant separation of quark and gluon contributions to nucleon spin is considered. We show that there is a wide number of gauge invariant spin decompositions each of them reduces to the canonical one in a special gauge. A class of physical gauge equivalent nucleon spin decompositions is selected by requirements of consistence with helicity notion described within E(2) little group representation theory and with gluon helicity \Delta g measured in experiment. 
P. M. Zhang and D. G. Pak
hep-ph hep-th nucl-th