Quark and Gluon Orbital Angular Momentum and Spin in Hard Processes
  We suggest a method of constructing gauge invariant quark and gluon distributions that describe an abstract QCD observable and apply this method to analyze angular momentum of a hadron. In addition to the known quark and gluon polarized structure functions, we obtain gauge invariant distributions for quark and gluon orbital angular momenta, and consider some basic properties of these distributions and their moments. 
S. V. Bashinsky and R. L. Jaffe (CTP, MIT)