Identification of Observables for Quark and Gluon Orbital Angular   Momentum
  A new debate has recently arisen on the subject of orbital angular momentum in QCD, in particular on its observability and on its partonic interpretation. Orbital momentum can be defined in QCD using two different decomposition schemes that yield a kinetic and a canonical definition, respectively. We argue that kinetic orbital angular momentum is intrinsically associated with twist three generalized parton distributions, and it is therefore more readily observable, while, due to parity constraints, canonical angular momentum, if defined as suggested in the literature in terms of generalized transverse momentum distributions, cannot be observed in scattering processes involving a single hadronic reaction plane. 
Aurore Courtoy, Gary R. Goldstein, J. Osvaldo Gonzalez Hernandez,   Simonetta Liuti, Abha Rajan
hep-ph