  One of the challenges of hadronic physics is to fully understand the
structure of the proton. In particular, there is nowadays a great interest in
the decomposition of its total angular momentum into orbital angular momentum
and intrinsic spin, as well as identifying contributions from valence quarks,
sea quarks and gluons. The most common decompositions of angular momentum are
the Jaffe-Manohar (canonical) and Ji (kinetic) decompositions, which differ in
the way contributions are attributed to quarks and gluons. Using perturbation
theory, explicit one-loop calculations found that the difference between such
decompositions vanishes. We justify within the diquark model in QED that the
difference appears at two-loop level, supporting the interpretation of such a
difference as originating from the torque exerted by the spectator system on
the struck quark.
