On possible contribution of partons’ orbital momentum to the proton spin

Virtual parton excitations of proton achieve status of real particles after transition into the infinite momentum frame. In particular, QCD evolution of them should respect the conservation laws which in turn should be a necessary consequence of symmetry properties. I argue that these simple reasons may cast light on the so-called proton spin puzzle and upon an orbital momentum contribution of quarks and gluons to the proton spin. An already fulfilled experiment with a hint on possible observation of orbital excitations of gluons is pointed out