Xiangdong Ji, Yang Xu, Yong Zhao
It is well known that in gauge theories, the spin (and orbital angular momentum) of gauge particles is not gauge-invariant, although the helicity is; neither are the canonical momentum and canonical angular momentum of charged particles. However, the simple appeal of these concepts has motivated repeated attempts to resurrect them as physical descriptions of gauge systems. In particular, measurability of the gluon-spin-contribution to the proton helicity in polarized proton scattering has generated much theoretical efforts in generalizing it and others as gauge-invariant quantities. In this work, we analyze constraints of gauge symmetry, the significance of gluon spin in the light-cone gauge, and what-is-possible-and-natural in QCD parton physics, emphasizing experimental observability and physical interpretation in the structure of bound states. We also comment on the measurability of the orbital angular momentum of the Laguerre-Gaussian laser modes in optics.
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http://arxiv.org/abs/1205.0156
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