The elementary particle angular momentum controversy: lessons from laser optics

Elliot Leader (IC)

The role of the gluon in the internal structure of the nucleon, and in particular its contribution to the spin of the nucleon, is of central interest in Elementary Particle Physics. Hence, the hullabaloo when a paper appeared in 2008, claim- ing that our entire understanding of the angular momentum of a gluon, and by implication, of a photon, was incorrect. Included, was the disturbing proclama- tion that all QED textbooks of the past 50 years were wrong in their stressing that the photon angular momentum cannot be split, in a gauge invariant way, into a spin and an orbital part. A workshop at the Institute for Nuclear Theory in Seattle in 2011, the aim of which was ”to resolve the controversy”, only succeeded in deepening it. My talk will give a brief summary of the angular momentum controversy in particle physics. I will also suggest that the form of the angular momentum, generally utilized in laser optics papers, based on the Poynting vector, is not the physically relevant one, and that laser optics experiments can possibly resolve the whole controversy.