While the naive constituent quark model accounts beautifully for the hadron spectrum, it fails to explain the inner spin structure of the nucleon. Indeed, the EMC experiment in 1987 revealed that the quark spin contribution turns out to be much smaller than expected. From a QCD perspective, it is clear that the gluon spin and the total orbital angular momentum need to be included in the picture. Then, the key question is: Is there a physically meaningful decomposition of the total angular momentum consistent with Quantum Mechanics, Special Relativity and gauge symmetry? It took more than 25 years to reach a consensus among experts. Interestingly, this question sits at the roots of Quantum Field Theory, and has important implications for other fields, including Quantum Optics, extensions of General Relativity, Quantum Gravity and String Theory. We propose a summary of the recent developments and point to interesting prospects.

Argonne Physics Division Seminar Schedule