Making sense of the "Proton Spin Crisis"

Question

"Proton spin crisis" has been quoted as one of the outstanding problems in nuclear physics. Within the Standard Model, the different approaches to solve the crisis are fairly straight forward: measure the contributions from the quark spin, the gluon spin, the orbital angular momentum of the quarks and the orbital angular momentum of the gluons and sum them. Their measurements are work in progress which might take another 20 years.

How is this problem perceived in the "Foundations of Quantum Mechanics" community? Were there credible attempts to solve this problem as "a simple problem within the foundation of quantum mechanics"?   

Answer 1

The proton spin crisis should be referred to as the proton spin puzzle. Nobody seriously doubts that after the different contributions have all been measured that they will add to 1/2. There is no conceivable reason for doubting the foundations of QM.

The proton spin puzzle is the fact that the quark contribution, which has been fairly well measured, is relatively small (although not as small as it was when first measured 25 years ago), $$\Sigma_q\simeq 0.3 \times \frac{1}{2}.$$  QCD is a strongly interacting field theory, and there is no particualar reason that $\Sigma_q$ should be close to 1/2. However, the success of the non-relativistic quark model had led people to expect $\Sigma_q\sim 1/2$, and the failure of this naive expectation is called the proton spin puzzle.