[5] A key question is how the nucleons' spins are distributed amongst their constituent parts ("partons": quarks and gluons).
Components of proton's spin are expectation values of individual sources of angular momentum.
Therefore, it was expected that the quark's wave function is the spherically symmetric s-wave with no spatial contribution to angular momentum.
[8] This work used relativistic effects together with other quantum chromodynamic properties and explained how they boil down to an overall spatial angular momentum that is consistent with the experimental data.
[12] However, in 2018 lattice QCD calculations indicated that it is the quark orbital angular momentum that is the dominant contribution to the nucleon spin.
Given various error bars on both theoretical calculations and on experiments, this too is consistent with the observed experimental quark spin contribution of around 30%.