The key question is how the nucleon's spin, whose magnitude is 1/2ħ, is carried by its constituent partons (quarks and gluons).
Global analysis of data from all major experiments confirmed the original EMC discovery and showed that the quark spin did contribute about 30% to the total spin of the nucleon.
A major topic of modern particle physics is to find the missing angular momentum, which is believed to be carried either by gluon spin, or by gluon and quark orbital angular momentum.
This fact is expressed by the sum rule, The gluon spin components
Quark and gluon angular momenta will be studied by measuring so-called generalized parton distributions (GPD) through deeply virtual compton scattering (DVCS) experiments, conducted at CERN (COMPASS) and at Jefferson Lab, among other laboratories.