  We present a detailed assessment of how well a future Electron-Ion Collider
could constrain helicity parton distributions in the nucleon and, therefore,
unveil the role of the intrinsic spin of quarks and gluons in the proton's spin
budget. Any remaining deficit in this decomposition will provide the best
indirect constraint on the contribution due to the total orbital angular
momenta of quarks and gluons. Specifically, all our studies are performed in
the context of global QCD analyses based on realistic pseudo-data and in the
light of the most recent data obtained from polarized proton-proton collisions
at BNL-RHIC that have provided evidence for a significant gluon polarization in
the accessible, albeit limited range of momentum fractions. We also present
projections on what can be achieved on the gluon's helicity distribution by the
end of BNL-RHIC operations. All estimates of current and projected
uncertainties are performed with the robust Lagrange multiplier technique.

High Energy Physics - Phenomenology