Unveiling the Proton Spin Decomposition at a Future Electron-Ion Collider
Elke C. Aschenauer, R. Sassot, M. Stratmann
TL;DR
The paper tackles the proton spin decomposition by constraining helicity PDFs with a future EIC. It employs an NLO global QCD analysis framework that incorporates realistic EIC polarized DIS pseudo-data and RHIC spin results, using a Lagrange multiplier method to quantify 90% C.L. uncertainties. It provides projections for $\Delta g(x,Q^2)$ and its integral $\Delta g(Q^2)$ and assesses implications for the total orbital angular momentum via the spin sum rule. It also discusses SIDIS fragmentation-function limitations and the role of complementary RHIC data (e.g., $W$ boson asymmetries) for flavor separation, underscoring the EIC's crucial contribution to mapping spin structure across $x$ and $Q^2$.
Abstract
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.