NUC In the literature: open

Total gluon spin contribution $\Delta G$ to the proton spin

In plain words

Gluons, the particles that bind quarks, can carry spin aligned with the proton's. Collider data show a positive contribution from medium-momentum gluons, but the part from very low-momentum gluons is unknown.

Precise statement

Determine $\Delta G(\mu^{2})=\operatorname{integral}_{0}^{1}\mathrm{d}x\ \Delta g(x,\mu^{2})$ at $\mu^{2}=10\ \mathrm{GeV}^{2}$, where $\Delta g$ is the gluon helicity distribution and x the gluon momentum fraction. RHIC polarized pp data give $\operatorname{integral}_{0.05}^{0.2}\Delta g\ \mathrm{d}x=0.20\ (+0.06,-0.07)$ (de Florian et al., 2014) under a positivity assumption; the open part is $x<0.01$, where no data exist, together with the residual dependence of the sign on high-x and $\mathrm{SU}(3)$ assumptions. Answer: $\Delta G$ with total uncertainty below 0.05.

What would settle it

Scaling violations of the polarized structure function $g_{1}$ measured at the EIC over $1e-4 < x < 0.5$, combined in a global fit with RHIC data.

Status in the literature

Unverified note

Fits without positivity admitted negative $\Delta g$ (JAM, 2022); lattice Ioffe-time data could not fix the sign (Karpie et al., 2023), but adding high-x polarized DIS data disfavored the negative solution (Hunt-Smith et al., 2024); the $x < 0.01$ contribution remained unmeasured as of 2026.