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 $\Delta G(\mu^{2})=\int_{0}^{1}\mathrm{d}x\ \Delta g(x,\mu^{2})$ 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 $\int_{0.05}^{0.2}\Delta g\ \mathrm{d}x=0.20\ (+0.06,-0.07)$ (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 $10^{-4} < x < 0.5$, combined in a global fit with RHIC data.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
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.
Parent problems
- Special case of How the proton spin divides among quarks and gluons · open · from revision 1 The parent was revised since revision 1: it now states another question.
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