Measuring quark and gluon orbital angular momentum in the proton
In plain words
Quarks and gluons circulate inside the proton, and this circulating motion carries part of its spin. No experiment has yet measured this orbital part directly.
Precise statement
Extract the quark orbital angular momentum $L_q = J_q - \Delta \Sigma_q/2$ with $J_q = (1/2)\,\int dx\, x\, [H_q(x,0,0) + E_q(x,0,0)]$, where $H$ and $E$ are generalized parton distributions (GPDs, functions encoding both momentum and transverse position of partons) measured in deeply virtual Compton scattering and exclusive meson production, together with the gluon $J_g$. Answer: $L_u, L_d\ \text{and}\ J_g$ at $\mu^2 = 4\,\mathrm{GeV}^2$ with uncertainties below 0.05, satisfying the spin sum rule.
What would settle it
Global GPD extraction from JLab 12 GeV and EIC exclusive data with controlled extrapolation to zero momentum transfer, compared with lattice QCD values.
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.
Lattice QCD provided values by 2020, but experimental extraction of $E_{q}$, needed for $J_{q}$, remained model dependent 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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