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Title: Measuring quark and gluon orbital angular momentum in the proton
Plain: 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: Extract the quark orbital angular momentum $L_q = J_q - \Delta \Sigma_q/2$ with $J_q = (1/2)\,\operatorname{integral} dx\, x\, [H_q(x,0,0) + E_q(x,0,0)]$, 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.
Settled by: Global GPD extraction from JLab 12 GeV and EIC exclusive data with controlled extrapolation to zero momentum transfer, compared with lattice QCD values.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status note: Lattice QCD provided values by 2020, but experimental extraction of $E_{q}$, needed for $J_{q}$, remained model dependent as of 2026.
Posed since: 1997