Measuring quark and gluon orbital angular momentum in the proton · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

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 · changed · core
Revision 1: 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.
Revision 2: 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.
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

Decisions