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Only the quark-spin part is measured well, at about 30 percent.","posed_since":"1988","precise":"In a fixed gauge-invariant decomposition (Jaffe-Manohar: $1/2=\\Delta \\Sigma/2+\\Delta G+L_{q}+L_{g}$, or Ji: $1/2=J_{q}+J_{g}$) at $\\mu^{2}=10\\,\\mathrm{GeV}^{2}$ in the MS-bar scheme, determine each term with uncertainty below 0.05. $\\Delta \\Sigma \\sim 0.3$ is known from polarized deep-inelastic scattering; the answer is the complete set of values from data, cross-checked by lattice QCD at the physical pion mass.","problem_ref":null,"references":"","settled_by":"Polarized electron-proton collisions at the EIC (Electron-Ion Collider, Brookhaven) covering x down to about 1e-4, combined with lattice QCD calculations of quark and gluon angular momenta at physical quark masses.","status_note":"$\\Delta \\Sigma$ near 0.3 has been fixed since the 1990s and lattice QCD gave full Ji decompositions at the physical pion mass by 2020 (Alexandrou et al.), but the gluon helicity and orbital terms remained experimentally unmeasured at the needed precision as of 2026.","title":"How the proton spin divides among quarks and 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