How the proton spin divides among quarks and gluons · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title
How the proton spin divides among quarks and gluons
Plain
The proton spin of $1/2$ (in units of $\hbar$) must be the sum of quark spins, gluon spins and the orbital motion of both. Only the quark-spin part is measured well, at about 30 percent.
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.
Settled by · changed · core
Revision 1: 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.
Revision 2: Polarized electron-proton collisions at the EIC (Electron-Ion Collider, Brookhaven) covering x down to about $10^{-4}$, combined with lattice QCD calculations of quark and gluon angular momenta at physical quark masses.
Kind
well-posed
Answer type
value
References
(empty)
Literature status
partially-resolved
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.
Posed since
1988

Decisions