The discovery that the proton carries total spin one half posed a major test for quantum chromodynamics and nucleon structure. Early expectations from simple quark models assigned most of the spin to the three valence quarks, but experiments showed a very different picture. J. Ashman European Muon Collaboration CERN reported that quark intrinsic spin contributes far less than expected. That result initiated a broad experimental and theoretical program to decompose the proton’s angular momentum into its parts.
Experimental findings and evidence
Subsequent measurements by the HERMES Collaboration with A. Airapetian DESY and the COMPASS Collaboration at CERN refined the size of the quark spin contribution and probed sea quark effects. Polarized proton collisions at Brookhaven National Laboratory studied by the STAR Collaboration and the PHENIX Collaboration have measured the gluon spin contribution through jet and hadron production asymmetries. These efforts indicate that gluons can contribute significantly, but together quark and gluon intrinsic spins do not fully account for the proton’s half unit of spin. Numbers vary by analysis and kinematic coverage, and global fits typically place the quark spin near about thirty percent while gluon contributions depend on the momentum fraction probed.
Theoretical decomposition and remaining components
Xiangdong Ji University of Maryland Lawrence Berkeley National Laboratory formulated a gauge-invariant decomposition that separates total proton spin into quark spin, quark orbital angular momentum, gluon spin, and gluon orbital angular momentum. This framework highlights that missing spin can reside in orbital motion and in gluon angular momentum that is not simply the gluon helicity measured in some experiments. Accessing those pieces requires measurements of generalized parton distributions and transverse momentum dependent distributions, quantities being pursued at Jefferson Lab and planned for the Electron Ion Collider being built at Brookhaven National Laboratory.
Because the decomposition and its extraction are complex, significant theoretical and experimental uncertainties remain. The consequence for fundamental physics is profound: a complete accounting of proton spin tests nonperturbative quantum chromodynamics and informs how angular momentum is stored in confined systems. The research is inherently collaborative and international, involving large detector collaborations and theory groups across CERN DESY Brookhaven National Laboratory Jefferson Lab and national laboratories worldwide, reflecting both a technical and cultural scientific effort to close the spin budget.