Gluon Spins Align with the Proton They’re In

The Science

Scientists have new evidence that gluons—the gluelike particles that hold quarks together within protons and neutrons—have a positive spin polarization. This means the spins of individual gluons are aligned in the same direction as the spin of the proton they are in. The results come from tracking direct photons—particles of light emitted by interactions of quarks and gluons when polarized protons collide. Researchers found that more direct photons were emitted when protons collide with their spins pointing at one another than in head-to-tail collisions. This indicates that gluons make a positive contribution to the overall spin of these particles.

The Impact

Whether and how much gluons contribute to proton spin has been an open question since experiments in the 1980s revealed that quarks alone cannot account for protons’ total spin value. Previous experiments have revealed that gluons are polarized, having spins that are aligned in a coordinated way. But those results did not indicate whether the gluon spins were aligned in the same direction as the proton spins, contributing to that value, or in the opposite direction, counteracting the quarks’ contributions to spin. These new results will help theorists refine their calculations of how much gluons contribute to proton spin.

Summary

Understanding quark and gluon contributions to proton spin, or intrinsic angular momentum, has been a major quest for nuclear physics. Scientists used to think a proton’s spin came from its three main constituent quarks. Experiments revealing that quarks account for only a portion of proton spin set off a “spin crisis” in physics. The Relativistic Heavy Ion Collider (RHIC), a Department of Energy Office of Science user facility at Brookhaven National Laboratory, was designed with additional components for colliding spin-polarized protons so scientists could measure gluons’ contribution to spin. Several analyses of polarized proton collisions at RHIC showed that gluons have a degree of polarization, or spin alignment. However, those analyses did not show whether the gluon polarization is positive (aligned with the spin of the proton) or negative (oppositely aligned). 

The new analysis from RHIC PHENIX detector compared the yields of photons emitted from interactions between a quark in one proton with a gluon in the other when protons collide with their spins pointing in opposite directions and when pointing in the same direction. The PHENIX results show more “direct photons” emitted from oppositely oriented protons, providing conclusive evidence of positive gluon polarization. The results provide input for more accurate calculations of gluons’ spin contribution. However, this does not quite solve the proton spin mystery. The final contribution to proton spin is likely the orbital motion of quarks and gluons within these composite particles. Future experiments will explore that contribution and increase the precision of gluon contribution measurements.

Funding

This analysis was funded in part by the Department of Energy (DOE) Office of Science, Nuclear Physics program, and by the funders of individual members of the PHENIX collaboration as listed in the related scientific paper. Relativistic Heavy Ion Collider operations are funded by the DOE Office of Science, Nuclear Physics program. Japan’s RIKEN laboratory provided significant contributions to the spin polarization capabilities at RHIC and the PHENIX detector.