The science

Newswise – Scientists have new evidence that gluons – the glue-like particles that hold quarks together inside Protons And Neutrons– have a positive spin polarization. This means that the spins of individual gluons are aligned in the same direction as the spin of the proton in which they are located. The results come from tracking direct photons – particles of light emitted by interactions Quarks and gluons when polarized protons collide. Researchers found that more direct photons are emitted when protons with aligned spins collide than in head-to-tail collisions. This suggests 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 showed that quarks alone were not responsible for the overall spin value of protons. Previous experiments have shown that gluons are polarized and their spins are coordinately aligned. However, these results gave no indication whether the gluon spins were aligned in the same direction as the proton spins, contributing to this value, or in the opposite direction, counteracting the spin contributions of the quarks. These new results will help theorists refine their calculations about how much gluons contribute to proton spin.


Understanding the contributions of quarks and gluons to proton spin or angular momentum has been an important concern in nuclear physics. Scientists previously assumed that the spin of a proton came from its three main components, the quarks. Experiments that showed that quarks make up only part of the proton spin triggered a “spin crisis” in physics. The Relativistic heavy ion accelerator (RHIC), a Department of Energy Office of Science User Facility at Brookhaven National Laboratory, was designed with additional spin-polarized proton collision components to allow scientists to measure the contribution of gluons to spin. Multiple analyzes of polarized proton collisions at RHIC showed that gluons exhibit some degree of polarization or spin alignment. However, these analyzes did not show whether the gluon polarization is positive (aligned with the proton’s spin) or negative (opposite).

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


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 detailed in the corresponding scientific paper. Operations of the Relativistic Heavy Ion Collider are funded by the DOE Office of Science Nuclear Physics Program. The Japanese RIKEN laboratory made significant contributions to the spin polarization capabilities at the RHIC and the PHENIX detector.

Journal link: Physical Review Letters, June 2023

By admin

Leave a Reply

Your email address will not be published. Required fields are marked *