Results from ‘golden measurement’ at RHIC’s PHENIX experiment show the spins of gluons align with the spin of the proton they’re in
STONY BROOK, N.Y., June 21, 2023 — New research findings published in Physical Review Letters provides theorists with new input for calculating how much gluons—the gluelike particles that hold quarks together within protons and neutrons—contribute to a proton’s spin. This work by the PHENIX Collaboration at the Relativistic Heavy Ion Collider (RHIC) provides definitive evidence that gluon “spins” are aligned in the same direction as the spin of the proton they’re in.
Whether and how much gluons contribute to proton spin—an intrinsic angular momentum that’s associated with the protons’ optical, magnetic, and other properties—has been an open question since 1987. That’s when an experiment at Europe’s CERN laboratory revealed that quarks, the other components of protons and neutrons, cannot account for a proton’s total spin value.
Abhay Deshpande, PhD – a SUNY Distinguished Professor in the Department of Physics and Astronomy in the College of Arts and Sciences at Stony Brook University, and a physicist at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, where RHIC operates as a DOE Office of Science user facility – was involved in the CERN experiment showing that quarks didn’t carry too much of the spin.
“That result was a big surprise; it ignited a ‘spin crisis,’” said Deshpande. “Obviously, the remaining spin had to come from somewhere else. So, at the time, there were ideas that gluons might carry the rest of the spin.”
He explains that protons and neutrons are fundamental observable particles in nature, but how they come about through the interactions of quarks and gluons – which make them – is still not understood, despite more than 70 years of efforts by scientists around the world.
The ‘spin crisis’ was addressed through collaborative work at Brookhaven Lab (BNL) to spin-polarize the proton beams at RHIC so scientists could investigate if the gluons carried the remaining 80 percent of the proton’s spin.
At the time, RHIC was taking shape to explore a different set of physics questions related to the way quarks and gluons existed in the early universe. But the spin mystery attracted a whole new set of collaborators—and significant contributions from Japan’s RIKEN laboratory—to expand RHIC’s capabilities. These included specialized accelerator components that allow RHIC to align and “flip” the spins of protons (making it the only polarized proton collider in the world), and additional equipment for making key spin measurements at the PHENIX experiment, one of RHIC’s first particle detectors.
“At the beginning of the RHIC project, we felt this is the place we must go,” says Hideto En’yo, a RIKEN physicist and one of the spokespersons of the first proposal for polarized proton acceleration at RHIC. “We were so excited when we realized that RHIC is polarizable, which enables us to resolve gluons’ role in the proton spin crisis.”
Deshpande, one of the early RIKEN Fellows who was employed by the RIKEN BNL Research Center (RBRC) from 2000 to 2003, continued to work on the project since 2004 when he became a faculty member at Stony Brook, and has since collaborated with his graduate and post doc students, along with the worldwide PHENIX collaboration and RIKEN scientists, to continue to try to solve the “gluon question.”
The new PHENIX result as described in the published paper is one of the “golden” measurements proposed as a key motivator for the RHIC spin physics program. It’s a comparison of the number of “direct photons” (particles of light) emitted when RHIC collides protons with their spins pointing in opposite directions (either at or away from each other) with the number of direct photons produced when the protons in the two beams are pointing in the same direction (colliding head to tail).
For reasons having to do with the way quarks and gluons can interact to emit photons (and knowing that net quark spins are positively aligned with proton spin), seeing a difference would indicate that gluon spins are also aligned, or polarized—and, importantly, in which direction. That is, the collision condition producing more direct photons would tell them whether the gluon spin alignment was positive—pointing in the same direction as the proton spin and contributing to that value—or negative (pointing in the opposite direction and counteracting the quarks’ contributions to spin).
“This is a very clean and simple result for interpretation,” says Brookhaven Lab physicist Alexander (Sasha) Bazilevsky, deputy spokesperson for the PHENIX Collaboration. “The beauty of the direct photons is that their production at RHIC is dominated by quark-gluon interactions.”
In other words, these particles of light come directly from the interaction of a quark in one proton beam with a gluon in the other—rather than from the decay of some other particle produced in the collision.
The PHENIX results, carried out by Stony Brook graduate student Zhongling Ji from the Deshpande research team (now a postdoctoral fellow at UCLA), show a clear difference in direct photon yields, with higher yields coming from the collisions where the proton spins are pointing in opposite directions (at or away from one another). This is clear evidence that the gluon spins are aligned in the same direction as the proton spin and make a significant contribution to the proton’s overall spin value.
The new analysis will now provide input for theorists’ calculations to determine just how much of a contribution that is. Does that finally solve the proton spin mystery? Not quite, the scientists say.
The final contribution to proton spin is likely the orbital motion of quarks and gluons within these composite particles. Those details will be measured by the Electron-Ion Collider being built at Brookhaven Lab.