New data from collisions of protons at the Relativistic Heavy Ion Collider (RHIC) indicate that "gluons," glue-like particles that bind the inner building blocks of each proton, play a substantial role in determining the proton's "spin," or intrinsic angular momentum. RHIC is the only machine in the world that can collide "polarized" protons—protons with their spins aligned in a particular direction—to help unravel the mystery of how the proton's building blocks, quarks and gluons, contribute to spin, a property that's put to use every day in MRI scans.
Exploring the sources of proton spin, one of the major scientific missions at RHIC, is helping to resolve the long-standing proton spin "mystery." The mystery originated when experiments in the 1980s revealed that a proton's spin—a property that influences these particles' optical, electrical, and magnetic characteristics—does not come solely from its quarks. With the ability to collide two beams of protons with their spins aligned in the same direction, and then with the polarization of one beam flipped so the spins are "antialigned," RHIC has conducted detailed studies to tease out the gluons' contribution to spin. The new results analyzed by RHIC's PHENIX collaboration indicate that the gluons' contribution is substantial, and might be even greater than the contribution from quarks.
RHIC physicists collide two beams of protons with their spins aligned in the same direction, and then "antialigned." The PHENIX detector measures the number of particles called pions that come out of the collision zone perpendicular to the colliding beams under these two conditions. Any difference in the production of pions between the two conditions is an indication of how much the gluons' spins are aligned with, and therefore contribute to, the spin of the proton. The latest data come from the highest-energy polarized proton collisions conducted to date and expand the area over which the PHENIX detector could look for pions emerging over a broader momentum range. These conditions allowed the scientists to look at the contributions of gluons that carry a lower fraction of the overall momentum of the proton—similar to the way a higher power microscope allows you to focus on smaller objects. The data show that these lower-momentum-fraction —a.k.a. "wimpy"—gluons play an outsized role in contributing to proton spin.
The reason, the nuclear physicists say, is that there are so many of them. Inside the proton, there's a sea of quarks and antiquarks and gluons changing and evolving. When you look with one resolution you see a certain number, but looking closer you can see that some of these particles have split to form new gluons, so there are actually more gluons there. The measurements of low-momentum-fraction gluon polarization, and these particles' large contribution to overall proton spin, have reduced the uncertainties about the overall size of the gluon contribution to spin, and indicate that gluons might contribute even more than the quarks and antiquarks. There's still room for improving these measurements by looking for contributions from even lower-momentum-fraction gluons and measuring particles emerging from RHIC collisions at more "forward" angles. Extending the momentum fraction range to lower values is one of the remaining goals of the RHIC spin program. Nuclear physicists hope to conduct even more detailed studies of the proton spin structure at a proposed future electron ion collider.
Research at RHIC is supported by the DOE Office of Science (NP) and these agencies and organizations. The Japanese RIKEN laboratory has provided significant funding for the PHENIX experiment, and RIKEN scientists played a key role in this analysis.
A. Adare et al. (PHENIX Collaboration), "Inclusive cross section and double-helicity asymmetry for ?0 production at midrapidity in p+p collisions at s?=510 GeV." Physical Review D 93, 011501(R) (2016). [DOI: 10.1103/PhysRevD.93.011501]