By analyzing the highest-energy proton collisions at the Relativistic Heavy Ion Collider (RHIC), a particle collider at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, nuclear physicists have gotten a glimpse of how a multitude of gluons that individually carry very little of the protons' overall momentum contribute to the protons' spin. The data described in a recently published paper indicate that these glue-like particlesâ??named for their role in binding the quarks that make up each protonâ??play a substantial role in determining the intrinsic angular momentum, or spin, of these building blocks of matter.
"These results confirm our suspicion that a lot of the gluons' contribution to proton spin comes from the gluons with relatively low momentum," said Ralf Seidl, a physicist from the RIKEN-BNL Research Center (RBRC) and a member of RHIC's PHENIX collaboration, which published these results. The results also suggest that gluons' overall contribution to spin might be even greater than the contribution from quarks.
Exploring the sources of proton spin is one of the major scientific missions at RHIC, a DOE Office of Science User Facility and the only machine in the world capable of colliding protons with their spins aligned in a chosen direction. Nuclear physicists from around the globe, including many supported by the Japanese RIKEN laboratory, come to RHIC to study these "polarized proton" collisions in an effort to solve the so-called proton spin puzzle. The RBRC was established at Brookhaven in collaboration with RIKEN to support young scientists engaged in this and other relevant research.
The proton spin mystery originated when experiments in the 1980s revealed that a proton's spinâ??a property that influences these particles' optical, electrical, and magnetic characteristicsâ??does not come solely from its quarks. To tease out the gluons' role, RHIC physicists 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." 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 observed in the production of these 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.
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Physicists zoom in on gluons' contribution to proton spin