This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: New results from the STAR detector at the Relativistic Heavy Ion Collider (RHIC) suggest that gluons, the glue-like particles that hold quarks together inside protons, play a central role in the conservation of baryon number—an essential part of a particle's quantum identity. The findings from energetic particle collisions at RHIC—a U.S.
Department of Energy (DOE) Office of Science user facility for nuclear physics research that operated at DOE's Brookhaven National Laboratory from 2000 to early 2026—suggest that baryon number is carried by a Y-shaped "junction" of gluons connecting the proton's three main quarks. The study, published in the journal Science, challenges a long-held view that baryon number is solely carried by those three quarks. "Traditionally, scientists have assumed that each of the three main 'valence' quarks inside a proton or neutron carries one-third of the baryon number," said Zhangbu Xu, a professor at Kent State University with a joint appointment at Brookhaven Lab.
The baryon junction, or gluon junction, was predicted by physicists in the 1970s to explain how gluons hold those valence quarks together within protons. Then, in 1996, four years before RHIC turned on, Dmitri Kharzeev, a theoretical physicist at Stony Brook University and Brookhaven Lab, proposed that the baryon junction could be the true carrier of baryon number, instead of the valence quarks. In the current paper, the STAR team describes its innovative strategy to test this idea.
"Using data collected from different types of particle collisions at RHIC, our results suggest that the baryon number is not simply carried by individual quarks," Xu added. "Our findings strongly support the idea that baryon number is more readily carried and transported by gluons, the particles that hold quarks together, when arranged in this special configuration." Identifying the carrier of baryon number has important implications. At the level of RHIC collisions, baryon number conservation ensures that the total number of baryons—three-quark particles such as protons and neutrons—remains the same before and after a collision.
But the idea of baryon number conservation extends to the entire universe. "Since the Big Bang, the number of protons and neutrons all together never changes as a function of time," said Nicole Lewis, a STAR physicist at Rice University who started this project as a postdoc at Brookhaven Lab in 2020. "The reasons for this conservation are not well understood.
It's one of the mysteries of the universe, related to why we have more matter than antimatter," she said. On an everyday practical level, baryon number conservation explains why protons, central building blocks of atomic nuclei, are so stable and don't decay. "It's believed that the lifetime of a proton is longer than the lifespan of the universe," Lewis said.
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