Physicists discover a hidden gluon structure inside protons that could rewrite textbooks | ScienceDaily

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Physicists discover a hidden gluon structure inside protons that could rewrite textbooks

Date:<br>August 16, 2026<br>Source:<br>Brookhaven National Laboratory<br>Summary:<br>Physicists may have uncovered a hidden feature inside protons that helps preserve one of matter’s most fundamental properties. RHIC collision data suggest baryon number is carried not simply by three quarks, but by a Y-shaped junction of the gluons connecting them. The finding challenges a decades-old textbook picture and could deepen our understanding of why protons—and ultimately matter itself—remain stable.<br>Share:

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FULL STORY

This image shows how, upon impact in a collision (left), a proton's three valence quarks (u, u, d) will continue to fly down the beampipe while the baryon junction, the Y-shaped configuration of gluons, is more easily stopped (right). The baryon junction will pull three new quarks out of the vacuum to become a new baryon, thus retaining the baryon number, while the "freed" quarks will each pair up with a new partner to form mesons. Credit: Valerie A. Lentz/Brookhaven National Laboratory

New findings from the STAR detector at the Relativistic Heavy Ion Collider (RHIC) are challenging a familiar picture of what gives protons one of their defining quantum properties. The results suggest that gluons, the particles that act as the glue holding quarks together, may play a key role in carrying and conserving baryon number.

The evidence comes from high-energy 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. According to the new study, published in Science, baryon number may be associated with a Y-shaped "junction" of gluons connecting the proton's three main quarks. If confirmed, that would challenge the long-standing assumption that baryon number belongs exclusively to those 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.

A Decades-Old Idea About Gluons

Physicists first proposed the baryon junction, also called a gluon junction, in the 1970s as a way to describe how gluons connect the valence quarks inside a proton. In 1996, four years before RHIC began operating, Dmitri Kharzeev, a theoretical physicist at Stony Brook University and Brookhaven Lab, proposed that this junction might do something even more fundamental. Rather than the valence quarks carrying baryon number, the junction itself could be responsible.

The STAR collaboration has now developed a way to test that possibility using several types of collisions produced at RHIC.

"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 favorably carried and transported by gluons, the particles that hold quarks together, when arranged in this special configuration."

Why Baryon Number Matters

Determining what actually carries baryon number matters far beyond the internal structure of a proton. In RHIC collisions, conservation of baryon number means that the total number of baryons, three-quark particles such as protons and neutrons, must remain unchanged before and after the collision. The same conservation principle also applies on the scale of the 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.

Baryon number conservation also has a much more tangible consequence. It helps explain the extraordinary stability of protons, which form a central part of atomic nuclei and do not appear to decay under ordinary circumstances.

"It's believed that the lifetime of a proton is longer than the lifespan of the universe," Lewis said. "This allows atomic nuclei to form and be stable -- which means matter, as we interact with it in the universe, can exist."

A More Complicated Proton

The possibility that gluons carry baryon number would overturn the standard simplified description found in many textbooks. In that picture, a proton has a baryon number of plus one, divided equally among its three main valence quarks. Each quark therefore carries plus one third of the baryon number, much as the proton's...