Proton Baryon Number May Be Carried by Gluon Networks, STAR Experiment Finds
For decades, physicists have investigated how protons carry baryon number—the quantum property that distinguishes matter from antimatter. New findings from the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) suggest that baryon number may not be carried solely by a proton’s three quarks. Instead, it may be transported by the network of gluons that binds the quarks together, a structure known as a baryon junction.
Quarks and gluons inside protons. Image credit: Brookhaven National Laboratory.
The traditional picture describes a proton as a particle made primarily of three valence quarks, with each quark contributing to its fundamental properties.
However, increasingly precise experiments show that a proton is a far more complex system. Many of its defining characteristics emerge from the powerful interactions between quarks and gluons, the particles that carry the strong nuclear force.
One of the central unanswered questions in particle physics is how baryon number is distributed and transported inside protons and other baryons.
“We found that the quantum number of baryons has been conserved since baryon formation in the early universe,” STAR physicists said.
“Traditionally, each fractionally charged valence quark is understood to carry one-third of the baryon number.”
“An alternative hypothesis proposes that baryon number is carried by baryon junctions—nonperturbative, Y-shaped gluon structures that connect the three quarks.”
To distinguish between these models, researchers analyzed particle collisions involving photons, gold nuclei, and other heavy ions at RHIC. The collider recreates conditions similar to those that existed shortly after the Big Bang, allowing scientists to study how quarks and gluons behave under extreme conditions.
The results showed that baryon number was transported farther and in a different manner than expected if it traveled with individual quarks alone. This observation provides stronger support for the gluon-coupled baryon-junction model.
“Our results, using data collected from different types of particle collisions at RHIC, suggest that baryon number is not simply carried by individual quarks,” said Professor Zhangbu Xu, a physicist at Kent State University and Brookhaven National Laboratory.
“Our findings strongly support the idea that when baryon numbers are arranged in this special configuration, they are more favorably carried and transported by gluons—the particles that hold quarks together.”
“Our work challenges the long-held idea that baryon number is simply divided among three quarks and transported by them,” said Brookhaven National Laboratory physicist Dr. Rongrong Ma.
“This new understanding reshapes how we think about the structure of matter and deepens our knowledge of the fundamental particles and forces responsible for the universe in its present form.”
The team’s paper was published in the journal Science.
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STAR Collaboration. 2026. “Tracking baryon number transport in nuclear collisions.” Science 393 (6812): 727–731. doi: 10.1126/science.ads5962
Source: www.sci.news