New experimental data from the STAR detector at the Relativistic Heavy Ion Collider (RHIC), published in the journal Science, reveal that gluons—not quarks—play the central role in transporting net baryon number during energetic particle collisions. This finding challenges the textbook model of subatomic physics, shifting our understanding of how quantum properties stabilize matter in the universe.
Rewriting the Standard Model of Subatomic Structure
For half a century, standard subatomic models taught a straightforward rule about protons and neutrons. Textbooks stated that a proton’s baryon number of plus one was divided evenly among its three primary valence quarks, with each individual quark carrying precisely one-third of that value. This naive quark model treated protons as simple bags containing three point-like constituents and little else.
Yet, quantum chromodynamics (QCD) paints a vastly more chaotic picture. Inside every nucleon, a dense sea of virtual quarks, antiquarks, and gluons constantly flickers into existence and vanishes. Tommy Tsang, formerly a postdoc at Kent State University and now at the Department of Energy’s Argonne National Laboratory, explained the complexity of this internal ecosystem. “In the naïve quark model, there are three quarks inside a proton, but nothing else,” Tsang noted. “But if we look at details inside, there are not only three quarks but also a lot of gluons interacting, connecting between those quarks, and there are also quarks and antiquarks that pop up from the vacuum, so it’s actually a really complex object.”
The Gluon Junction Hypothesis Put to the Test
The theoretical foundation for this discovery dates back to the 1970s, when physicists first theorized the existence of a Y-shaped “junction” of gluons that physically binds the three valence quarks together. In 1996, Dmitri Kharzeev—a theoretical physicist at Stony Brook University and Brookhaven National Laboratory—proposed that this baryon junction might be the actual carrier of baryon number, rather than the valence quarks themselves. That proposal waited decades for sufficient experimental muscle to test it.
That testing ground became RHIC, a U.S. Department of Energy Office of Science user facility for nuclear physics research operated at Brookhaven National Laboratory from 2000 until early 2026. Zhangbu Xu, a professor at Kent State University with a joint appointment at Brookhaven Lab, pointed out the traditional assumptions of the field. “Traditionally, scientists have assumed that each of the three main ‘valence’ quarks inside a proton or neutron carries one-third of the baryon number,” Xu said.
Using data collected from different types of particle collisions gathered by the STAR collaboration at RHIC, researchers evaluated how baryon number moved through high-energy environments. “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 stated. “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 Conservation Matters for the Universe
Identifying the exact carrier of baryon number is not merely an academic exercise in particle physics. At the scale of RHIC collisions, baryon number conservation ensures that the total number of baryons—three-quark particles like protons and neutrons—remains identical before and after a high-energy crash. On a cosmic scale, however, this conservation law touches upon one of the deepest mysteries in modern physics: the profound asymmetry between matter and antimatter.
Nicole Lewis, a STAR physicist at Rice University who initiated this project as a postdoc at Brookhaven Lab in 2020, emphasized the scale of the mystery. “Since the Big Bang, the number of protons and neutrons all together never changes as a function of time,” Lewis said. “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.”
This strict conservation law also guarantees everyday stability. Because baryons do not readily decay, protons remain remarkably stable across cosmic timescales. “It’s believed that the lifetime of a proton is longer than the lifespan of the universe,” Lewis added. “This allows atomic nuclei to form and be stable — which means matter, as we interact with it in the universe, can exist.” With RHIC concluding its operational run in early 2026, these final datasets from the STAR detector have successfully resolved a fifty-year-old question about the internal architecture of matter.
