A gluon junction may explain where a proton’s baryon number comes from

A proton looks so simple, made of three quarks held together by gluons. However, this appearance could be a deception. New measurements suggest that one of its most fundamental properties may not belong to those three quarks at all. The measurements came from the STAR (Solenoidal Tracker) detector at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC), the same facility that recently completed its final collisions after 25 years of probing quarks and gluons. The results suggest that baryon number—the quantum property that distinguishes matter such as protons and neutrons from other particles—may be carried by a hidden Y-shaped structure made of gluons. “In the naïve quark model, there are three quarks inside a proton, but nothing else. 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,” Tommy Tsang, one of the researchers and a scientist from Argonne National Laboratory, said. If confirmed, these findings would challenge the standard picture of how one of nature’s basic conservation laws is stored inside matter. The particle property hiding in plain sight Every proton and neutron has a baryon number of +1. For decades, physicists have conventionally treated that number as being divided equally among the three valence quarks inside each particle: each quark carries one-third. However, there is a problem. Protons contain far more than three quarks. Their interiors are a constantly shifting sea of gluons, quarks and antiquarks, all governed by quantum chromodynamics, the theory of the strong force. Scientists therefore have never directly established whether baryon number belongs to the quarks or to the gluonic structure connecting them. One alternative, proposed theoretically in the 1970s and developed as a possible explanation for baryon-number transport in high-energy collisions in 1996, is the baryon junction—a Y-shaped arrangement of gluon fields linking the three quarks. The STAR Collaboration has now found evidence favoring that picture. Turning collisions into a test The researchers did not simply look inside a proton. Instead, they used RHIC as a kind of particle-scale stress test. STAR analyzed several kinds of high-energy collisions, including isobar nuclear collisions and photonuclear collisions, and compared where baryons and their antimatter counterparts emerged after the collisions. The team also compared the new results with earlier measurements from Au+Au collisions. The key clue was an unexpected excess of baryons. “In the STAR detector, we consistently see an excess of baryons coming out of the collisions perpendicular to the direction of the colliding beams,” Tsang added. If the three valence quarks were solely responsible for baryon number, the amount of baryon excess should track the electric charge carried by those quarks. Electric charge provided the researchers with an independent way to estimate how many valence quarks had been stopped in the collision. Instead, STAR found about twice the baryon yield that would be expected from the amount of valence-quark stopping inferred from the measured electric charge. In the study, this appeared as a larger B/∆Q ratio than predicted by models in which valence quarks carry the baryon number. That mismatch is difficult to explain if baryon number simply travels with the three fast-moving quarks. The baryon-junction model provides another possibility At RHIC energies, a proton contains a huge number of gluons. As gluons split and multiply, the three valence quarks retain much of the proton’s forward momentum, while the gluonic junction can carry relatively little. During a collision, the junction may therefore be easier to stop than the quarks. In this picture, the stopped junction can contribute to the production of new baryons while the original quarks continue down the beam direction. As a baryon junction connects three color charges, it can effectively recruit three new quarks and form a new baryon. This could explain why baryon number appears far away from the original quarks. The researchers found another clue in net-proton yields measured across rapidity in photonuclear collisions. The distribution was less asymmetric than expected from models assigning baryon number to valence quarks. STAR also found that the beam-energy dependence of mid-rapidity net-hyperon yields was largely flavor blind after accounting for the suppression of strange-quark production, another feature consistent with baryon-junction transport. Together, these observations, along with previous Au+Au measurements, disfavor the picture of valence quarks as the sole carriers of baryon number. They also add to a growing body of RHIC research showing how collisions can reveal unexpected behavior of quarks and gluons under extreme conditions, including the strange properties of the quark-gluon plasma created in these collisions. A clue, not the final word The result does not mean scientists have completely solved the mystery. The evidence comes from collision patterns and comparisons with theoretical models, rather than from directly observing a baryon junction inside a proton. Still, the implications reach far beyond RHIC. Baryon-number conservation is tied to the stability of ordinary matter and to one of cosmology’s deepest puzzles: why the universe contains far more matter than antimatter. Experiments with antimatter are still trying to explain how matter survived the Big Bang, making any better understanding of baryon number potentially important for cosmology. Baryon number can be conserved even when individual protons and neutrons transform into other baryons; what remains unchanged is the total baryon number. RHIC itself ceased operations in early 2026, so future tests will have to rely on existing data and other facilities capable of probing the strong force. One important successor will be the Electron-Ion Collider, which is being designed to probe how quarks and gluons build the structure of matter. The next challenge is to determine whether the same gluon-driven mechanism consistently explains baryon transport across different collision systems and energies. If it does, the humble proton may have to be rethought—not as three quarks carrying a shared identity, but as three quarks connected by a gluonic structure that may help define what the particle fundamentally is. The study is published in the journal Science. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Rupendra Brahambhatt is an experienced writer, researcher, journalist, and filmmaker. With a B.Sc (Hons.) in Science and PGJMC in Mass Communications, he has been actively working with some of the most innovative brands, news agencies, digital magazines, documentary filmmakers, and nonprofits from different parts of the globe. As an author, he works with a vision to bring forward the right information and encourage a constructive mindset among the masses.