What Holds Matter Together? Physicists Find Breakthrough Evidence After 50 Years

According to research published in Science, new evidence from the Relativistic Heavy Ion Collider (RHIC) suggests that a proton’s baryon number is carried by a Y-shaped “baryon junction” of massless gluons rather than its three valence quarks, potentially resolving a decades-old particle physics mystery regarding matter and antimatter.

Unraveling the Proton’s Structural Mystery at RHIC

Physics operates like water flowing downhill, where particles and systems spontaneously tumble toward lower energy states. Heavy particles decay into lighter ones, and protons sit firmly at the bottom as the lightest baryon. For generations, textbook physics taught us that a proton’s defining stability relies on baryon-number conservation. Traditionally, physicists have presumed that the three valence quarks comprising baryons like protons and neutrons are responsible for holding the baryon number. Yet, according to work by the STAR Collaboration published in Science, this simple valence-quark picture is likely wrong.

The core dilemma stems from how subatomic building blocks are organized. As the researchers note, “The three ends of the baryon junction are connected to valence quarks, making it indistinguishable for most physical processes whether the junction or valence quarks carry the baryon number.” Because of this overlap, neither competing theory could be verified for decades. To break the deadlock, scientists turned to the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York, utilizing high-energy particle collisions to track where this critical quantum property actually hides.

Isobar and Photonuclear Collisions Put Theories to the Test

To find out what happens to matter under extreme pressure, researchers analyzed specific types of particle smashups. According to Prithwish Tribedy, a physicist at Brookhaven National Laboratory who worked on the new result, when two protons collide, the baryon number starts at two and remains two despite a chaotic blaze of created and destroyed particles. Tribedy frames the central question simply: What should we actually track to find out where it goes?

To answer this, the STAR Collaboration examined isobar nuclear collisions and photonuclear collisions. Isobar collisions involved smashing ruthenium and zirconium atoms together. Because ruthenium has four more protons and four fewer neutrons than zirconium while sharing the same mass number, it allowed scientists to measure net baryon transport against net electric charge. Valence quarks carry electric charge, but the gluon junction carries zero electric charge. The team discovered that baryons travel farther through the dense collision zone than electric charge does. This movement proves the junction isn’t slowed as much as the electrically charged valence quarks.

Photonuclear collisions provided another clean look by racing gold nuclei near the speed of light. These ions generate an intense electromagnetic field, surrounding themselves with virtual photons. Zhangbu Xu, a physicist at Kent State University and member of the STAR Collaboration, explains this setup: So, when the ions are moving close to the speed of light, there are a bunch of photons surrounding the gold nucleus, traveling with it like a cloud. Because these makeshift photons carry zero baryon number, any net baryon number measured afterward must originate directly from the gold nucleus.

Ultimately, These results, supported by data previously reported by STAR from Au+Au collisions at a variety of beam energies, disfavor the valence quark picture, the researchers write. These experimental outcomes match Regge theory predictions, which account for a baryon junction far better than older models.

Connecting Subatomic Gluons to Cosmic Matter Asymmetries

While the data marks a massive leap forward, caveats remain. Mississippi State University physicist Wenliang Li points out in an accompanying Science Perspective piece that while these fresh measurements point to gluon involvement in baryon-number transport, they lack a direct and strictly controlled assessment of the exact process at play. Still, Dmitri Kharzeev, a physicist at Stony Brook University who was not involved with the study, calls it one of the most significant results achieved by the RHIC program, adding that it reshapes our understanding of baryon structure and how baryonic matter emerged.

Solving this puzzle reaches far beyond standard particle cataloging. It dives straight into why the universe is filled with anything at all rather than empty, sterile energy fields. During the initial split second of the Big Bang, matter and antimatter should have annihilated each other completely. Instead, a minuscule excess of matter survived. Understanding whether quarks or gluons transport the baryon number helps researchers figure out how the strong force organizes stable matter and what caused that primordial matter-antimatter imbalance.

Future particle accelerators, such as Brookhaven’s upcoming Electron-Ion Collider, will push these investigations even further. As the study’s authors conclude, Further investigations into existing and alternative theories are warranted; to be viable, such theories must simultaneously explain all observed phenomena, noting that and so far, only the baryon junction framework remains qualitatively consistent.