A decades-old question about what makes a proton a proton has taken a significant step toward an answer. New results from the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) indicate that the proton’s baryon number may be carried not only by its three constituent quarks, but also by the invisible gluon field that binds them together. The evidence supports a striking picture in which baryon identity is associated with a Y-shaped configuration called a baryon junction, a structure predicted by quantum chromodynamics but never before tested so directly in experiments.
Baryon number is one of the fundamental quantum properties used to distinguish ordinary matter from antimatter. Protons and neutrons each carry a baryon number of one, while their antimatter counterparts carry minus one. In everyday conditions, baryon number appears to be conserved: when particles collide or decay, the total amount of baryon number remains unchanged. Yet conservation does not explain where that number resides inside a proton. For much of modern particle physics, the simplest answer was that it belongs to the proton’s three valence quarks—two up quarks and one down quark. The new findings suggest that this familiar description may be incomplete.
The proton is not a static trio of pointlike particles. According to quantum chromodynamics, or QCD, the theory of the strong force, quarks are bound together by gluons, the force-carrying particles of the strong interaction. Gluons themselves interact with one another, creating a constantly fluctuating field filled with virtual quark-antiquark pairs and gluonic excitations. At high energies, the proton behaves less like a compact object with three clearly separated ingredients and more like a dynamic quantum system. Its mass, spin, internal momentum and other properties emerge from the combined behavior of quarks and gluons. The origin of baryon number has remained one of the most difficult pieces of this internal puzzle.
The alternative explanation tested at RHIC is known as the baryon junction model. In this picture, the three color fields associated with the proton’s quarks meet at a common gluonic connection, forming a Y-shaped topology. The junction carries the proton’s baryon number, while the quarks can move away from it or be rearranged during a violent collision. This is fundamentally different from treating baryon number as a label permanently attached to the three valence quarks. If the junction picture is correct, baryon number can travel through the gluon field even when the original quarks are displaced, absorbed into a larger system or separated from the structure that initially contained them.
Testing the two possibilities requires more than simply counting protons after a collision. Electric charge and baryon number are related but not identical. Quarks carry both electric charge and baryon number, whereas gluons carry color charge but no electric charge or baryon number in the conventional particle accounting. By comparing how electric charge and baryon number move through the debris of high-energy collisions, physicists can search for evidence that the two quantities are transported by different mechanisms. The STAR Collaboration used several complementary collision environments at RHIC, including interactions between photons and gold nuclei as well as central and peripheral collisions between heavy nuclei.
RHIC accelerates atomic nuclei to speeds approaching that of light and brings them into collision, briefly creating matter under extreme temperatures and densities. In head-on gold-gold collisions, the energy density can become high enough to produce a quark-gluon plasma, a state in which quarks and gluons are no longer confined inside individual protons and neutrons. Glancing collisions create different geometries and particle densities, while photon-induced interactions provide a cleaner way to probe the internal structure of a nucleus and its constituents. Across these conditions, the STAR researchers tracked the distribution of particles carrying baryon number and compared it with the movement of electric charge.
The results consistently showed that baryon number was transported farther and behaved differently from what would be expected if it were tied exclusively to the proton’s valence quarks. In particular, baryon number appeared to move more readily than electric charge under the collision conditions studied. This separation is difficult to explain using a simple three-quark picture, because the valence quarks carry both properties together. The observations instead align with a mechanism in which the gluonic junction can transport baryon number independently of the detailed paths followed by the quarks. The measurements do not mean that quarks are irrelevant, but they indicate that the gluon field plays a direct and essential role in preserving the proton’s identity during violent rearrangements.
The finding is especially important because it connects an abstract feature of QCD with measurable behavior in nuclear collisions. Baryon junctions have appeared in theoretical calculations and models of high-energy particle production for decades, but their role has been difficult to isolate experimentally. The STAR measurements provide a consistent pattern across different collision systems, strengthening the case that the junction is more than a mathematical convenience. By studying how baryon number is shifted away from its original rapidity region—the particle-physics measure of motion along the beam direction—researchers can determine how efficiently it is transported through the strongly interacting medium. The observed transport favors junction-based dynamics over a model in which only the original valence quarks carry the proton’s baryon number.
The implications reach beyond the structure of a single proton. Understanding how baryon number is stored and transported may improve models of the quark-gluon plasma, the dense matter created in neutron-star interiors and the earliest moments of the Universe. It may also clarify how the strong interaction helps produce the remarkable stability of ordinary matter. The matter-antimatter imbalance presents an even larger mystery: the known Universe contains vastly more matter than antimatter, although many fundamental processes appear to treat the two similarly. The baryon junction results do not solve that imbalance, but they offer a more precise view of how baryonic matter is organized and how its defining quantum number survives when matter is heated, compressed and broken apart.
The researchers emphasize that the evidence favors the baryon junction model rather than establishing that all baryon number is carried exclusively by gluons. Future experiments will be needed to determine how the junction shares responsibility with quarks and how its behavior changes when the proton’s internal arrangement is modified. More precise measurements at RHIC and other high-energy facilities could reveal whether baryon number follows specific gluonic configurations, how it is redistributed in quark-gluon plasma and whether similar transport occurs in other baryons. As Wenliang Li notes in an accompanying Perspective, identifying whether quarks, gluons or a combination of both transport baryon number could expose hidden aspects of the proton’s identity. What began as a question about a tiny subatomic particle may ultimately help explain how the strong force organizes stable matter across the Universe.
Subject of Research: The transport and origin of baryon number inside the proton, with a focus on the baryon junction model and gluonic contributions to proton structure.
Article Title: Tracking the baryon number with nuclear collisions
News Publication Date: 13-Aug-2026
Web References: https://doi.org/10.1126/science.ads5962
References: Science, “Tracking the baryon number with nuclear collisions,” DOI: 10.1126/science.ads5962
Keywords
Baryon number, proton structure, baryon junction, gluons, quarks, quantum chromodynamics, RHIC, STAR Collaboration, heavy-ion collisions, quark-gluon plasma, nuclear physics, antimatter asymmetry
Tags: baryon number conservation in particle physicsexperimental evidence of gluon contributiongluons carrying baryon numberimplications for matter antimatter asymmetryproton baryon number distributionproton structure and internal compositionquantum chromodynamics baryon junctionquantum properties of protonsRelativistic Heavy Ion Collider (RHIC) studiesrole of gluons in proton stabilitySTAR Collaboration proton researchvalence quarks vs gluons in protons
