A series of high-energy particle collision experiments tests a decades-old hypothesis about the proton’s baryon number – a conserved quantum property that distinguishes matter from antimatter. The findings offer support for the idea that baryons are likely carried by a Y-shaped gluonic structure known as a baryon junction, and offer insights into the origin of the proton’s identity and how matter is organized throughout the Universe. The proton – the fundamental building block of atoms – was once thought to be simply a collection of three quarks. However, increasingly precise experiments have revealed that it is a far more complex system, with many of its defining properties emerging from the interactions between quarks and the gluons that bind them together. One of the biggest remaining mysteries is how the proton carries its baryon number. The traditional model holds that baryon number is carried by the proton's three quarks, while an alternative theory proposes that it is instead associated with the gluon field connecting those quarks through a structure known as the baryon junction. Yet, to date, neither hypothesis has been experimentally confirmed.

Using the Relativistic Heavy Ion Collider (RHIC) to study particle collisions involving photons and gold nuclei, as well as head-on and glancing heavy-ion collisions, the STAR Collaboration was able to track baryon number independently of electric charge under conditions where protons break apart. Across these experiments, the results consistently favored the baryon junction model over the traditional three-quark picture. The researchers found that baryon number was transported farther and differently than would be expected if it were carried solely by the proton's valence quarks. In particular, the baryon number moved more readily than electric charge, supporting the idea that the proton's identity is carried, at least in part, by the gluon field that binds its quarks together rather than by the quarks alone. “Future measurements could reveal hidden aspects of the proton’s identity – how its baryon number is carried when its quarks and gluons are rearranged,” writes Wenliang Li in a related Perspective. “Determining whether quarks or the gluon field transports baryon number could contribute to understanding how strong interaction between subatomic particles organizes stable matter and what causes the imbalance between matter and antimatter in the Universe.”