New evidence suggests the structure that holds protons together also dictates the quantum property that separates matter from mysterious antimatter.
This property, known as the baryon number, and its related structure—the baryon junction—comprise two elements that lie at the core of protons, one of the building blocks of atoms.
The STAR Collaboration of physicists is behind the new work, providing new evidence for a decades-old theory and insight into the matter-antimatter imbalance. The researchers revealed their findings, the result of high-energy particle collision experiments, in a recent paper published in Science.
Matter in the Universe
Down to the level of the atom, classical physics generally explains how our universe works, but when we dive into the subatomic realm, quantum physics takes over. The new work provides insights into how matter in our universe is organized at these microscopic levels, including the atomic building blocks known as baryons. Those atomic building blocks include protons and neutrons.
Physicists once suspected that baryons were a loose collection of three quarks, the smallest elementary pieces of matter, indivisible into smaller elements. This new work supports the theory that a gluonic structure called a baryon junction holds these quarks together to form a proton. This presents a more complex picture of matter at these tiny scales, influenced not just by the quarks themselves but also by the gluons that hold them together.
Matter and Antimatter
An issue physicists have struggled to resolve is how a proton carries its baryon number, the conserved quantum number obtained by subtracting the antiquarks from one-third of its total number of quarks. If this number is negative, the particle is antimatter, while positive numbers indicate normal matter.
Traditionally, physicists have assumed this number is dictated by the three quarks themselves, yet an alternative hypothesis suggests the baryon junction’s gluon field is responsible. Although the first hypothesis is more common, no experimental evidence had previously been captured to support either proposition.
That was until the team of physicists behind the new paper began work to investigate this mysterious gluon structure at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider in Upton, New York. The two massive, round tunnels of metal vacuum tubes and superconducting magnets measure 2.4 miles in circumference and allow for the collision of heavy ions at near-light speeds.
Particle Collisons
Their work involved collisions with photons and gold nuclei, as well as glancing and head-on heavy-ion collisions. The team draws its name from the STAR detector, an instrument at the site designed to study the quark-gluon plasma of the early universe.
These researchers tracked the protons’ baryon numbers, which were separated from their electric charge during events in which these tiny particles broke apart. In every instance that these experiments were run, the results agreed with the baryon junction model. The distance over which the baryon number was transported and its behavior did not correlate with expectations of the quark model.
Particularly notable was how the baryon number was easier to move than the electric charge, suggesting that the quarks are not the only factor.
The researchers say that this finding may explain not just the tiny inner workings of the smallest elements of an atom, but the very organization of matter on a universal scale. These interactions between subatomic particles lie at the root of the matter-antimatter imbalance, and future measurements could reveal even more hidden parts of how protons derive their identity.
The paper, “Tracking the Baryon Number with Nuclear Collisions,” appeared in Science on August 13, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org.