Using particle accelerators, researchers recreate the processes that occurred in the earliest moments of the Universe to better understand fundamental questions in physics, such as why matter dominates over antimatter, the structure of quarks, and the conservation of baryon number. A recent analysis of collisions at the RHIC collider has provided strong evidence that baryon number is not carried by quarks, but rather by a gluon junction inside protons.

Baryons and the Law of Baryon Number Conservation

Baryons are elementary particles composed of three quarks, with protons and neutrons being the most well-known examples. For these particles, the law of baryon number conservation has been established empirically: the number of baryons in a system is determined by the difference between the number of quarks and antiquarks that make up the antiparticles. Although this law lacks a strict theoretical foundation, it is supported by all observed physical processes and is linked to the fact that there is more matter than antimatter in the Universe.

The Role of Gluons and the Gluon Junction

Inside baryons, in addition to quarks, there are gluons—carriers of the strong force. According to quantum chromodynamics, gluons in a proton form a Y-shaped structure known as the baryon (or gluon) junction. Until recently, the existence of this structure had not been confirmed experimentally.

Current theories suggest that the baryon number is evenly distributed among the three quarks. However, since the 1990s, a hypothesis has emerged that the gluon junction itself could be the true carrier of baryon number. Testing this hypothesis is challenging because observing quarks and gluons individually is extremely difficult.

Experimental Data from RHIC

Experiments conducted with the STAR detector at the Relativistic Heavy Ion Collider (RHIC) involved an international team of physicists who presented data indicating that the Y-shaped junction acts as the carrier of baryon number. These results were published in the scientific journal Science.

During collider experiments, an excess of baryons was observed flying perpendicular to the direction of the colliding beams. To explain this phenomenon, it would require all three quarks from a single proton to stop, with their energy going into the creation of new particles. However, calculations showed that other processes also play a role: quarks carry electric charge, while the junction does not. Comparing the observed baryon number in different types of nuclear collisions with the redistribution of electric charges revealed that collisions generate twice as many baryons as would be expected based on the measured charges of stopped quarks.

The Mechanism of Baryon Formation

At high energies, gluons inside the proton split and multiply, each carrying a smaller fraction of the proton’s energy, while the contribution from quarks remains constant. According to the STAR detector team, the gluon baryon junction becomes energetically light and is easier to stop than the three quarks inside the proton. After the junction stops, the quarks continue moving through the collider. Quarks and gluons cannot exist in isolation and quickly combine with other particles: each quark can pair with an antiquark to form a meson, while the gluon junction attracts three new quarks from the vacuum to form a baryon. In reality, the process is more complex: the collision of nuclei, each consisting of hundreds of protons and neutrons, leads to the creation of thousands of new particles. The more particles that are produced, the greater the observed excess of baryons.

Model Comparison and Future Research

Analysis of the experimental data showed that it aligns better with the gluon junction model than with the model where baryon number is carried by quarks. Data collected at the collider from 2000 to 2026 are considered compelling evidence for the existence of the baryon junction. However, independent confirmation from other research groups is needed before a definitive conclusion can be drawn.