U.S. Heavy-Ion Collision Research Suggests Protons and Neutrons May Not Be Just "Three Quarks"

The STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy's Brookhaven National Laboratory recently proposed, based on heavy-ion collision data studies, that baryons such as protons and neutrons may not be simply composed of three valence quarks, and that their baryon number may be more closely related to the "baryon junction" in the gluon field. The findings were published in the journal *Science*.

In the traditional valence quark model, each quark is assigned a baryon number of 1/3, so baryons such as protons and neutrons, composed of three quarks, carry a baryon number of +1. However, this model is not a fundamental requirement of quantum chromodynamics (QCD). As early as the 1970s, alternative explanations had been proposed in the theoretical community; in 1996, Dmitri Kharzeev proposed a model suggesting that the baryon number is not necessarily carried by the quarks themselves, but may instead be concentrated at the Y-shaped junction formed by the gluon field—the "baryon junction."

In this study, the STAR Collaboration focused on comparing the different predictions of the baryon junction model and the valence quark model in heavy-ion collisions. The researchers analyzed head-on collisions of gold nuclei, as well as nuclear breakup processes induced by virtual photon exchange in near-miss collisions. The results showed that as beam energy increased, the number of net scattered baryons did decrease, but at a slower rate than predicted by the existing valence quark model. This observation is more consistent with the picture that "baryon junctions scatter more readily than valence quarks."

The study also compared debris from collisions of zirconium-96 and ruthenium-96 nuclei. Both have the same number of nucleons and the same baryon number, but differ in proton number and therefore in electric charge. If both baryon number and charge moved primarily with the quarks, the outcomes of the two types of collisions should be well predictable within a certain energy range. However, the experiments showed that the magnitude of baryon number migration was significantly larger than that of charge migration, further supporting the conclusion that "quarks do not simultaneously carry both baryon number and charge."

Nuclear physicist Anselm Vossen, who was not involved in the study, noted that this result is consistent with the overall understanding that the internal structure of the proton is far more complex than "three valence quarks." He pointed out that questions regarding the proton's spin and mass have long suggested a richer dynamical structure within the proton. The research team hopes to continue analyzing data from other collision experiments in the future and looks forward to more precise baryon detection results from the Electron-Ion Collider.