Water runs downhill, and heavy particles decay into lighter ones for much the same reason: systems drift toward lower energy. The proton sits at the bottom of that slope. It is the lightest baryon, and nobody has ever seen one decay.
That stability is tied to a rule called baryon-number conservation, which helps tell matter from antimatter. Textbooks have long assigned the baryon number to the three valence quarks inside protons and neutrons. A study published in Science now argues that picture may be wrong.
Quarks or a Y-shaped junction
An older idea, proposed in the 1970s, puts the baryon number somewhere else: in a Y-shaped “baryon junction” made of gluons, the massless particles that bind quarks together. The trouble was testing it. “The three ends of the baryon junction are connected to valence quarks, making it indistinguishable for most physical processes whether the junction or valence quarks carry the baryon number,” the researchers write. For that reason, they add, “neither scenario has been unequivocally verified experimentally.”
The STAR Collaboration went after the question with data from the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York, now retired. The team studied two kinds of collision and compared the results with several computer models.
Two experiments
In the isobar runs, nuclei of ruthenium and zirconium, which share a mass number, were smashed together. Valence quarks carry electric charge and the junction does not, so the two ideas predict different patterns. STAR measured how far baryon number travelled compared with electric charge. Baryons got farther through the dense collision zone than charge did, which fits a junction that is slowed less than charged quarks.
The photonuclear runs used gold nuclei passing close to each other at near light speed. “When the ions are moving close to the speed of light, there are a bunch of photons surrounding the gold nucleus, traveling with it like a cloud,” said Zhangbu Xu, a physicist at Kent State University and a STAR member. One of those photons hits the other nucleus. Because a photon carries no baryon number, any net baryon number in the debris has to come from the gold.
The results match predictions from Regge theory, which includes a baryon junction, better than they match simulations built on valence quarks. “These results, supported by data previously reported by STAR from Au+Au collisions at a variety of beam energies, disfavor the valence quark picture,” the team writes.
A cautious reading
Wenliang Li, a physicist at Mississippi State University, wrote an accompanying Science Perspective. He stressed that the measurements suggest gluons may play a part in moving baryon number around, but they are not a direct, tightly controlled measurement of the mechanism underneath.
Li also tied the work to the question of why the universe holds more matter than antimatter. Working out whether quarks or the gluon field carries baryon number, he wrote, “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.”
The STAR authors say other theories still need to be examined, since any viable one must explain every observation. “So far, only the baryon junction framework remains qualitatively consistent,” they conclude. Brookhaven’s planned Electron-Ion Collider is next in line for further tests.
