The proton looks very simple: it consists of three quarks held together by gluons. However, this appearance can be deceiving.
New measurements suggest that one of its fundamental properties may not belong to those three quarks at all.
The measurements came from the STAR (Solenoidal Tracker) detector at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC), the same facility where the discovery was recently made. It completed its final collisions After 25 years of exploring quarks and gluons.
The results suggest that the baryon number — the quantum property that distinguishes matter such as protons and neutrons from other particles — may be carried by a hidden Y-shaped structure made of gluons.
“In the naive quark model, there are three quarks inside the proton, but nothing else. But if we look at the details inside, not only are there three quarks, but there are also a lot of gluons interacting, communicating between those quarks, and there are also quarks and antiquarks emerging from the vacuum, so it’s actually a really complex object,” said Tommy Tsang, one of the researchers and a scientist from Argonne National Laboratory. He said.
If these results are confirmed, they will challenge the standard picture of how one of nature’s fundamental conservation laws is stored within matter.
A property of particles hiding in plain sight
Every proton and neutron has a baryon number of +1. For decades, physicists have traditionally treated this number as being evenly divided between the three valence quarks within each particle: each quark carries a third.
However, there is a problem. Protons contain much more than three quarks. Its interiors are a constantly changing sea of gluons, quarks and antiquarks, all governed by quantum chromodynamics, the theory of the strong force.
Therefore, scientists have never been able to determine whether the baryon number belongs to the quarks or to the gluon structure that connects them.
One alternative, it was theoretically proposed in the 1970s and developed as a possible explanation for the transition of baryon numbers at high energy 1996 collisionsis a baryon junction – a Y-shaped arrangement of gluon fields that connects the three quarks.
The STAR Collaboration has now found evidence to support this picture.
Turn collisions into a test
The researchers didn’t simply look inside the proton. Instead, they used RHIC as a type of particle-level stress test.
STAR analyzed several types of high-energy collisions, including isobaric nuclear collisions and photonuclear collisions, and compared where baryons and their antimatter counterparts appear after the collisions. The team also compared the new results with previous measurements from Au+Au collisions.
The key was the presence of an unexpected excess of baryons. “At the STAR detector, we constantly see an excess of baryons emerging from collisions perpendicular to the direction of the colliding beams,” Tsang added.
If the three valence quarks are solely responsible for the baryon number, then the amount of excess baryon should follow the electric charge carried by those quarks. The electric charge provided researchers with an independent way to estimate how many valence quarks were stopped in the collision.
instead of, A star has been found About twice the baryon yield that would be expected from the valence stopping quantities and quarks inferred from the measured electric charge. In the study, this emerged as greater for/∆S The proportion predicted by models in which valence quarks have the baryon number.
This mismatch is difficult to explain if the baryon number simply moves with the three fast-moving quarks.
The baryon junction model offers another possibility
At RHIC energies, the proton contains a large number of gluons. As gluons split and multiply, the three valence quarks retain much of the proton’s forward momentum, while the gluon coupling can carry relatively little. During a collision, it may be easier to stop the link than to stop the quarks.
In this picture, the stalled connection can contribute to the production of new baryons while the original quarks continue in the direction of the beam. Since the baryon bond connects three color charges, it can effectively recruit three new quarks and form a new baryon. This could explain why the baryon number appears to be far from the original quarks.
The researchers found further evidence in the net proton yield measured via velocity in photonuclear collisions. The distribution was less variable than expected from models determining the baryon number for valence quarks.
STAR also found that the beam energy dependence of the net-average hyperion yield was largely blind after accounting for the suppression of strange quark production, another feature consistent with baryon coupling transport.
Together, these observations, along with previous Au+Au measurements, do not support the picture of valence quarks as the sole bearers of the baryon number.
It also adds to a growing body of RHIC research showing how collisions can reveal unexpected behavior of quarks and gluons under extreme conditions, including Strange properties of quark-gluon The plasma created in these collisions.
An idea, not the final word
The result does not mean that scientists have completely solved the puzzle. The evidence comes from collision patterns and comparisons with theoretical models, not from direct observation of baryon intercalation within a proton.
However, the implications reach far beyond RHIC. The conservation of the baryon number is related to the stability of ordinary matter and to one of the deepest mysteries in cosmology: why the universe contains so much more matter than antimatter.
Experiments with antimatter They are still trying to explain how matter survived the Big Bang, making any better understanding of the baryon number important for cosmology. The baryon number can be conserved even when individual protons and neutrons are transformed into other baryons; What remains unchanged is the total baryon number.
RHIC itself ceased operations in early 2026, so future tests will have to rely on existing data and other facilities capable of examining the robust force. One of the important successors will be Electron-ion colliderwhich is designed to explore how quarks and gluons build the structure of matter.
The next challenge is to determine whether the same gluon-driven mechanism consistently explains baryon transport across different systems and collision energies.
If so, the humble proton might have to be reconsidered, not as three quarks with a common identity, but as three quarks connected by a gluon structure that might help define what a particle essentially is.
the He studies It was published in the magazine sciences.
