Physicists have discovered a hidden gluon structure inside protons that could rewrite textbooks

The evidence comes from high-energy particle collisions at RHIC, a DOE Office of Science user facility for nuclear physics research that operated at DOE’s Brookhaven National Laboratory from 2000 to early 2026. According to the new study, published in sciencesThe baryon number may be related to a Y-shaped “junction” of gluons connecting the proton’s three main quarks. If confirmed, it would challenge the long-standing assumption that the baryon number belongs exclusively to those quarks.

“Traditionally, scientists assumed that each of the three main valence quarks inside a proton or neutron carries a third of the baryon number,” said Zhangbo Xu, a professor at Kent State University who works at Brookhaven Laboratory.

A decades-old idea about gluons

Physicists first proposed the baryon coupling, also called the gluon coupling, in the 1970s as a way to describe how gluons bind to valence quarks within a proton. In 1996, four years before RHIC became operational, Dmitry Kharzev, a theoretical physicist at Stony Brook University and Brookhaven Laboratory, suggested that this crossover might do something more fundamental. Instead of valence quarks carrying the baryon number, the bond itself could be responsible.

The STAR collaboration has now developed a way to test this possibility using several types of collisions produced at RHIC.

“Using data collected from different types of particle collisions at RHIC, our results indicate that the baryon number is not simply carried by individual quarks,” Xu added. “Our findings strongly support the idea that the baryon number is best carried and transported by gluons, the particles that hold quarks together, when they are arranged in this special configuration.”

Why does the baryon number matter?

Determining what actually carries the baryon number has importance far beyond the internal structure of the proton. In RHIC collisions, conservation of baryon number means that the total number of baryons, which are three-quark particles like protons and neutrons, must remain unchanged before and after the collision. The same conservation principle also applies on a cosmic scale.

“Since the Big Bang, the number of protons and neutrons together has never changed as a function of time,” said Nicole Lewis, a STAR physicist at Rice University who began this project as a postdoctoral researcher at Brookhaven Laboratory in 2020. “The reasons for this conservation are not well understood. It is one of the mysteries of the universe, and is related to why we have more matter than antimatter.”

Preserving the baryon number also has a much more tangible consequence. It helps explain the extraordinary stability of protons, which form a central part of atomic nuclei and do not appear to decay under normal conditions.

“The lifetime of the proton is thought to be longer than the lifetime of the universe,” Lewis said. “This allows the atomic nucleus to form and stabilize – meaning that matter, when we interact with it in the universe, can exist.”

Proton is more complex

The possibility that gluons have a baryon number would upend the standard simplistic description found in many textbooks. In this picture, the proton has a baryon number plus one, evenly divided between the three main valence quarks. Thus each quark carries plus a third of the baryon number, just as the electric charge of the proton is distributed among the three valence quarks.

But real protons are much more complex than the simplified model suggests.

“In the naive quark model, there are three quarks inside the proton, but nothing else,” said Tommy Tsang, a former postdoctoral researcher at Kent State University, now at the Department of Energy’s Argonne National Laboratory. “But if we look at the details inside, there are not only three quarks, there are also a lot of interacting gluons, connecting those quarks, and there are also quarks and antiquarks emerging from the vacuum, so it’s actually a really complex object.”

Quantum chromodynamics (QCD), the theory used to describe these interactions, has had great success in explaining the strong force operating between quarks and gluons. However, QCD-inspired models often need additional assumptions to reproduce some of the particle patterns observed when RHIC smashes nuclei together at close to the speed of light.

An unexpected excess of baryons

One observation in particular caught the attention of the STAR team. The detector repeatedly records more baryons than antibaryons emerging laterally from collisions, perpendicular to the direction of the incoming beams.

“At the STAR detector, we constantly see an excess of baryons emerging from collisions perpendicular to the direction of the colliding beams,” Tsang said. “The fact that we end up having more baryons than anti-baryons — or more matter than antimatter — is not surprising because our collisions start with matter,” he said.

These highly energetic collisions transform huge amounts of energy into thousands of newly created molecules. What puzzled the researchers wasn’t just that more baryons were produced than anti-baryons. This was where the excess baryons appeared.

If valence quarks alone had the baryon number, explaining the increase away from the beamline would require that all three valence quarks from a colliding proton stop near the center of the detector. They would then have to undergo a transformation from matter to energy and back to matter, producing new baryons that move outward perpendicular to the beam.

STAR researchers suspect there may be another explanation.

Electrical charging provides a test

The team found a way to investigate the mystery by taking advantage of another property of valence quarks: electric charge. The scientists compared the net baryon count measured in various RHIC nuclear collisions with the way electrical charge was redistributed in those same events.

“Measuring the electric charge leaving perpendicular to the collision gives you a specific way to measure how many quarks have stopped and turned into new particles,” said Zibo Tang, a professor at the University of Science and Technology of China who led a group of students analyzing the data and simulating the models.

The comparison revealed a stark mismatch. The researchers observed nearly twice as many baryons as should have been produced based on the electrical charge associated with parked quarks.

According to QCD-based models, this means that too few quarks are stopped to account for all the baryons that appear in the detector.

This left an important question: what carries the extra baryon number?

STAR physicists argue that gluons provide a possible answer, specifically the three-pronged gluon connection that connects the proton’s valence quarks.

How can a gluon intersection have a baryon number?

The proposed mechanism is based on what happens when protons inside colliding nuclei reach enormous energies. According to the STAR team, stopping the “gluon conductor” or “baryon conductor” that connects the quarks in a collision may be much easier than stopping the three quarks themselves.

If the link is stopped, its energy can be converted into newly produced baryons which travel outward in directions perpendicular to the beams. At the same time, the valence quarks that were previously connected by the junction can continue to move forward along the beam tube.

Understanding why requires looking at the changing internal structure of the proton as its energy increases.

“The baryon junction is always there even when protons are accelerated to higher and higher energy,” said Prithwesh Tribidi, a STAR physicist at Brookhaven Laboratory. “But at high energy, the gluons inside the proton split and multiply.”

As the number of gluons increases, the proton’s momentum is spread among more of them. Thus each individual gluon, including those forming the coupling, carries a smaller fraction of the proton’s total momentum. However, the valence quarks continue to carry most of the proton’s forward motion.

As a result, when a collision occurs, it should be easier to stop the connection of relatively slower three-axial gluons and turn them into new particles than fast-moving quarks.

Stopping a single connected structure is also easier than stopping three separate quarks, making such an interaction more likely, according to Tripidi.

“In the collision, the baryon bond stays behind, and the quarks continue on their way,” he noted.

Building new molecules after collision

Quarks and gluons cannot remain isolated, so after collision they quickly combine with other particles.

In a simplified example, a quark that continues through the beam tube can join an antiquark and form a two-quark particle called a meson. Meanwhile, the three-pronged gluon coupling can act a bit like a Y-shaped magnet, pulling three newly created quarks out of the vacuum and producing a new baryon.

Actual RHIC collisions are considerably more violent and complex.

“Even though we start with a nucleus containing approximately 100 protons and 100 neutrons, these collisions create thousands of new particles; 99% of the energy is transformed into new particles,” said Rongrong Ma, a physicist at Brookhaven Laboratory.

The STAR team found that collisions that produced larger numbers of particles also showed a larger increase in “medium-velocity” baryons than predictions based on the simpler picture in which quarks alone carry the baryon number.

The fact that many of these baryons appear perpendicular to the beamline provides strong evidence, according to the researchers, that baryon intersection exists and plays an important role in baryon number transfer.

Reconsidering a fundamental property of matter

The results suggest that one of the defining quantum properties of the proton may not lie solely in the three valence quarks. Instead, the structure of the gluons that bind those quarks could be fundamental to how the baryon number is transferred during energetic collisions.

“Our research challenges the long-held idea that the baryon number is simply divided between the three quarks and holds them,” Ma said. “This new understanding reshapes the way we think about the structure of matter and deepens our knowledge of the fundamental element responsible for the universe in its current form.”

The research was supported by the Department of Energy’s Office of Science, the US National Science Foundation (NSF), and several international agencies and organizations listed in the paper. The researchers also used the Open Science Network, which is directly supported by NSF, along with computing resources at data and scientific computing facilities at Brookhaven Laboratory and the National Energy Research Scientific Computing Center (NERSC), another DOE Office of Science user facility located at DOE’s Lawrence Berkeley National Laboratory.