A Science paper that could rewrite the textbooks
On August 14, 2026, *Science* published a paper from the STAR Collaboration: "Tracking the baryon number with nuclear collisions", Science 393 (6812): 727-731; doi:10.1126/science.ads5962.
Its conclusion in one sentence:
> The proton's baryon number may, more accurately, be carried by the Y-shaped "gluon junction" that connects the three quarks—rather than being split evenly among the quarks themselves.
This matters because baryon number is one of the strictest conservation laws in physics: since the Big Bang, the total number of protons and neutrons in the universe has never changed. It underlies two cosmic-scale questions—why matter outnumbers antimatter, and why protons remain stable for longer than the age of the universe.
Where "1/3 + 1/3 + 1/3" came from—and its blind spot
The standard assignment dates to the 1960s-70s quark model:
- Proton = 2 up quarks + 1 down quark, baryon number = +1
- Neutron = 1 up + 2 down, baryon number = +1
- Antiproton, baryon number = -1
- Meson = quark + antiquark, baryon number = 0
- Baryon number on quarks → quarks can decay → baryon number could change → total matter could change
- Baryon number on a gluon junction → the Y-shaped topology is conserved → baryon number strictly conserved → matter is stable
- Brookhaven Lab: Gluons May Play Central Role in Baryon Number Conservation
- ScienceDaily: Physicists discover a hidden gluon structure inside protons that could rewrite textbooks
- Science: A new explanation of why the universe's supply of nuclear matter is fixed?
- Sci.News: Physicists Find Evidence for How Protons Carry Their Identity
- Rice University News: Scientists uncover new clue to how protons maintain their identity
- Mechanism Me: New Discoveries at RHIC Suggest Gluon Junctions Rather Than Quarks Carry Proton Baryon Number
"Three quarks make a baryon, so each quark naturally carries 1/3" seemed obvious—but theorists realized in the 1970s that because gluons interact with each other, three gluon fields can meet at a central point, forming a Y-shaped structure (a baryon junction or gluon junction). In 1996, Dmitri Kharzeev (then at Stony Brook University and Brookhaven Lab) proposed that the baryon number might be carried entirely by this junction. No experiment could test it then. Thirty years later, one could.
Three smoking guns from STAR
The STAR detector (1,200 tons) ran at RHIC for 25 years. The analysis was led by Zhangbu Xu (Kent State University) and Rongrong Ma (Brookhaven Lab), with Nicole Lewis (Rice University) initiating the project. The logic: if baryon number rides on quarks, it should keep flying along the beam direction; if it sits on a gluon junction, the junction is more easily "stopped," pulls three new quarks out of the vacuum, and new baryons emerge preferentially perpendicular to the beam.
| Evidence | Meaning | | --- | --- | | Far more baryons than antibaryons in transverse flow in gold nucleus–photon collisions | Baryon number is being stopped in the transverse direction | | No analogous transverse flow of net charge | Net charge still rides on quarks | | Spectator quarks keep flying along the beam | Quarks themselves are not stopped |
The third point is decisive: baryon number stopping while charge does not is an asymmetry only explicable if the two are carried by different objects. As Zhangbu Xu put it:
> "If the baryon number flows with the quarks, I don't think you will ever be able to explain this data."
External theorists backed the result. Kharzeev: "I'm delighted the STAR collaboration was able to do this. It is very difficult and intricate measurement." Zohar Komargodski (Stony Brook Center for Geometry and Physics) noted that from the theoretical side, the existence of gluon junctions was already established.
Why it matters for the cosmos
Nicole Lewis: "Since the Big Bang, the number of protons and neutrons all together never changes as a function of time... It's one of the mysteries of the universe, related to why we have more matter than antimatter."
The causal chain:
The second picture explains why the proton's lifetime is thought to exceed the age of the universe: a proton cannot decay unless its topologically protected gluon junction is destroyed.
RHIC's final gift
This paper is among RHIC's last results before its retirement in early 2026, after 25 years and millions of gold-gold collisions recorded by STAR. The analysis used the Open Science Grid, Brookhaven Scientific Computing and Data Facilities, and NERSC at Lawrence Berkeley National Laboratory.
What to watch in the next 6-12 months
1. Reproduction and extension by STAR: whether additional collision systems (lead-lead, copper-copper) provide independent verification; the Electron-Ion Collider (construction starting 2026, operations in the 2030s) will push precision further. 2. Textbook revisions: which mainstream particle-physics texts update the "baryon number carrier" chapters by 2027 H1. 3. Theory follow-up: whether Kharzeev's three decades of baryon-junction work triggers a rewrite of the QCD baryon-number chapters. 4. Cosmology: if baryon number lives on junctions, "why are there more gluon junctions than anti-junctions?" becomes a new cosmological question. 5. LHC cross-checks: whether ATLAS/CMS design dedicated triggers to test the stopping probability of baryon junctions in heavy-ion collisions.