Science, August 14: A 53-Year-Old Textbook Assumption Shaken by Hard Experimental Evidence
On August 14, 2026, *Science* published a paper: STAR Collaboration, "Tracking the baryon number with nuclear collisions", Science 393 (6812): 727-731; doi:10.1126/science.ads5962.
Its conclusion can be stated in one sentence:
> The 'baryon number' inside a proton is, more accurately, likely carried by the Y-shaped 'gluon junction' that connects the three quarks.
The weight of this conclusion starts with the phrase 'baryon number.' It is one of the strictest conserved quantities in particle physics—since the Big Bang, the total count of protons plus neutrons in the universe has never changed. It is also at the root of two cosmic-scale questions: 'why is there more matter than antimatter' and 'why can protons remain stable longer than the age of the universe.'
Chaining together the physical consequences: if baryon number really resides in the gluon junction, then half a century of textbooks must be revised, and the pictures of proton stability and the origin of matter must be redrawn.
Where 'Baryon Number = 1/3 + 1/3 + 1/3' Came From—and Its Blind Spot
This framework dates to when the quark model was established in the 1960s-70s:
- Proton = 2 up quarks + 1 down quark, baryon number = +1
- Neutron = 1 up quark + 2 down quarks, baryon number = +1
- Antiproton = 2 anti-up + 1 anti-down, baryon number = -1
- Meson = quark + antiquark, baryon number = 0
- Baryon number on quarks → quarks can decay → baryon number can change → total matter can change
- Baryon number on the gluon junction → the junction (a Y-shaped topology) is conserved → baryon number strictly conserved → total matter stable
- Open Science Grid (NSF-supported distributed computing)
- Brookhaven Scientific Computing and Data Facilities
- NERSC (a DOE Office of Science user facility at Lawrence Berkeley National Laboratory)
- 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
'Since three quarks make a baryon, each quark naturally gets 1/3 of the baryon number'—this 'sharing' assumption seemed natural in the 1970s.
But theorists realized early on that 'natural' does not mean 'correct.' Gluons do not only exist between quarks; they also interact with each other. This fact allows gluons to form independent topological structures beyond the quarks themselves—
> Three gluon fields converge at a central point, forming a Y-shaped structure linking the three quarks. Theorists named this Y-junction the 'baryon junction' or 'gluon junction.'
In 1996, theorist Dmitri Kharzeev (then at Stony Brook University + Brookhaven Lab) made a radical conjecture:
> The baryon number may be carried entirely by the 'Y-shaped gluon junction' connecting the three quarks.
But when proposed in 1996, no experiment could test it at RHIC. Until 30 years later.
Three Smoking Guns from STAR: Transverse Flow, Net Charge, Spectator Quarks
The STAR detector (1,200 tons) has run at RHIC for 25 years. The research team was led by Professor Zhangbu Xu of Kent State University and Dr. Rongrong Ma of Brookhaven Lab; Nicole Lewis of Rice University initiated the project in 2020.
Their method was conceptually simple—collide different systems (gold-gold, gold ion + photon, lead) and see where the baryon number goes.
If baryon number 'rides on quarks,' then after the collision quarks keep flying down the beam pipe, so baryon number continues along the beam direction—the forward flow of baryon number would be highly concentrated.
If baryon number sits on the gluon junction, the situation reverses—the junction is more easily 'stopped.' A stopped gluon junction pulls three new quarks out of the vacuum to form a new baryon, and these new baryons emerge with high probability in the perpendicular (transverse) direction.
The STAR team reported three independent smoking guns:
| Evidence | Meaning | | --- | --- | | Far more baryons than antibaryons in transverse flow in gold + photon collisions | Baryon number is stopped in the transverse direction | | Net charge distribution shows no similar transverse flow | Net charge still rides on quarks | | Spectator quarks continue along the beam pipe | The quarks themselves were not stopped |
The third point is the most decisive, because the asymmetry of 'baryon number stopping while charge does not' can only be explained if baryon number and charge are carried by different carriers. If baryon number truly rode on quarks, this asymmetry could not appear.
Zhangbu Xu's words:
> "If the baryon number flows with the quarks, I don't think you will ever be able to explain this data."
Why the 'Gluon Junction' Matters: 30 Years of Theory Waiting for Experiment
When Kharzeev proposed his conjecture in 1996, RHIC had not yet turned on. Verifying 'the probability that a Y-shaped gluon junction gets stopped' requires extremely high collision energies, violent enough to shake quarks loose and eject the gluon junction on its own.
The STAR team designed a series of measurements, amplifying statistical fluctuations in 'baryon number flow direction' to observable scales. Across 50,000+ gold-gold collisions and hundreds of gold-photon collisions, they saw:
> "Our findings strongly support the idea that baryon number is more favorably carried and transported by gluons, the particles that hold quarks together, when arranged in this special configuration."
Crucially, external theorists reviewing the paper before publication reinforced the result. Kharzeev himself, upon seeing it, said:
> "I'm delighted the STAR collaboration was able to do this. It is very difficult and intricate measurement."
Theorist Zohar Komargodski of Stony Brook's Center for Geometry and Physics also endorsed: from a theoretical standpoint, we already knew gluon junctions exist.
Why This Ties Into the Ultimate Question: 'Why Does the Universe Have Matter?'
Scaling this up to the universe:
> "Since the Big Bang, the number of protons and neutrons all together never changes as a function of time. The reasons for this conservation are not well understood. It's one of the mysteries of the universe, related to why we have more matter than antimatter." — Nicole Lewis, Rice University
Baryon number conservation is one of the deepest physical explanations for 'why we exist.' If baryon number conservation comes from 'quark sharing,' then quark decays could quietly change baryon number; if it comes from 'the gluon junction cannot decay,' then matter's stability has a more fundamental guarantee.
Straightening the causal chain:
The latter picture explains why the proton's lifetime is thought to exceed the age of the universe: for a proton to decay, the gluon junction must first be destroyed—and the junction is a topologically protected object of QCD, more stable than quark flavor conservation.
Experimentally verifying that baryon number sits on the gluon junction amounts to finding a deeper source for matter's most fundamental 'indestructibility.'
Why RHIC Had to Retire: The 1,200-Ton Detector's Last Gift
This paper is one of RHIC's final gifts (turned on in 2000, decommissioned in early 2026). Over 25 years, RHIC recorded millions of gold-gold collisions, with STAR online throughout.
The team also used:
This 'distributed computing + large experimental facility' model distilled 25 years of data into four pages in *Science* on August 14.
What to Watch in the Next 6-12 Months
1. Replication and extension of the STAR paper: The EIC (Electron-Ion Collider, construction starting 2026, operation in the 2030s) will push 'post-collision gluon junction flow' measurements to higher precision. Will STAR publish independent verification in more collision systems (lead-lead, copper-copper)? 2. Textbook revisions: Which mainstream particle physics textbooks will update their 'baryon number carrier' chapters before 2027 H1? 3. Theoretical follow-up: Will Kharzeev's 30 years of baryon-junction work trigger a wave of theorists 'rewriting the QCD baryon number chapter'? 4. Cosmology link: If baryon number really sits on the gluon junction, the question 'why are there more gluon junctions than anti-junctions' enters the agenda—a new cosmological topic. 5. LHC comparison experiments: CERN's LHC also does heavy-ion collisions, but ATLAS/CMS focus on bulk quark-gluon plasma properties. Will this paper prompt LHC teams to design dedicated triggers probing 'gluon junction stopping probability'?