Background
On August 18, *Science* published a result from the STAR international collaboration — led by the University of Science and Technology of China (USTC), Kent State University, and Brookhaven National Laboratory — that may revise particle physics textbooks. Using the Relativistic Heavy Ion Collider (RHIC), the team precisely measured net baryon number and net charge in high-energy nuclear collisions, and found that nucleons must contain a basic structure that carries baryon number but carries neither electric charge nor color charge. This poses a significant challenge to the naive quark model.
How the experiment worked
STAR did not "look" directly inside the proton. Instead, RHIC was used as a particle-scale stress test platform:
- Several types of high-energy collision events were analyzed, including isobar nuclear collisions and photon-nuclear collisions.
- The post-collision spatial distributions of baryons and their corresponding antiparticles were systematically compared.
- These new results were cross-validated against previous gold-gold collision measurements.
- Gluons cannot simply connect the three valence quarks pairwise into a triangle.
- They must form a Y-shaped field structure, with the central confluence point being the baryon junction.
The decisive clue came from an unexpected "baryon excess": the STAR detector repeatedly observed large numbers of baryons being released along the direction transverse to the colliding beams.
Why the naive quark model fails
The naive quark model is simple: a proton consists of three valence quarks (uud), each carrying +1/3 baryon number and either +2/3 or -1/3 electric charge. Combined, the three quarks yield a baryon number of +1 and a charge of +1.
If baryon number truly belonged only to these three valence quarks, then the total number of slowed baryons in a collision would imply a fixed total number of slowed quarks, and therefore a predictable total charge. But the measured charge was only half of the prediction. This means that, in addition to quarks and gluons, the nucleon must contain a fundamental structure that carries baryon number but not electric charge — a feature that neither the valence quarks nor any single gluon in the traditional quark model possesses.
The Y-shaped baryon junction
The only picture that self-consistently describes both sets of experimental results is the Y-shaped gluon junction (baryon junction). This topological structure was theorized in the 1970s:
Why "baryon excess" is decisive
RHIC's isobar nuclear collisions and photon-nuclear collisions together cover different modes of nucleon stopping. If three valence quarks alone carry baryon number, the number of slowed baryons and the number of slowed charges should be in a fixed ratio (the B/Q ratio per quark is derivable from the quark model). However, the measured B/Q is far higher than the naive-model prediction — meaning that when a "baryon is slowed," something else is carried along with it, but that something does not contribute charge.
The geometry of the Y-shaped baryon junction fits this picture exactly: the gluon junction carries no charge, but can "repackage" a baryon by pulling three quarks from the vacuum — and the pulled quarks do not contribute charge in any systematic way.
Why baryon-number conservation deserves scrutiny
Baryon number (B) is one of the oldest and most stable conservation laws in particle physics. Since the Big Bang, the total baryon number in the visible universe has barely changed — we exist in a world of matter made of protons and neutrons precisely because this conservation law holds. However, "baryon-number conservation" is still only an experimental observation; it has not yet received a dynamical explanation comparable to that of charge conservation.
The Y-shaped baryon-junction picture relocates the conservation mechanism from "an internal property of quarks" to "a topological structure of gluons." If the experimental finding continues to be confirmed, the explanation of baryon-number conservation will move closer to QCD's non-Abelian symmetries rather than the statistical weight of quarks.
The culmination of Brookhaven's 25-year program
RHIC began operating in 2000 and will complete its final collisions in early 2026. STAR's new result is not an isolated highlight but the culmination of 25 years of accumulated experimental data at RHIC — gold-gold, copper-copper, uranium-uranium, isobar, and photon-nuclear collision modes all converge on a consistent picture. This cross-mode consistency provides a statistical strength that no single experiment could fabricate, and pulls the long-standing open question "Is the naive quark model sufficient?" close to an answer for the first time.
What changes now
STAR's evidence for the Y-shaped gluon junction opens a new window in particle physics: baryon-number conservation may not be a byproduct of quark statistics, but a geometric necessity of gluon topology. This means future heavy-ion collision experiments, the Electron-Ion Collider (EIC), and even high-energy cosmic-ray observations will all need to re-examine the question "Where does baryon number come from?"
The experiment, led by the USTC team of Tang Zebo, has moved Chinese fundamental-physics research in core QCD questions from "important participant" to "leading player" — a set of numbers worth remembering in RHIC's 25-year finale.