Protons May Not Be Just Three Quarks: STAR's Final RHIC Collisions Hint at a Gluon Y-Junction Carrying Baryon Number

STAR's Final Collision Results at RHIC

On August 17, the STAR detector at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) released preliminary analyses from the final round of collision data in its 25-year run. The headline finding is potentially revolutionary: the proton's classic property of baryon number +1 may not be distributed evenly among its three valence quarks — a portion may be hidden in a Y-shaped structure made of gluons.

From the Naive Quark Model to a Boiling Sea

The textbook picture, established in the 1970s as the "naive quark model," holds that the proton consists of two up quarks and one down quark bound by gluons, with each quark carrying +1/3 baryon number. This model has been extremely successful in explaining hadron spectra, deep inelastic scattering, and jet physics.

But STAR's research points out that the proton contains far more than three valence quarks: vast numbers of gluons interact between quarks, and vacuum fluctuations continuously spawn quark-antiquark pairs. The proton is actually a boiling ocean of quarks, gluons, and antiquarks — and where baryon number "lives" within it had never been directly measured.

The Measurement and the Y-Junction

Using polarized proton-proton collisions at RHIC, STAR analyzed the angular momentum distributions and spin correlations of final-state particles, separating the quark and gluon contributions to baryon number. The results show:

• The gluon contribution to baryon number is nonzero.
• The gluon distribution forms a Y-shaped structure — not a uniform Λ-shaped (pairwise) connection, but a Y-shaped junction linking all three quarks, with a nontrivial carrier structure for baryon number along this junction.

The significance lies not in the precise percentage (still under analysis), but in providing the first experimentally distinguishable alternative narrative. A "Y-junction" or "three-gluon junction" model proposed in the 1970s predicted exactly this structure as a geometric origin of color confinement: since the color field is shared through a Y-shaped node, breaking it requires all three quarks to be reconnected simultaneously. Dormant for half a century due to lack of direct observation, the theory has now moved from "theoretical toy" to "physically real with possible data support."

> "In the naive quark model, there are only three quarks inside the proton and nothing else. But if we look at the internal details, we see far more than three quarks: lots of gluons interacting between them, and quarks and antiquarks popping out of the vacuum — it's actually a very complex object." > — Tommy Tsang, Argonne National Laboratory

Implications for Physics

1. Proton spin crisis: Quark spins account for only part of the proton's total spin; if baryon number also resides in a gluonic Y-structure, gluons play a more active role than assumed, redefining their weight in QCD dynamics. 2. Collider phenomena: Strangeness enhancement and baryon-antibaryon asymmetries, hard to fully explain with the naive model, may require topological contributions from the Y-junction. 3. Astrophysics and cosmology: Cosmic-ray origins, QCD matter inside neutron stars, and quark-gluon plasma evolution in the early universe may all need to incorporate the Y-junction as a physical variable.

Outlook

STAR's final collisions close RHIC's 25-year story, but the question it raised — "where does baryon number actually live?" — ensures RHIC's scientific legacy endures. Future facilities such as the Electron-Ion Collider (EIC) and FAIR at GSI are expected to make the Y-junction a key measurement target, while lattice QCD teams begin new methodological efforts to identify Y-junctions on the lattice. The proton is not simply three quarks — the frontier is shifting back toward a balanced picture in which gluons are just as essential.