Proton Structure | Why 3-Quark Models Are Incomplete

Reevaluating the Internal Structure of Protons and Neutrons

Textbook descriptions of protons and neutrons as simple collections of three quarks are under fire. New findings from the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) suggest these particles are more accurately described as baryon junctions. This model moves the location of baryon number from the individual quarks to the Y-shaped gluon field that connects them. The distinction is not just theoretical. It changes how physicists model particle interactions in high-energy environments.

Since 1938, physicists have treated baryon number as a conserved quantity. The standard valence quark model, which gained traction after Murray Gell-Mann introduced the concept of quarks in 1963, assigns one-third of a baryon number to each constituent quark. While this model works for basic interactions, it struggles to explain behavior in extreme states like quark-gluon plasma. Dmitri Kharzeev proposed the alternative baryon junction model in 1996, suggesting that the Y-shaped gluon structure acts as the true anchor for baryon number. The recent data from Brookhaven National Laboratory provides strong evidence that this alternative picture matches experimental reality more closely.

Challenging the Valence Quark Model

High-energy accelerators usually rely on perturbative quantum chromodynamics (QCD) to model collisions. In these conditions, quarks behave almost like free particles, making the valence quark model and the junction model appear equivalent. However, inside the dense environment of a proton or during a heavy-ion collision, the interaction landscape changes. As Anselm Vossen of Duke University notes, lattice QCD calculations become necessary because every interaction diagram carries equal weight. This is where the standard model begins to fail.

The STAR collaboration observed this discrepancy by colliding gold nuclei at various energy levels. They found that the rate at which baryons scatter at large angles declines more slowly than the valence quark model predicts. Because a baryon junction is easier to stop during a collision than independent valence quarks, the results align better with Kharzeev’s 1996 proposal. The data effectively contradicts the idea that quarks remain the sole carriers of both charge and baryon number throughout the collision process.

Implications for Future Nuclear Physics

Further evidence emerged when the researchers compared collisions involving ruthenium-96 and zirconium-96. These nuclei share the same baryon count but possess different electric charges. According to the valence quark model, both properties should redistribute among fragments in similar ways. Instead, the team discovered that baryon number moved with far greater ease than electric charge. This decoupling reinforces the conclusion that quarks do not hold the full responsibility for both properties.

This study, published in the journal Science, confirms long-held suspicions within the nuclear physics community. Scientists have known for years that the proton is a complex entity, citing the famous proton spin crisis as a clear example that three valence quarks cannot account for all the particle's properties. While this research clarifies the nature of the baryon junction, the next steps will involve looking for more precise data. Researchers expect the future Electron-Ion Collider to provide even cleaner measurements of these internal structures. This shift in understanding forces a move away from simplified diagrams toward a more nuanced view of the strong nuclear force at work.