Protons and neutrons are more than just groups of three quarks
Comparison between theoretical models and experimental data favours alternative “baryon junctions” picture The post Protons and neutrons are more than just groups of three quarks appeared first on Physics World .
Protons and neutrons are often viewed as collections of three quarks in textbooks, but physicists argue they should be considered as "baryon junctions" instead. This discrepancy arises from studies of heavy-ion collisions, which contradict the traditional textbook descriptions and validate the belief that these models are oversimplified.
Baryon number conservation, initially stated in 1938, predates the development of quantum chromodynamics (QCD) and the quark concept introduced by Murray Gell-Mann in 1963. The valence quark model, which assigns a baryon number of 1/3 to each quark and -1/3 to each antiquark, was a simple way to incorporate baryon number conservation in QCD theories.
However, this model is not fundamental to QCD, and alternative approaches have been considered since the 1970s. In 1996, Dmitri Kharzeev proposed the baryon junction model, suggesting that baryon number should be assigned to a Y-shaped junction in the gluon field that mediates quark interactions rather than to the quarks themselves.
At high-energy accelerators like the Large Hadron Collider (LHC), quark interactions can be treated as isolated particles, making the two models effectively equivalent. In contrast, within lattice QCD, the models' predictions can diverge. The STAR collaboration at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) investigated these differences by comparing predictions from the baryon junction model with those from the valence quark model in various scenarios and comparing them against experimental results.
They observed that the rate of scattering of net baryons decreased more slowly than the valence quark model would predict, suggesting that baryon junctions scatter more easily than valence quarks. Additionally, when comparing gold nucleus collisions with zirconium-96 and rubidium-96 nuclei, the researchers found that baryon number moved more easily than electric charge in collisions, indicating that quarks are not the carriers of both baryon number and electric charge.
The STAR collaboration hopes to study collisions from other experiments and anticipate that the Electron-Ion Collider, currently planned to replace RHIC, will provide more precise baryon probes and offer a definitive answer.
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