Physicists Discover Hidden Gluon Structure in Protons

Physicists have uncovered a hidden feature inside protons that may help preserve one of matter’s fundamental properties. Data from the Relativistic Heavy Ion Collider (RHIC) suggest that baryon number is carried by a Y-shaped junction of gluons. This challenges the long-held assumption that baryon number is solely carried by the three quarks within a proton. The finding could deepen understanding of proton stability and the existence of matter.

This evidence comes from high-energy particle collisions at RHIC, a U.S. Department of Energy (DOE) Office of Science user facility. The study, published in *Science*, indicates that the gluon junction, rather than the valence quarks, might be responsible for baryon number. Traditionally, scientists assumed each of the three main valence quarks inside a proton or neutron carried one-third of the baryon number. This new research suggests a more complex mechanism.

The concept of a baryon junction, also known as a gluon junction, was first proposed in the 1970s. It described how gluons connect the valence quarks within a proton. In 1996, theoretical physicist Dmitri Kharzeev suggested this junction might carry the baryon number itself. The STAR collaboration at RHIC developed a method to test this possibility using various collision types. The results support the idea that baryon number is carried and transported by gluons when arranged in this specific configuration.

Understanding what carries baryon number extends beyond a proton's internal structure. In RHIC collisions, baryon number conservation means the total number of baryons remains constant before and after impact. This principle also applies to the universe at large. The reasons for this conservation are not fully understood, and it relates to the imbalance of matter over antimatter. Baryon number conservation also explains the extraordinary stability of protons, which form atomic nuclei and do not decay under normal conditions.

Protons are more complex than simplified models suggest. The naive quark model depicts three quarks inside a proton. However, real protons contain numerous gluons interacting and connecting these quarks. They also contain quarks and antiquarks that emerge from the vacuum. Quantum chromodynamics (QCD), the theory describing these interactions, has successfully explained the strong force. However, QCD-inspired models often require additional assumptions to reproduce observed particle patterns from RHIC collisions.

One observation that caught the STAR team's attention was an excess of baryons over antibaryons emerging sideways from collisions. If valence quarks alone carried baryon number, explaining this excess would require all three valence quarks from one proton to stop near the detector's center. They would then convert from matter to energy and back, producing new baryons moving perpendicular to the beam. The STAR physicists propose that the gluon junction offers an alternative explanation. The three-pronged gluon junction, which links the quarks, may be easier to stop in a collision than the quarks themselves. If the junction stops, its energy can convert into new baryons that travel outward. The valence quarks connected by the junction can continue moving forward along the beam pipe.