Physicists Discover Hidden Gluon Structure Inside Protons

## A Hidden Feature Inside Protons Physicists have made a groundbreaking discovery that could rewrite the textbooks on the fundamental properties of matter. Using data from the Relativistic Heavy Ion Collider (RHIC), researchers have found evidence that gluons, the particles that hold quarks together, may play a key role in carrying and conserving baryon number. The study, published in *Science*, suggests that baryon number may be associated with a Y-shaped "junction" of gluons connecting the proton's three main quarks. This finding challenges the long-standing assumption that baryon number belongs exclusively to those quarks. ## Why Baryon Number Matters Determining what actually carries baryon number is crucial for understanding the stability of protons and ultimately, the universe. Baryon number conservation is a fundamental principle that has been observed since the Big Bang, and it's essential for explaining why we have more matter than antimatter. The conservation of baryon number also has a tangible consequence: it helps explain the extraordinary stability of protons. Protons are the building blocks of atomic nuclei, and their stability is essential for the existence of matter as we know it. ## A More Complicated Proton The possibility that gluons carry baryon number would overturn the standard simplified description found in many textbooks. In this picture, a proton has a baryon number of plus one, divided equally among its three main valence quarks. However, real protons are much more complicated than this simplified model suggests. Quantum chromodynamics (QCD), the theory used to describe the interactions among quarks and gluons, has been highly successful in explaining the strong force. However, models inspired by QCD often need additional assumptions to reproduce some of the particle patterns observed when RHIC smashes nuclei together at nearly the speed of light. ## An Unexpected Excess of Baryons One observation in particular caught the attention of the STAR team: the detector repeatedly records more baryons than antibaryons emerging sideways from the collisions, perpendicular to the direction of the incoming beams. This excess of baryons is not surprising, given that the collisions start with matter. However, the fact that the excess baryons appear in a specific direction is puzzling. The researchers suspected that there might be another explanation for the excess baryons, one that does not rely on the valence quarks carrying the baryon number. They found a way to investigate the mystery by taking advantage of another property of valence quarks: electric charge. ## Electric Charge Provides a Test The team compared the net baryon number measured in different RHIC nuclear collisions with the way electric charge was redistributed in those same events. The comparison revealed a striking mismatch: researchers observed roughly twice as many baryons as should have been produced based on the electric charge associated with stopped quarks. This left an important question: what was carrying the extra baryon number? The STAR physicists argue that gluons offer a possible answer, specifically the three-pronged gluon junction that connects the proton's valence quarks. ## How the Gluon Junction Could Carry Baryon Number The proposed mechanism depends on what happens when protons inside colliding nuclei reach enormous energies. According to the STAR team, the gluon junction or baryon junction that links the quarks may be much easier to stop in a collision than the three quarks themselves. If the junction is stopped, its energy can be converted into newly produced baryons that travel outward in directions perpendicular to the beams. Meanwhile, the valence quarks that were previously connected by the junction can continue moving forward along the beampipe. Understanding why requires looking at the changing internal structure of a proton as its energy increases. The baryon junction is always there even as protons are accelerated to higher and higher energy. However, at high energy, gluons within the proton split and multiply. As the number of gluons increases, the proton's momentum becomes spread among more of them. Each individual gluon, including those forming the junction, therefore carries a smaller portion of the proton's total momentum. The valence quarks, however, continue to carry much of the proton's forward motion.