Physicists from the STAR collaboration at the RHIC accelerator at Brookhaven National Laboratory have presented data that challenge the conventional understanding of proton structure. According to new results published in Science, baryon number—a fundamental property determining matter's stability—may be carried not only by three valence quarks but also by a Y-shaped configuration of gluons connecting them.
Traditionally, it was believed that each of the three quarks within a proton carried one-third of the baryon number. However, an analysis of nuclear collisions at RHIC revealed that this simplified view does not account for the observed excess of baryons emitted perpendicular to the beam direction.
Researchers compared the distribution of baryon number with the redistribution of electric charge in the same events. They found that approximately twice as many baryons were produced as would be expected from only the stopped quarks. This discrepancy points to an additional mechanism for baryon number transport.
Scientists attribute this to a 'baryon junction'—a gluon structure first proposed in the 1970s. In 1996, Dmitri Kharzeev suggested that this Y-shaped configuration, rather than the quarks themselves, might be responsible for baryon number conservation. The new STAR data support this hypothesis.
In high-energy collisions, the gluon junction is more easily stopped than valence quarks. Once stopped, it 'pulls' three new quarks from the vacuum to form a new baryon, while the original quarks continue moving along the beam direction. This mechanism better explains the excess of baryons in the mid-rapidity region of the detector.
Baryon number conservation is one reason why protons remain stable for periods exceeding the age of the universe. It is also linked to the asymmetry between matter and antimatter in the cosmos. If gluons truly play a key role in this process, then the understanding of the proton as a simple object composed of three quarks requires revision.
“Our results challenge the long-held idea that baryon number is simply shared among three quarks,” notes physicist Rongrong Ma from Brookhaven. Further research will reveal exactly how this discovery will impact quantum chromodynamics models and our understanding of the early universe.
