What did the STAR Collaboration at RHIC discover about how gluons contribute to the proton's baryon number, and how does this finding relate to the broader understanding of gluonic matter?

New results from the STAR Collaboration at RHIC, published in Science, provide the first experimental evidence that the proton's baryon number is carried not by its three valence quarks, but by a Y-shaped "gluon junction" — a non-perturbative topological configuration of the gluon field .

• Three independent measurements — isobar collisions (Ru+Ru vs. Zr+Zr), photon-gold collisions, and net-proton yields in Au+Au collisions — all showed significantly more baryon transport to mid-rapidity than predicted if valence quarks carried the baryon number individually .
• The data directly disfavor the traditional picture where each valence quark carries B=1/3, and instead support the "baryon junction" hypothesis proposed in the 1970s, in which the gluonic junction itself carries the full baryon number (B=1) and zero electric charge .
• In collisions, this gluon junction is stopped much more easily than the high-momentum valence quarks; its energy is converted into new baryons that spray out perpendicular to the beamline, while the quarks themselves continue forward .