An international team led by the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) has made significant progress in understanding the fundamental properties of strongly interacting matter. Researchers from Central China Normal University (CCNU), together with Kent State University, Brookhaven National Laboratory and the University of Science and Technology of China, investigated a basic question in physics: whether the baryon number is carried by quarks or by gluons. Their paper, Tracking the Baryon Number with Nuclear Collisions, was published in Science on Aug. 14, 2026. Professor Luo Xiaofeng of CCNU’s School of Physical Science and Technology and his doctoral student Liu Hui, now a faculty member at Huzhou Normal University, were among the main contributors.
In the naive quark model, the baryon number of a proton or neutron is shared equally among its three valence quarks. Quantum chromodynamics, however, predicts complicated gluon fields inside the baryon: the three valence quarks may be linked by a Y-shaped gluon topology known as the “baryon junction.” A key hypothesis holds that it is this gluonic structure, rather than the valence quarks themselves, that carries and transports the baryon number.
The STAR Collaboration analyzed isobar collisions of ruthenium-96 and zirconium-96 at a center-of-mass energy of 200 GeV per nucleon pair. Because the two nuclei have the same mass number but different proton numbers, they share the same total baryon number while differing in charge — offering a unique way to compare charge and baryon-number transport. The experiment found that baryon-number transport at midrapidity is markedly stronger than the valence-quark transport inferred from charge transport, and cannot be explained by valence-quark “stopping” alone. Measurements of net-proton rapidity distributions in photon–gold nuclei interactions likewise agree with predictions incorporating the baryon-junction mechanism. The two independent observations mutually corroborate each other, providing important experimental evidence that the baryon number is mainly carried and transported by the Y-shaped gluon topology.
The CCNU team played a key role in reconstructing net baryon number. Since neutrons and antineutrons are electrically neutral and hard to detect directly, the researchers inferred their yields from the production of protons, antiprotons, deuterons and antideuterons. Under Luo’s supervision, Liu Hui completed systematic measurements of proton, deuteron and other light-nuclei production in 200 GeV ruthenium and zirconium collisions, providing essential experimental input for the reconstruction.
The study suggests that viewing a baryon merely as three valence quarks may be incomplete; the gluon fields connecting the quarks and their nonperturbative topology may help define and transmit the baryon's fundamental quantum numbers. The work sheds new light on the nonperturbative properties of QCD, the internal structure of hadrons and baryon-number transport in high-energy nuclear collisions. CCNU is a long-standing member of the STAR Collaboration, with its group working on the QCD phase structure, conserved-charge fluctuations, light nuclei and hypernuclei production, particle correlations and hadron interactions. The research was supported by the National Natural Science Foundation of China and the Ministry of Science and Technology.
Chinese