Measurements from the STAR Collaboration suggest that a defining quantum property of protons may reside in the gluons binding their quarks, rather than in the quarks themselves. The result, published on August 13 in Science and reported by Science News, found that particle patterns from high-energy nuclear collisions favor a neutral, Y-shaped gluon structure called a baryon junction as the carrier of baryon number. The researchers and independent physicists stressed that the evidence is important but not yet definitive.
Baryon number distinguishes protons and neutrons, which have a value of 1, from antibaryons, which have a value of minus 1. Its conservation across every observed reaction helps explain why protons do not simply decay and why ordinary atoms endure. The conventional account assigns one third of the number to each of a proton's three valence quarks. The competing proposal, developed decades ago, assigns the complete number to the junction formed by gluons, the carriers of the strong force that binds the quarks.
STAR tested the two pictures at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York. The collaboration analyzed about 2 billion ruthenium-96 collisions and 2 billion zirconium-96 collisions recorded in 2018 at 200 billion electron volts per nucleon pair. Both nuclei have mass number 96, but ruthenium has 44 protons and zirconium has 40. That difference allowed the team to compare the transport of baryon number with the transport of electric charge near the midpoint of the colliding beams under nearly identical detector conditions.
In the 10 percent most central collisions, the measured baryon-to-charge transport ratio, after scaling for the nuclear difference, was 1.84, with statistical, systematic and neutron-estimation uncertainties of 0.02, 0.09 and 0.16. A simple valence-quark picture predicts a value near 1, while the UrQMD transport model without baryon junctions produced values from 0.5 to 0.7. The STAR paper explains that low-momentum gluon junctions should stop near the collision midpoint more readily than valence quarks, which retain a larger share of the incoming proton's momentum.
The collaboration also used photon-gold interactions at a collision energy of 54.4 billion electron volts and earlier gold-gold data as independent checks. The net-proton distribution in photon-gold events had an exponential rapidity slope of 1.04 plus or minus 0.22, while gold-gold measurements across several energies averaged 0.64 plus or minus 0.05. Both are compatible with the junction framework's predicted range of 0.42 to 1, although the authors said present measurements cannot settle every dependence on collision conditions and alternative theories still require study.
Independent physicists cited by Science News offered measured assessments. Spencer Klein of Lawrence Berkeley National Laboratory said the work shows for the first time that experiments can distinguish the two pictures. Chun Shen of Wayne State University said the evidence strongly favors the junction interpretation but does not yet amount to a decisive demonstration, while Zi-Wei Lin of East Carolina University argued that the quark and junction descriptions may be compatible. STAR said future measurements at the planned Electron-Ion Collider at Brookhaven could directly map baryon-number distribution inside nuclei, extending the legacy of RHIC after its final collisions in February.
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