Abstract
Baryon quantum number is found to be conserved since baryogenesis in the early Universe. Conventionally, each fractionally charged valence quark is understood to carry one-third of a baryon number. An alternative hypothesis posits that baryon number is instead carried by a baryon junction-a nonperturbative, Y-shaped gluonic configuration. Neither scenario has been verified experimentally. The STAR Collaboration reports measurements at mid-rapidity of baryon number (
) over the electric charge number difference (∆
) in isobar nuclear collisions, and the net-proton yield along rapidity in photonuclear collisions. A larger
/∆
ratio and less asymmetric net-proton yield are observed than predicted from models assigning baryon number to valence quarks. These findings, corroborated by previous measurements in Au+Au collisions, disfavor the valence quark picture.