Tracking the baryon number with nuclear collisions.

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 (<i>B</i>) over the electric charge number difference (∆<i>Q</i>) in isobar nuclear collisions, and the net-proton yield along rapidity in photonuclear collisions. A larger <i>B</i>/∆<i>Q</i> 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.