Tracking the baryon number with nuclear collisions

Abstract

Baryon quantum number is believed to be conserved since baryogenesis in the early Universe. While fractionally charged valence quarks are understood conventionally to each carry a baryon number of 1/3, the baryon junction, a non-perturbative Y-shaped topology of neutral gluons, has also been proposed as an alternative entity tracing the baryon number. Neither scenario has been verified experimentally. The STAR Collaboration reports measurements at mid-rapidity of baryon number ($\boldsymbol{B}$) over the electric charge number difference ($\boldsymbol{\Delta Q}$) in isobar nuclear collisions, and the net-proton yield along rapidity in photonuclear collisions. A larger $\boldsymbol{B/\Delta Q}$ 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.