Tracking the baryon number with nuclear collisions

TL;DR

This paper reports experimental measurements of baryon number distribution in nuclear collisions, providing evidence against the valence quark model and supporting the baryon junction hypothesis.

Contribution

It presents the first experimental verification that challenges the valence quark assignment of baryon number, favoring the baryon junction model.

Findings

Higher B/ΔQ ratio observed than valence quark models predict

Net-proton yield is less asymmetric than expected from valence quark models

Results disfavor the valence quark picture of baryon number assignment

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.