Tracking the baryon junction in heavy-ion collisions

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 (B) over the electric charge number difference (ΔQ) in isobar nuclear collisions, the net-proton yield along rapidity in photonuclear collisions and an exponential rapidity distribution of baryon transport. A larger B/ΔQ ratio and less asymmetric net-proton yield are observed than predicted from models assigning baryon number to valence quarks. These findings disfavor the valence quark picture. I will also discuss other measurements from EMC and LHC experiments with perspectives in future experiments.