CPF Seminar with Rongrong Ma (BNL)

Baryon number is one of the best-tested conservation laws in nature. Yet a basic question about its microscopic carrier remains open. Conventionally, each valence quark carries one-third of a unit of baryon number. An alternative proposed in the 1970s assigns it instead to a baryon junction, a non-perturbative, Y-shaped configuration of gluons. For a baryon at rest the two pictures are almost indistinguishable. They differ in how baryon number is transported in high-energy collisions: fast valence quarks are hard to stop, but a junction built from soft gluons is easier to stop.

I will present three measurements from the STAR experiment at RHIC in identifying baryon number carrier: i) the rapidity dependence of net-protons in gamma+Au collisions, ii) the scaling of mid-rapidity net-proton yields across the Au+Au beam energy scan, and iii) a direct comparison of baryon and electric-charge transport in Ru+Ru and Zr+Zr isobar collisions. All three disfavor the valence-quark picture and are consistent with junction-based expectations. I will close with prospects for junction studies at the LHC and future EIC.

Bio:
Rongrong Ma is a tenured Physicist at Brookhaven National Laboratory. He received his B.S. from Peking University in 2008 and his Ph.D. from Yale University in 2013, where he measured jets in pp and Pb–Pb collisions with the ALICE experiment at the LHC. He joined BNL in 2014, first as a Goldhaber Distinguished Fellow. His research uses nuclear collisions to study QCD, spanning quarkonium suppression, jet quenching, heavy flavor and, most recently, baryon number transport. He served as Physics Analysis Coordinator of the STAR Collaboration from 2020 to 2023 and as convener of the Jets and Heavy Flavor working group for the ePIC experiment.