Understanding baryon stopping at the BNL Relativistic Heavy Ion Collider top energies

TL;DR

The paper addresses how baryon number is transported in high-energy nuclear collisions, testing whether valence-quark stopping or a gluon-baryon junction dominates by using the PYTHIA-8 event generator with Angantyr for heavy ions. It systematically compares three CF/CR configurations—CF-1/CR-1, CF-1/CR-2, and CF-2/CR-3—to study their impact on baryon stopping in γ+p and isobar collisions, incorporating $dN/dy$ and the isobar ratio $R({ m Isobar})$ (defined via net-baryon and net-charge observables). The results show that γ+p observables exhibit only weak sensitivity to CF/CR details, while isobar data are more discriminating: schemes with junctions (CF-2/CR-3) predict enhanced mid-rapidity baryon transport and $R({ m Isobar})>1$, closer to STAR findings, whereas the default CF-1/CR-1 underpredicts transport. The study emphasizes that $R({ m Isobar})$ alone cannot fully distinguish between CF models and advocates using the net-baryon rapidity distribution as a complementary observable to constrain baryon transport mechanisms in high-energy collisions.

Abstract

The nucleon exhibits a rich internal structure governed by Quantum Chromodynamics (QCD), where its electric charge arises from valence quarks, while its spin and mass emerge from complex interactions among valence quarks, sea (anti-)quarks, and gluons. At the advent of QCD, an alternative hypothesis emerged suggesting, at high energies, the transport of a nucleon's baryon number could be traced by a non-perturbative configuration of gluon fields connecting its three valence quarks, forming a $Y$-shaped topology known as the gluon junction. Recent measurements by the STAR experiment are compatible with this scenario. In light of these measurements, this study aims to explore the mechanisms of baryon transport in high-energy nuclear collisions using the PYTHIA-8 framework, which incorporates a state-of-the-art hadronization model with advanced Color Flow (CF) and Color Reconnection (CR) mechanisms that mimic signatures of a baryon junction. Within this model setup, we investigate (i) the rapidity slope of the net-baryon distributions in photon-included processes ($γ$+p) and (ii) baryon over charge transport in the isobaric (Ru+Ru and Zr+Zr) collisions. Our study highlights the importance of the CF and CR mechanisms in PYTHIA-8, which play a crucial role in baryon transport. The results show that the CF and CR schemes significantly affect the isobaric baryon-to-charge ratio, leading to different predictions for baryon stopping and underscoring the need to account for CF and CR effects in comparisons with experimental measurements.

Figures (6)

• Figure 1: Schematic illustration of two types of baryon number carriers: panel (a) valence quarks and panel (b) baryon junction.
• Figure 2: Schematic illustration of the baryon number carriers and their transport mechanisms based on STAR:2024lvy. On the left: a depiction of the case when valence quarks carry the baryon number. On the right is a representation of the case when the baryon junction carried the baryon number. Unstopped valence quarks (solid spheres) can form mesons near beam rapidity. Open spheres represent produced quarks and anti-quarks.
• Figure 3: The net-baryon rapidity distributions for $\gamma_{\rm e}$+p collisions, corresponding to the different PYTHIA-8 modes, are shown. The lines represent exponential fits to the PYTHIA-8 calculations in the rapidity range $y - Y_{\text{Beam}} = -2.5$ to $-5.0$.
• Figure 4: The R(Isobar) as a function of $N_{\rm part}$ at 200 GeV from the PYTHIA-8 model with (i) $CF-1$$CR$-Off ($CR-0$) and (ii) $CF-1~CR-1$ schemes.
• Figure 5: The same as Fig. \ref{['fig:fig3']} but for different PYTHIA-8 schemes given in Sec. \ref{['sec:PYTHIA']}.