Disentangling baryon stopping and neutron skin effects in heavy-ion collisions

TL;DR

This work develops a thermal-hadronization framework to disentangle baryon stopping from neutron-skin effects in heavy-ion collisions. By introducing a centrality-dependent proton participant fraction and an excess baryon-stopping parameter $\\gamma_B$, the authors connect midrapidity charge and baryon yields within a constrained HRG description, enabling robust extraction of $\\gamma_B$ from isobar data and proposing an oxygen-baseline observable $r^{OX}$ to probe neutron-skin thickness via the robust ratio $\\mathcal{R}^{OX}$. The key finding is $\\gamma_B\\approx 1.61$ for Ru+Ru and Zr+Zr at $\\sqrt{s_{NN}}=200$ GeV, with neutron-skin effects driving centrality dependence and bias in naive estimates. The proposed $r^{OX}$ observable shows a near-linear sensitivity to $\\Delta R_{np}$ across RHIC and LHC energies, suggesting that heavy-ion measurements can constrain nuclear skin properties. Overall, the framework offers a controlled methodology to separate transport dynamics from initial-state structure, with implications for understanding baryon transport mechanisms and informing neutron-skin constraints from collider data.

Abstract

We analyse the net baryon (B) and net electric charge (Q) stopping in heavy-ion collisions using the statistical model. Focusing first on isobar collisions $\rm{Ru}+\rm{Ru}$ and $\rm{Zr}+\rm{Zr}$ at $\sqrt{s_{\rm NN}}=200$~GeV, we show that the excess baryon-stopping parameter $γ_B \equiv (N_B/N_Q)\,(Z/A)$ can be quantitatively extracted from forthcoming RHIC measurements of charge- and baryon-stopping ratios. We then generalize the approach to other collision systems at RHIC and LHC energies and introduce an oxygen-baseline observable, $r^{OX}$, whose central-to-peripheral ratio exhibits strong and systematic sensitivity to the neutron-skin thickness of the target nucleus $X$.

Figures (4)

• Figure 1: Participant proton fraction $p_{\rm frac}(c)$ obtained from the 3D Glauber model using the Woods--Saxon parameters in Table \ref{['table:WSParams']}, shown as a function of centrality and normalized by the global charge fraction $Z/A$. Panel (a): $p_{\rm frac}(c)/(Z/A)$ for $\rm O$, $\rm Cu$, $\rm Ru$, $\rm Zr$, $\rm Au$ and $\rm U$. Panel (b): the same quantity for Pb for two choices of neutron-skin thickness, $\Delta R_{\rm np}\simeq 0.17$ fm from ab initio calculations Hu:2021trw and $\Delta R_{\rm np}\simeq 0.28$ fm from PREX-II PREX:2021umo.
• Figure 2: Centrality dependence of the isobar ratio $r^{\rm Zr \rm Ru}(\gamma_B^{\rm opt})$ at $\sqrt{s_{\rm NN}}=200$ GeV. The three curves correspond to the implementations indicated in the legend: (i) Multiplicity scaling with the corrections in Eq. \ref{['eq:DeltaPhi']}; (ii) Multiplicity scaling without corrections ($\epsilon^{\rm Ru}=\beta^{\rm Zr\rm Ru} = 0$); and (iii) the density matched $\mathrm{d}V^{\rm Ru}/\mathrm{d}y=\mathrm{d}V^{\rm Zr}/\mathrm{d}y$, and net-baryon density $n_B^{\rm Ru}=n_B^{\rm Zr}$ by adjusting $\mu_B$. Black points (with the gray band) show the hydrodynamic proxy from Ref. Pihan:2024lxw. The dashed horizontal line at $r=1$ indicates the GCC expectation. The quoted values for $\gamma_B^{\rm opt}$ are the optimal values found from $\chi^2$ minimization in each case.
• Figure 3: Centrality dependence of the oxygen-based ratio $r^{OX}$, evaluated using Eq. \ref{['eq:ratio']} with multiplicity scaling. Panel (a): $X=\rm Cu,\rm Ru,\rm Zr,\rm Au,\rm U$ at RHIC. Panel (b): $X=\rm Pb$ at LHC for two benchmark neutron-skin thicknesses, $\Delta R_{\rm np}\simeq 0.17$ fm and $\Delta R_{\rm np}\simeq 0.28$ fm (Table \ref{['table:WSParams']}).
• Figure 4: Central-to-peripheral oxygen-based ratio ${\mathcal{R}^{OX}=r^{OX}(0\text{--}5\%)/r^{OX}(60\text{--}80\%)}$ as a function of neutron-skin thickness $\Delta R_{\rm np}$. Markers indicate literature values from Table \ref{['table:WSParams']}. Multiplicative factors are applied for visual clarity.