A Method to Constrain Preferential Emission and Spectator Dynamics in Heavy-Ion Collisions

TL;DR

This work addresses how longitudinal particle production in heavy-ion collisions arises from competing mechanisms: preferential emission by participating nucleons and spectator fragmentation. It proposes a data-driven observable, the Pearson correlation $\rho(\alpha_{\mathrm{sp}},\alpha_{\mathrm{ch},\eta})$, linking spectator asymmetry to the pseudorapidity-odd component of final-state production, with a modified variance $\mathrm{Var}(\alpha_{\mathrm{ch},\eta})_{\mathrm{mod}}$ to ensure independence from bin width $\delta\eta$. Using AMPT simulations of Au+Au at $\sqrt{s_{NN}}=200$ GeV, the study validates that the correlator captures the imprint of preferential emission and is sensitive to spectator fragmentation, exhibiting a centrality-dependent suppression when fragmentation is included. The results motivate experimental measurements at RHIC and LHC to constrain spectator dynamics and to refine understanding of the three-dimensional initial state and its translation into longitudinal final-state distributions, potentially extending to identified hadrons and different energies.

Abstract

Longitudinal particle production in heavy-ion collisions is influenced both by preferential emission from participating nucleons and by the breakup of spectator matter, yet quantifying these effects experimentally remains challenging. We introduce a Pearson correlation between spectator and charged-particle forward-backward asymmetries as an experimental probe of these phenomena. Using Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV simulated with A Multi-Phase Transport (AMPT) model, we validate that this correlator provides a robust, pseudorapidity-differential measure of the influence of preferential emission on the longitudinal structure of particle production. We further demonstrate that the correlation strength is sensitive to fluctuations in spectator number, which in experiments arise from evaporation and fragmentation of the spectator remnants. The proposed observable therefore offers a data-driven handle for constraining models of preferential emission and spectator breakup, thereby improving our understanding of the mechanisms that shape the final-state longitudinal distributions in heavy-ion collisions.

Figures (9)

• Figure 1: (a) Distribution of $a_{\mathrm{sp}}$ in 10-20% central Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}=200$ GeV. Regions 1, 2, and 3 denote $a_{\mathrm{sp}}\ll 0$, $a_{\mathrm{sp}}\approx 0$, and $a_{\mathrm{sp}}\gg 0$, respectively. (b) Comparison of the $P(\eta)$ for events in each region in panel a. (c) Ratios of the $P(\eta)$ for events with asymmetric participant configurations (Regions 1 and 3) to those with symmetric configurations (Region 2).
• Figure 2: The correlation coefficient $\rho(\alpha_{\mathrm{sp}},\alpha_{\mathrm{ch},\eta})$ for $\delta\eta=0.2$ in 10--20% central Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}=200$ GeV from the AMPT model, exhibiting an odd dependence on $\eta$ characteristic of preferential emission.
• Figure 3: (a) $\mathrm{Var}(\alpha_{\mathrm{ch},\eta})_{\mathrm{mod}}$, (b) $-\mathrm{Cov}(\alpha_\mathrm{sp},\alpha_{\mathrm{ch},\eta})$, and (c) $\rho(\alpha_\mathrm{sp},\alpha_{\mathrm{ch},\eta})$ as a function of $\eta$ for different bin-widths $\delta\eta$ in 10-20% central Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 200$ GeV from the AMPT model. The quantities are arbitrarily scaled for better visibility.
• Figure 4: The components of the correlator for different centrality intervals in Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}=200$ GeV from the AMPT model: (a) modified variance of charged-particle asymmetry, $\mathrm{Var}(\alpha_{\mathrm{ch},\eta})_{\mathrm{mod}}$. (b) covariance, $-\,\mathrm{Cov}(\alpha_{\mathrm{sp}},\alpha_{\mathrm{ch},\eta})$. (c) correlation coefficient, $\rho(\alpha_{\mathrm{sp}},\alpha_{\mathrm{ch},\eta})$. The horizontal lines in (a) show $\mathrm{Var}(\alpha_{\mathrm{sp}})$ for each centrality.
• Figure 5: (a) The number of free-neutron spectators ($N_{\mathrm{spec}}^{fn}$) versus total participants ($\hbox{$N_{\mathrm{part}}$}$) for Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 200$ GeV, simulated using the mean and sigma from the PHENIX measurements Tarafdar:2014ouaPHENIX:2004vdg. (b) The estimated correlation between forward-going free neutrons ($N_{\mathrm{spec,F}}^{fn}$) and forward-going participants ($N_{\mathrm{part,F}}$), which is used to model spectator fragmentation.