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

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.

💡 Research Summary

The authors address a long‑standing challenge in relativistic heavy‑ion physics: disentangling the contributions of preferential emission from participant nucleons and the breakup of spectator matter to the longitudinal (pseudorapidity) distribution of final‑state particles. While forward–backward multiplicity correlations, flow decorrelations, and baryon stopping have provided indirect evidence of these mechanisms, a direct, experimentally accessible observable that simultaneously probes both effects has been missing.

To fill this gap, the paper introduces a Pearson correlation coefficient, ρ(α_sp, α_ch, η), that links the event‑by‑event forward–backward asymmetry of the spectator system (α_sp) with the forward–backward asymmetry of charged particles measured in a narrow η window (α_ch, η). The spectator asymmetry α_sp is constructed from the energy deposited in the forward and backward Zero‑Degree Calorimeters (ZDCs), which primarily detect spectator neutrons. The charged‑particle asymmetry α_ch, η is defined as the difference of charged‑particle multiplicities in symmetric η intervals around ±η, normalized by their sum. Both quantities are further normalized to remove first‑order detector‑efficiency effects.

A key technical innovation is the introduction of a modified variance, V ar(α_ch, η) · δη, which compensates for the trivial scaling of statistical fluctuations with the η‑window width δη. This renders the correlation coefficient invariant under changes of δη, an essential property for practical measurements where the granularity of the detector may vary. The final observable is

ρ(α_sp, α_ch, η) = − Cov(α_sp, α_ch, η) /