Free spectator nucleons in ultracentral relativistic heavy-ion collisions as a probe of neutron skin

TL;DR

This paper proposes using the ratio of free spectator neutrons to protons in ultracentral heavy-ion collisions as a sensitive probe of neutron skin thickness and symmetry energy slope, supported by theoretical modeling and simulations.

Contribution

It introduces the $N_n/N_p$ ratio as a novel, more sensitive observable for neutron skin and symmetry energy, validated through Skyrme-Hartree-Fock-Bogolyubov and Glauber model calculations.

Findings

Larger neutron skin thickness increases $N_n/N_p$ ratio.

Double ratio cancels detection efficiency uncertainties.

Effects of deformation and electromagnetic excitation are subdominant.

Abstract

Besides the yield ratio of free spectator neutrons produced in ultracentral $^{96}$Zr+$^{96}$Zr to $^{96}$Ru+$^{96}$Ru collisions, we propose that the yield ratio $N_n/N_p$ of free spectator neutrons to protons in a single collision system at RHIC and LHC can be a more sensitive probe of the neutron-skin thickness $\Delta r_{\mathrm{np}}$ and the slope parameter $L$ of the symmetry energy. The idea is demonstrated based on the proton and neutron density distributions of colliding nuclei obtained from Skyrme-Hartree-Fock-Bogolyubov calculations, and a Glauber model that provides information of spectator matter. The final spectator particles are produced from direct emission, clusterization by a minimum spanning tree algorithm or a Wigner function approach, and deexcitation of heavy clusters by GEMINI. A larger $\Delta r_{\mathrm{np}}$ associated with a larger $L$ value increases the isospin asymmetry of spectator matter and thus leads to a larger $N_n/N_p$, especially in ultracentral collisions where the multiplicity of free nucleons are free from the uncertainties of cluster formation and deexcitation. We have further shown that the double ratio of $N_n/N_p$ in isobaric collision systems or in collisions by isotopes helps to cancel the detecting efficiency for protons. Effects from nuclear deformation and electromagnetic excitation are studied, and they are found to be subdominant compared to the expected sensitivity to $\Delta r_{\mathrm{np}}$.