Collision geometry effect on free spectator nucleons in relativistic heavy-ion collisions

TL;DR

This study investigates how nuclear deformation and collision geometry influence free spectator nucleon yields and neutron-to-proton ratios in relativistic heavy-ion collisions, highlighting the potential to probe neutron skin properties.

Contribution

It introduces a detailed analysis of the impact of nuclear deformation and collision orientation on spectator nucleon yields and neutron skin effects at RHIC energies.

Findings

Tip-tip collisions produce fewer spectator nucleons.

Neutron-to-proton ratio is sensitive to neutron skin thickness and symmetry energy.

Collision geometry effects can rival symmetry energy effects in magnitude.

Abstract

Based on the deformed nucleon distributions obtained from the constrained Skyrme-Hartree-Fock-Bogolyubov calculation using different nuclear symmetry energies, we have investigated the effects of the neutron skin and the collision geometry on the yield of free spectator nucleons as well as the yield ratio $N_n/N_p$ of free spectator neutrons to protons in collisions of deformed nuclei at RHIC energies. We found that tip-tip (body-body) collisions with prolate (oblate) nuclei lead to fewest free spectator nucleons, compared to other collision configurations. While the $N_n/N_p$ ratio is sensitive to the average neutron-skin thickness of colliding nuclei and the symmetry energy, it is affected by the polar angular distribution of the neutron skin in different collision configurations. We also found that the collision geometry effect can be as large as 50% the symmetry energy effect in some collision systems. Due to the particular deformed neutron skin in $^{238}$U and $^{96}$Zr, the symmetry energy effect on the $N_n/N_p$ ratio is enhanced in tip-tip $^{238}$U+$^{238}$U collisions and body-body $^{96}$Zr+$^{96}$Zr collisions compared to other collision orientations in the same collision system. Our study may shed light on probing deformed neutron skin by selecting desired configurations in high-energy collisions with deformed nuclei.