Measuring deformed neutron skin with free spectator nucleons in relativistic heavy-ion collisions

TL;DR

This paper proposes a method to measure the angular distribution of neutron skins in deformed nuclei using free spectator nucleons in relativistic heavy-ion collisions, with potential application in RHIC experiments.

Contribution

It introduces a novel approach to extract the deformed neutron skin's angular distribution through spectator nucleon yield ratios in specific collision configurations.

Findings

Spectator neutron-to-proton yield ratios can indicate neutron skin deformation.

Probing neutron skin deformation is feasible in certain nuclei like $^{96}$Ru and $^{197}$Au.

Method is applicable with existing detector setups at RHIC.

Abstract

The neutron skin in deformed nuclei is generally not uniformly distributed but has an angular distribution, depending on both the spin-dependent nuclear interaction and the nuclear symmetry energy. To extract the information of the deformed neutron skin, we have explored the possibility of using free spectator nucleons in central tip-tip and body-body collisions at top RHIC energy with four typical deformed nuclei. The density distributions of neutrons and protons are consistently obtained from the Skyrme-Hartree-Fock-Bogolyubov calculation, and the angular distribution of the neutron skin can be varied by adjusting the strength of the nuclear spin-orbit coupling. With the information of spectator nucleons obtained based on a Monte-Carlo Glauber model, the free spectator nucleons are generated from a multifragmentation process. By investigating the results from different systems and with different collision configurations, we found that although it is difficult to probe the deformed neutron skin in $^{96}$Zr and $^{238}$U by their collisions, it is promising to extract the polar angular distributions of the neutron skin in $^{96}$Ru and $^{197}$Au by comparing the yield ratios of free spectator neutrons to protons in their central tip-tip and body-body collisions. The proposed observables can be measured by dedicated zero-degree calorimeters in heavy-ion collision experiments that have been carried out in recent years by RHIC.