Evidence of Hexadecapole Deformation in Uranium-238 at the Relativistic Heavy Ion Collider

TL;DR

This paper shows that including hexadecapole deformation of uranium-238 in hydrodynamic models resolves previous discrepancies with experimental data on particle flow at RHIC, highlighting the importance of detailed nuclear shape modeling.

Contribution

The study introduces a refined modeling approach that incorporates hexadecapole deformation of uranium-238, improving the accuracy of hydrodynamic simulations of heavy-ion collisions.

Findings

Correcting for hexadecapole deformation aligns simulations with RHIC data.

Neglecting nuclear deformation details causes discrepancies in flow predictions.

Inclusion of detailed nuclear shape effects enhances understanding of QGP behavior.

Abstract

There is strong evidence of the failure of hydrodynamic simulations of the quark-gluon plasma (QGP) to reproduce data on the elliptic flow of particles in relativistic collisions of $^{238}$U nuclei at the BNL Relativistic Heavy Ion Collider (RHIC). We demonstrate that this failure is caused by an inappropriate implementation of well-deformed ions, such as $^{238}$U, in the hydrodynamic framework. Past studies have identified the deformation of the nuclear surface with that of the nuclear volume, though these are different concepts. In particular, a volume quadrupole moment can be generated by both a surface hexadecapole and a surface quadrupole moment. This feature was so far neglected in the modeling of heavy-ion collisions, and is particularly relevant for nuclei like $^{238}$U, which is both quadrupole- and hexadecapole-deformed. With rigorous input from Skyrme density functional calculations, we show that correcting for such effects in the implementation of nuclear deformations in hydrodynamic simulations restores agreement with BNL RHIC data. This brings consistency to the results of nuclear experiments across energy scales, and demonstrates the impact of the hexadecapole deformation of $^{238}$U on high-energy collisions.