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

State-of-the-art hydrodynamic simulations of the quark-gluon plasma are unable to reproduce the elliptic flow of particles observed at the BNL Relativistic Heavy Ion Collider (RHIC) in relativistic 238U thorn 238U collisions when they rely on information obtained from low-energy experiments for the implementation of deformation in the colliding 238U ions. We show that this is due to an inappropriate treatment of well -deformed nuclei in the modeling of the initial conditions of the quark-gluon plasma. 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 238U, which is both quadrupole deformed 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 238U on high-energy collisions.

Automated summary

State-of-the-art hydrodynamic simulations cannot reproduce the observed elliptic flow of particles at RHIC due to inappropriate treatment of nuclear deformations in the modeling of quark-gluon plasma initial conditions. Past studies neglected the relationship between hexadecapole deformation and surface quadrupole moment deformation. Correcting for this effect restores agreement with RHIC data.