Hydrodynamic Simulations of Au-Au Collisions from the STAR Experiment

High energy nuclear physics is an experimental branch of physics whose subject matter has rapidly evolved in recent years. This is due to the emergence and development of particle accelerators like RHIC (Relativistic Heavy Ion Collider) at Brookhaven National Laboratory, which produce high energy collisions between nuclei in order to probe the substructure of nuclear matter and study emergent properties of nuclear systems at high temperatures. In particular, hadrons like protons and neutrons are actually made up of a dynamical system of quarks, elementary particles that carry color charge, and gluons, elementary particles that propagate (and carry) color charge. Collisions of heavy ions like gold or lead can cause the participating nucleons to undergo a phase transition, or "melt", into an exotic phase of matter, the quark gluon plasma (QGP). Properties of the QGP can be extracted by simulating the QGP phase of the collision as a fluid undergoing hydrodynamic expansion. By optimizing the best-fit parameters for a simulation using machine learning to reproduce experimental data, the viscosity and other characteristics of a fluid can be determined. In addition, the model can be validated by comparing the thermodynamic properties from fluid calculations to lattice quantum chromodynamic (QCD) simulations. This process has been implemented for high energy (5.02 TeV) Pb-Pb collisions [1]. Applying this process to experimental data [2] from Au-Au collisions at a range of much lower energies (7.7, 11.5, 14.5, 19.6, 27.0, 39.0, and 62.4 GeV) can push the limits of a hydrodynamic model and reveal possible changes of fluid characteristics as the energy dips below the threshold for QGP formation. Preliminary results for the 39.0 GeV and 62.4 GeV runs have been calculated. Future work involves analysis and comparison across the rest of the energies.

[1] Nature Physics 16, 615-619 (2020)

[2] Phys. Rev. Lett. 121, 032301 (2018)