Relativistic heavy ion collisions generate nuclear-sized droplets of quark-gluon plasma (QGP) that exhibit nearly inviscid hydrodynamic expansion. Smaller collision systems such as $p+\mathrm{Au}, d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$ at the BNL Relativistic Heavy Ion Collider, as well as $p+\mathrm{Pb}$ and high-multiplicity $p+p$ at the CERN Large Hadron Collider may create even smaller droplets of QGP. If so, the standard time evolution paradigm of heavy ion collisions may be extended to these smaller systems. These small systems present a unique opportunity to examine pre-hydrodynamic physics and extract properties of the QGP, such as the bulk viscosity, where the short lifetimes of the small droplets make them more sensitive to these contributions. Here, we focus on the influence of bulk viscosity, its temperature dependence, and the implications of negative pressure and potential cavitation effects on the dynamics in small and large systems using the publicly available hydrodynamic codes sonic and music. We also discuss pre-hydrodynamic physics in different frameworks including anti–de Sitter/conformal field theory strong coupling, ip-glasma weak coupling, and free streaming.

• Received 21 November 2019
• Accepted 10 March 2020

DOI:https://doi.org/10.1103/PhysRevC.101.044902