Relativistic heavy ion collisions produce nuclei-sized droplets of quark-gluon plasma whose expansion is well described by viscous hydrodynamic calculations. Over the past half decade, this formalism was also found to apply to smaller droplets closer to the size of individual nucleons, as produced in $p+p$ and $p+A$ collisions. The hydrodynamic paradigm was further tested with a variety of collision species, including $p+\mathrm{Au},d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$ producing droplets with different geometries. Nevertheless, questions remain regarding the importance of pre-hydrodynamic evolution and the exact medium properties during the hydrodynamic evolution phase, as well as the applicability of alternative theories that argue the agreement with hydrodynamics is accidental. In this work we explore options for new collision geometries including $p+\mathrm{O}$ and $\mathrm{O}+\mathrm{O}$ proposed for running at the Large Hadron Collider, as well as ${}^4\mathrm{He}+\mathrm{Au},\mathrm{C}+\mathrm{Au},\mathrm{O}+\mathrm{Au}$, and ${}^{7,9}\mathrm{Be}+\mathrm{Au}$ at the Relativistic Heavy Ion Collider.

• Received 31 December 2018

DOI:https://doi.org/10.1103/PhysRevC.99.044904