# Exploring New Small System Geometries in Heavy Ion Collisions

## TL;DR

This paper investigates new small-system collision geometries in heavy ion experiments, aiming to understand quark-gluon plasma formation and the applicability of hydrodynamic models in these novel configurations.

## Contribution

It proposes and explores new collision geometries such as p+O, O+O, and light ion+heavy ion systems for heavy ion collision experiments.

## Key findings

- Assessment of potential collision geometries for future experiments.
- Discussion on the relevance of hydrodynamics in small systems.
- Identification of experimental setups to test medium properties.

## Abstract

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$$+$Au, $d$$+$Au, and $^{3}$He$+$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$$+$O and O$+$O proposed for running at the Large Hadron Collider, as well as, $^{4}$He$+$Au, C$+$Au, O$+$Au, and $^{7,9}$Be$+$Au at the Relativistic Heavy Ion Collider.