Collective Phenomenon in Photonuclear and Other Small Collision Systems

In the ultrarelativistic collisions of heavy nuclei at the Large Hadron Collider, a large amount of energy and nuclear matter is deposited at the collision point. This forms a hot dense state, where quarks and gluons, usually bound in hadrons, are deconfined, transporting heat and momentum across relatively large distance scales. This strongly-coupled fluid hydrodynamically expands, cooling back into hadrons that enter particle detectors. This thesis presents experimental studies of the hydrodynamic behavior of the smallest droplets of the so-called quark-gluon plasma, with relativistic photon-nucleus collision data, recorded by the ATLAS detector. Also presented are studies of the energy deposition during the earliest times of the collision, through the analysis of proton-proton and xenon-xenon collisions.