Characterization of Photonuclear Collisions and the Search for Quark-Gluon Plasma

Ultra-relativistic heavy-ion collisions create tiny droplets of Quark-Gluon Plasma (QGP), a hot, dense state of matter where quarks and gluons are in a deconfined state. Experiments at the CERN Large Hadron Collider have observed the presence of this strongly interacting matter in Pb+Pb collisions and, interestingly, in small collision systems like $\it{p}$+Pb and $\it{pp}$. This raised many important questions in the field, particularly: What is the smallest system that is necessary to create a tiny droplet of QGP? Photonuclear collisions, a subset of Ultra Peripheral Collisions (UPCs) , are one such exotic small system and occur when the electromagnetic field of one nucleus acts as a photon and breaks apart the other nucleus. The ATLAS Collaboration has recently observed flow-like features, a QGP signature suggesting the collective expansion of the fluid in photonuclear collisions. Thus, it is imperative to check these events for other potential QGP signatures, including radial flow, strangeness enhancement, and enhanced production of baryons relative to mesons. Furthermore, modeling photonuclear collisions is particularly challenging due to their extremely asymmetric nature and the fluctuating photon energies between events. This dissertation presents two new measurements of photonuclear collisions using 5.02 TeV Pb+Pb data collected in 2018 by ATLAS, with a dedicated photo-nuclear event trigger. The first measurement reports unidentified-charged-hadron yields, and the second measurement provides the identified-hadron yields, such as the $K^0_S$, $\Lambda$, and $\Xi^-$, in Pb+Pb UPCs. The yields are shown as a function of pseudorapidity and transverse momentum in different categories of event multiplicity. The results are compared with 5.02 TeV $\it{p}$+Pb data collected in 2016 by ATLAS at the same event multiplicities. Additionally, they are compared with calculations by the DPMJET Monte Carlo model and hydrodynamic- based models. These comparison enable detailed characterizations of photonuclear collision properties and evaluate the potential formation of small QGP droplets.