Novel Probes of Quark-Gluon Plasma Evolution in Heavy-Ion Collisions With the ATLAS Detector: From Structure of Colliding Nuclei to Collective Expansion of Medium

Understanding the properties of the quark-gluon plasma (QGP) offers insights into the strong interaction and the conditions of the early universe.Since the QGP cannot be observed directly, its properties must be inferred from the particles emitted as it cools. This dissertation introduces a suite of novel probes based on event-by-event multi-particle correlations allowing us to work backward from final-state particles to constrain the collective evolution, transport properties, and initial conditions of the QGP. The studies utilize $^{208}\mathrm{Pb}+{}^{208}\mathrm{Pb}$ and $^{129}\mathrm{Xe}+{}^{129}\mathrm{Xe}$ collisions recorded by ATLAS detector at the LHC. First, evidence for the collective nature of the QGP's radial expansion is provided using a transverse momentum ($p_{\mathrm{T}}$)-differential observable, $v_0(p_{\mathrm{T}})$. This observable exhibits genuine long-range correlations and factorizes into a single-particle property. Its strong sensitivity to bulk viscosity providing more direct constraints on this medium property than traditional measures. Second, the sources of event-wise fluctuations in radial flow within the initial state are disentangled by analyzing higher moments of the event-wise mean-$p_{\mathrm{T}}$ distribution. The average of this distribution is shown to constrain the speed of sound in the QGP. Finally, nuclear deformations, which control overlap area geometry, are constrained by comparing correlations between the anisotropic flow, $v_{n}$ and mean-$p_{\mathrm{T}}$ in spherical Pb+Pb and deformed Xe+Xe collisions. This comparison provides the first experimental evidence for triaxial deformation in $^{129}$Xe. Together, these three complementary analyses significantly improve our understanding of the initial conditions, transport properties, and collective behavior of the QGP, establishing new avenues for precision studies of QGP.