Investigations of $p$+Pb Collisions at Perturbative and Non-Perturbative QCD Scales

Abstract

High energy nuclear collisions manifest a variety of interesting phenomena over a broad range of energy scales. Many of these phenomena are related to the formation of a hot and dense state of deconfined quarks and gluons known as the quark gluon plasma (QGP). Chief among these are the observed near inviscid hydrodynamic expansion of the QGP, as measured through azimuthal anisotropy coefficients of low-$p_\mathrm{T}$ final state hadrons $v_\mathrm{n}$, and the loss of energy of high-$p_\mathrm{T}$ color charges as they traverse the QGP which is observed as a quenching of strongly-interacting final state objects like jets. Observations of these phenomena in Au+Au and Pb+Pb collisions at RHIC and the LHC provide compelling evidence of QGP formation. Small collision systems, like $p$+Pb, also show evidence for the creation droplets of QGP through the observation of anisotropic flow; however, these systems show no signs of the energy loss observed in large collision systems. Thus, small systems are an ideal venue to explore the relationship between high- and low-$p_\mathrm{T}$ QGP phenomena. Furthermore, the low ambient energy of $p$+Pb compared to A+A collisions allow for the precise determination of perturbative process rates which can be used to understand the nuclear modification of nucleon parton densities. This dissertation explores $p$+Pb data collected with the ATLAS detector at the LHC. The cross section and nuclear modification factor for prompt, isolated photons are measured and compared to predictions from perturbative QCD calculations and a model of initial state energy loss. Additionally, the charged hadron azimuthal anisotropy coefficients, are measured via two-particle correlations as a function of particle $p_\mathrm{T}$ and event centrality. Results are shown from minimum bias events and events selected because of the presence of a high-$p_\mathrm{T}$ jet.