A thesis for the degree of Doctor of Philosophy defended August 2018.
The PhD School of Science, Faculty of Science, Discovery Center, Niels Bohr Institute, University of Copenhagen
Jens Jørgen Gaardhøje
Investigations on collectivity in small and large collision systems at the LHC with ALIC
Collisions of heavy ions at ultrarelativistic energies aim to recreate a hot and dense form of matter, called the Quark-Gluon Plasma (QGP), where quarks and gluons are in a deconfined state. One of the most suitable probes to study the properties of the QGP are the flow coefficients vn, which quantifies the anisotropic particle distributions in the final state. Experimental measurements of vn together with their good description by hydrodynamic calculations demonstrate that the QGP is an almost ideal fluid which undergoes a collective expansion into the surrounding vacuum. This collectivity exhibits itself in the form of long-range multi-particle correlations, which originate from a common source. Small collision systems, such as proton-proton and proton-lead, aim to provide the reference data for collisions of heavy nuclei. However, inspection of high multiplicity pp and p-Pb interactions revealed surprising features, usually attributed to collective effects in heavy-ion collisions. Thus, one of the main goals of this thesis is to understand these unexpected observations. In this work, measurements of flow coefficients and their correlations using the cumulant method are obtained as a function of multiplicity in pp collisions at 13 TeV, p-Pb collisions at 5.02 TeV, Xe-Xe collisions at 5.44 TeV, and finally Pb-Pb collisions at 5.02 TeV, from the LHC Run 2 data taking using the ALICE experiment. The ability of the novel subevent technique to suppress non-flow effects was examined with the experimental measurements and found to be crucial for the interpretation of the results in small collision systems. Exploration of the measurements presented in this thesis further implies that long-range multi-particle correlations prevail in small collision systems. Comparison of the pp data to PYTHIA 8 calculations shows that non-flow cannot fully explain the observed collectivity. Hydrodynamic calculations IP-Glasma+MUSIC+UrQMD could not reproduce the pp results either, while they provided a successful description of Pb-Pb, Xe-Xe and p-Pb collisions. The broad spectrum of measured observables from various collision systems and their comparison to theoretical models provide an exhaustive set of information, which sheds more insight into the mechanisms responsible for the collectivity seen in different collision systems.