Lund University, Faculty of Science, Department of Physics
Abstract
In high energy heavy ion collisions, the QCD matter undergoes a phase transition to a hot and dense strongly coupled Quark Gluon Plasma, where quarks and gluons are deconfined in a volume of nuclear dimensions. At intermediate pT, 2 10 GeV/c), the particle production is dominated by jet fragmentation, where it is understood that these jets have suffered large energy losses propagating through the dense QGP. The goal of this thesis is to experimentally investigate the baryon-to-meson anomaly at intermediate pT, and to determine its origin, i.e. if it is an effect arising from the soft, collective, part (the bulk) of the medium, or from the hard processes (modified jet fragmentation). This will be done by analyzing central Pb–Pb collisions at sqrt(sNN) = 2.76 TeV from the ALICE experiment recorded in the 2011 heavy ion run period. For this analysis, a novel two-particle correlation technique called the η-reflection method is developed, where a separation can be made of the contributions from Λ and K0s particles produced in the soft underlying events from those which are produced in association with a high-pT trigger particle, representing a jet-like environment. The aim of this analysis is to separate the hadron production associated with the jet from that of the bulk, and to measure the Λ/K0s ratio at intermediate pT in the bulk and jet-like environment, to see how the baryon-to-meson anomaly differs in the two regions. The results show that at intermediate pT the hadron formation is dominated by the expanding and cooling Quark Gluon Plasma, giving rise to the anomalous overabundance of Λ over K0s which characterizes the inclusive results. The production of K0s and Λ associated with a high-pT trigger particle (presumably a leading hadron in a jet) is quite similar to that observed in pp collisions, i.e. unaffected by the processes in the dense, colored medium
