Christian Bourjau

A thesis for the degree of Doctor of Philosophy defended June, 2018.

The PhD School of Science, Faculty of Science, Discovery Center, Niels Bohr Institute, University of Copenhagen

Supervisor:
Jens Jørgen Gaardhøje

Factorization of two-particle distributions measured in Pb-Pb collisions at √s = 5.02 TeV with the ALICE detector

The distribution of particles governed by the evolution and freeze-out of a Quark-Gluon Plasma (QGP) can be expressed as the product of single-particle distributions – a property referred to as factorization.

Based upon this factorization assumption flow describes the collective phenomena in the observed particles due to the initial state of the collision and the subsequent evolution of the QGP. The distribution of particles in a single event can always be expressed in a factorized form, but fluctuations from collision to collision may break the factorization assumption for multi-particle distributions averaged over many events. Measuring to which degree multi-particle distributions can be expressed as products of single-particle distributions provides insights into the fluctuations of the initial conditions of a collision as well as the amount of non-flow. This thesis presents a method to precisely measure two-particle distributions and to asses the agreement of the observed data with a factorization assumption with respect to pseudorapidity η. A minimal η-separation between two particles is identified beyond which non-flow effects become negligible. Decorrelation effects caused by fluctuations are quantified under the assumption that the decorrelation between two particles increases with increasing separation in η. The data is found to be in agreement with similar measurements from other collaborations and can be reproduced in simulations using the AMPT model.