Rotating Planes, Fluctuating Shapes: Anisotropic Flow and Magnetic Fields in Heavy-Ion Collisions

Abstract

In this dissertation, we present a set of measurements of anisotropic flow in heavy-ion collisions, aimed at investigating the properties of hot and dense QCD matter. We analyse data collected by the ALICE experiment at the LHC between 2010 and 2015, colliding lead ions (208Pb) at a center-of-mass energy per nucleon of a few TeV. Anisotropic flow quantifies the degree of anisotropy in the azimuthal distribution of momenta of particles produced in the collision and provides information on collective dynamics of the system. In particular, we study how anisotropic flow, on average, depends on the collision energy of the ions, on how much they overlap, and on particle kinematic variables (e.g. velocity). We then analyse how anisotropic flow, in particular elliptic flow, changes between different collisions, which is mostly determined by the variations in the shape of the QCD system. We found all aforementioned results to be essentially compatible with expectations, thus confirming our current understanding of heavy-ion collisions. These results also constitute a large and comprehensive data set that will hopefully help to constrain even further a few important parameters in theoretical models, such as the specific shear viscosity and its temperature dependence. We then study the strong magnetic fields created in such collisions, which could provide information on so far unexplored features of QCD and on the electromagnetic response of the system. More precisely, we search for evidence of the Chiral Magnetic Effect, measuring again azimuthal distribution of particle momenta. We use simulations to understand the expected correlation between these magnetic fields and the shape of the QCD system. This helps us to decouple the contributions to anisotropic flow coming from magnetic fields and from other phenomena, unrelated to such fields. We conclude that no significant contribution of the Chiral Magnetic Effect is present in the azimuthal distribution of particle momenta and we quantify this statement with an upper limit. We also investigate another possible effect of the magnetic field, namely a difference in directed flow according to particle electric charge. Again, we find no significant evidence of such phenomenon, although results are suggestive; future work will provide a definitive answer. Finally, we conclude exploring the prospects for future measurements of anisotropic flow with the ALICE experiment. We also present the results of two exploratory studies and discuss their potential and limitations.