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Probing the early-time dynamics of heavy-ion collisions

Garcia Montero, Oscar Jesus

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The objective of this thesis is to contribute to the understanding of the phenomenology of the early stages of Heavy Ion Collision (HIC) experiments. In such settings, it is assumed that two nuclei collide at energies high enough to compress the nucleons into a volume of deconfined quarks and gluons. This \textit{fireball} rapidly thermalizes and expands against the surrounding vacuum. Nevertheless, the question of when and how the fireball thermalizes has not been completely answered, even though wide progress has been done. This work aims to tackle two small pieces of the overarching puzzle. How can we use photons radiated from the medium to probe the initial and pre-equilibrium stages? And how do adding conserved charges, given at initial time of the collision, change the path to thermal equilibrium?

The first question is addressed in three different avenues. We compute the change of the spectrum of photons produced at initial time, by understanding how quarks multiply scatter from a dense hadron, which is taken as a shockwave of gluons. For this, we use the framework of the Color Glass Condensate Effective field Theory. In this work, we present a comparison with proton-proton collision data and predictions for the new run at the Large Hadron Collider (LHC), at center of mass energies of $\sqrt{s} =13\TeV$.

For the case of collisions with heavy nuclei, a medium of quarks and gluons is created, and photons are radiated throughout the collision. We compute the photon spectrum for the case of the \textit{bottom-up} scenario, which parametrically describes the thermalization of the quark-gluon-plasma. Building on recent developments, we find the non-equilibrium case dominates. We compare it with an early hydrodynamical scenario, where a thermalized quark gluon plasma is assumed to settle at the same initial time as the pre-equilibrium case.

However, the yields cannot be dissected experimentally, and we need an extra observable to distinguish if the photons measured come from earlier or later times. For this, we will use interferometry of photons, in particular Hanbury-Brown-Twiss (HBT) correlations. We compute the associated \textit{HBT radii}, which give information on the spatial extent of the sources, and use them to propose a qualitative method to discriminate between scenarios with and without pre-equilibrium photons.

The second question we ask is motivated by the Chiral Vortical Effect (CVE), and the discovery of the imbalance of $\Lambda/ \bar{\Lambda}$ hyperons in STAR at the Relativistic Heavy Ion Collider (RHIC). We aim at understanding the role of angular momentum in the collision, and the road of the gluon medium to thermalization. To kickstart this objective, we use a 2D scalar toy model to explore non-vanishing angular momentum in an overoccupied Quantum Field Theory.

Item Type: Dissertation
Supervisor: Berges, Prof. Dr. Jürgen
Place of Publication: Heidelberg
Date of thesis defense: 17 October 2019
Date Deposited: 11 Nov 2019 09:20
Date: 2019
Faculties / Institutes: The Faculty of Physics and Astronomy > Institute for Theoretical Physics
Subjects: 530 Physics
Controlled Keywords: High Energy Physics, Quantum Field Theory, Heavy Ion Collision, Photon, Non-equilibrium phenomena
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