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Thermal leptogenesis, in the framework of the standard model with three additional heavy Majorana neutrinos, provides an attractive scenario to explain the observed baryon asymmetry in the universe. It is based on the out-of-equilibrium decay of Majorana neutrinos in a thermal bath of standard model particles, which in a fully quantum field theoretical formalism is obtained by solving Kadanoff-Baym equations. So far, the leading two-loop contributions from leptons and Higgs particles are included, but not yet gauge corrections. These enter at three-loop level but, in certain kinematical regimes, require a resummation to infinite loop order for a result to leading order in the gauge coupling. In this work, we apply such a resummation to the calculation of the lepton number density. The full result for the simplest “vanilla leptogenesis” scenario is by O(1) increased compared to that of quantum Boltzmann equations, and for the first time permits an estimate of all theoretical uncertainties. This step completes the quantum theory of leptogenesis and forms the basis for quantitative evaluations, as well as extensions to other scenarios.
In this thesis, the early time dynamics in a heavy ion collision of Pb-Nuclei at LHC center-of-mass energies of 5 TeV is studied. Right after the collision the system is out-of-equilibrium and essentially gluon dominated, with their density saturating at a specific momentum scale Q_s. Based on a separation of scales for the soft and hard gluonic degrees of freedom, the initial state is given from an effective model, known as the Color Glass Condensate. Within this model, the soft gluons behave classical to leading order, making it possible to study their dynamics in gauge invariant fashion on a three dimensional lattice, solving Hamiltonian field equations of motion, keeping real time. Quark-Antiquark pairs are produced in the gluonic medium, known as the Glasma and manifest themselves as a source of quantum fluctuations.
They enter the dynamics of the gluons as a current, making the system semi-classical. In lattice simulations, the non-equilibrium system is tested for pressure isotropization, which is a necessary ingredient to reach a local thermal equilibrium (LTE), making a hydrodynamical description at a later stage possible. In addition, the occupation of energy modes is studied with its implications on thermalization and classicality.