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Rust, Felix (2009): In-medium effects in the holographic quark-gluon plasma. Dissertation, LMU München: Faculty of Physics
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Abstract

In this dissertation we use the gauge/gravity duality to investigate various properties of strongly coupled gauge theories, which we interpret as models for the quark-gluon plasma (QGP). In particular, we use variants of the D3/D7 setup as an implementation of the top-down approach of connecting string theory with phenomenologically relevant gauge theories. We focus on the effects of finite temperature and finite density on fundamental matter in the holographic quark-gluon plasma, which we model as the N=2 hypermultiplet in addition to the N=4 gauge multiplet of supersymmetric Yang-Mills theory. As a key ingredient we develop a setup in which we can describe vector meson spectra in the holographic plasma at finite temperature and either baryon or isospin density. The resulting spectral functions are valid for all values of quark mass and temperature. They show the expected features of meson melting at high temperatures and are in agreement with the previously derived spectra for the zero temperature and zero density limit. Moreover, we are able to give a description of in-medium effects of finite particle density which are in qualitative agreement with phenomenological models and experimental observations. The description of vector meson excitations furthermore allows for a demonstration of the splitting of their spectrum at finite isospin chemical potential. In the effort to better understand transport processes in the QGP, we then study various diffusion coefficients in the quark-gluon plasma, including their dependence on temperature and particle density. In particular, we perform a simple calculation to obtain the diffusion coefficient of baryon charge and we derive expressions to obtain the isospin diffusion coefficient. Furthermore, we make use of an effective model to study the diffusion behavior of mesons in the plasma by setting up a kinetic model. The setup we chose allows to carry out computations at weak and strong coupling which we compare in order to estimate the effects of the coupling strength on mesonic diffusion and therewith equilibration processes in the QGP. Finally, we observe the implications of finite temperature and finite baryon or isospin density on the phase structure of fundamental matter in the holographic plasma. As one consequence we find a phase transition in the baryon diffusion coefficient which vanishes at a critical value of the particle density. The critical density we quantify matches the values of the according critical densities previously found in the phase transitions of other quantities. More important, we observe a new phase transition occurring when the isospin chemical potential excesses a critical bound, which depends on the temperature of the medium. Beyond this point we observe an instability of the system under consideration. In this way we trace out the border of a new phase in the phase diagram of fundamental matter in the holographic plasma.