Browsing by Subject "theoretical Physics"

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  • Säppi, Saga (Helsingin yliopisto, 2020)
    This Thesis covers research conducted at the University of Helsinki in the field of thermal field theory, a framework for describing quantum fields in a medium, in particular at finite temperature and chemical potential. In the included Publications, there is a strong emphasis on describing field theories in a dense medium, at large chemical potentials, as well as on thermal resummation methods. The central focus and inspiration for the research is the study of elementary particle physics, in the realm of relativistic quantum fields. Specific motivations include the desire to better understand the behaviour of dense, strongly interacting matter possibly present in the cores of neutron stars, push forward high-order perturbative calculations in thermal field theory, as well as to gain some analytic insight on nonperturbative physics by studying a simple low-dimensional model. The main results emerging from the research carried out for this Thesis include a method for reducing zero-temperature finite-density Feynman loop integrals into a sum of vacuum integrals and its proof, the determination of a new high-order contribution to the weak-coupling perturbative expansion of the pressure of cold and dense Quantum Chromodynamics, as well as a study of a three-dimensional thermal quantum field theory using a novel nonperturbative method. The research also paved the way for future determination of the full next-to-next-to-next-to leading order pressure, which is currently well under way. All of this research was theoretical, and involved primarily analytic and in some cases numerical calculation methods. In addition to peer-reviewed Publications, the Thesis contains an Introduction that builds the foundation of some key concepts—gauge theory and eventually Quantum Chromodynamics as well as thermal field theory, in particular in the imaginary-time formalism—required for understanding the included research. It also includes a more focussed Chapter on Quantum Chromodynamics at finite density, also covering Hard Thermal Loop theory.