Quantum kinetic theory with nonlocal coherence

In this thesis we develop a novel approximation scheme (eQPA), where the effects of nonlocal coherence are included in the kinetic approach to nonequilibrium quantum dynamics. The key element in our formalism is the finding of new singular shell solutions, located at $k_{0,z} = 0$ in the phase space of 2-point Wightman function, which describe the nonlocal quantum coherence between the ``opposite'' mass-shell excitations for spatially homogeneous and static planar symmetric problems, respectively. This phase space structure leads to a closed set of transport equations for the corresponding on-shell distribution functions $f$, providing an extension to the standard quantum Boltzmann equation. We have considered a number of applications to demonstrate the use of our formalism, including the Klein problem, quantum reflection from a CP-violating mass wall and coherent production of (fermionic and scalar) particles in an oscillating background. Our formalism should be of relevance for many problems in particle physics and cosmology, including baryogenesis and neutrino flavour oscillations in an inhomogeneous background.