Nonequilibrium phenomena in quantum field theory: From linear response to dynamical renormalization group.

摘要

This thesis is devoted to studying aspects of real-time nonequilibrium dynamics in quantum field theory by implementing an initial value formulation of quantum field theory. The main focus is on the linear relaxation of mean fields and quantum kinetics in nonequilibrium multiparticle quantum systems with potential applications to ultrarelativistic heavy ion collisions, cosmological phase transitions and condensed matter systems. We first study the damping of fermion mean fields in a fermion-scalar plasma with a view towards understanding baryon transport phenomena during electroweak baryogenesis. We obtain a fully renormalized, retarded and causal equation of motion that describes the relaxation of the mean field towards equilibrium and allows an unambiguous identification of a novel damping mechanism and the corresponding damping rate. Secondly, we apply and extend the renormalization group method to study nonequilibrium dynamics in a self-interacting scalar theory, the O(4) linear sigma model and a hot QED plasma, with the goals of constructing a quantum kinetic description that goes beyond usual Boltzmann kinetics and understanding anomalous (nonexponential) relaxation associated with infrared phenomena. Remarkably, within this framework the quantum kinetic equations and equations of motion for mean fields have the interpretation as the dynamical renormalization group equation which describes the dynamical evolution of a multiparticle system that is insensitive to microscopic details. By solving these equations, we effectively integrate out fast motion dynamics, and are left with an effective theory for slow motion dynamics. As a by-product, the issue of pinch singularities in nonequilibrium quantum field theory is resolved naturally in real time from a quantum kinetic point of view. The final part of this thesis presents a real-time kinetic analysis of direct photon production from a quark-gluon plasma created in ultrarelativistic heavy ion collisions. We show that the direct photon yield is significantly enhanced by the lowest order energy-nonconserving processes originated in the transient lifetime of the quark-gluon plasma. In particular, transverse momentum distribution of direct photons, which features a power law spectrum in the experimentally relevant regime, is proposed as a nonequilibrium signature of the quark-gluon plasma.