Perturbative description of dissipation in nonequilibrium field theory

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Abstract
Dissipative effects, such as the relaxation of quasiparticle occupation numbers, arise from absorptive parts of Green’s functions, which typically appear first at the second order of perturbation theory. Within the closed-time-path formalism, it is shown, using a generalized renormalization technique, that these absorptive parts may be approximately resummed so as to appear in unperturbed propagators. In this way, it becomes possible to study, in low-order perturbation theory, the evolution in time of a field theory which is driven away from thermal equilibrium by the presence in its Hamiltonian of explicitly time-dependent parameters. Particular attention is given to a scalar field with time-dependent mass, which is relevant to the dynamics of phase transitions in the very early Universe. Under favorable conditions, the analysis leads to a kinetic equation of the Boltzmann type, and an approximate numerical solution of this equation is presented for illustrative purposes.