Dissipative perturbation theory for quantum fields

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Abstract
A perturbative method is proposed for studying the evolution in time of quantum field theories which are driven away from thermal equilibrium by the presence of explicitly time-dependent parameters in their Hamiltonians. For the case of a scalar field with time-dependent mass, it is shown within the closed-time-path formalism that absorptive parts of higher-order contributions to propagators can be approximately resummed by the addition and subtraction of a local but non-diagonal quadratic counterterm. In this way, lowest-order propagators are obtained which mimic the dissipative behaviour of the complete propagators and thereby facilitate the incorporation of finite relaxation times in low-order calculations. Under highly favourable conditions, a Boltzmann equation can be approximately derived.