Decoherence in a field and time-symmetry breaking

As an application of the complex spectral representation of the Liouvillian that gives a rigorous approach to irreversible processes, a problem of decoherence for a particle coupled with a field in the thermodynamic limit is considered. We especially focus our attention on a time evolution of the field, which is commonly neglected in phenomenological approachs to the decoherence problem. In contrast to the usual hypothesis, there is a non-negligible time evolution of the field. The result shows that the decoherence in the field is an important consequence of irreversible processes that leads dynamically to a mixed state from a pure state through a secular effect due to the resonance effect between the particle and the field.