  We study the decoherence of a renormalised quantum field theoretical system.
We consider our novel correlator approach to decoherence where entropy is
generated by neglecting observationally inaccessible correlators. Using
out-of-equilibrium field theory techniques at finite temperatures, we show that
the Gaussian von Neumann entropy for a pure quantum state asymptotes to the
interacting thermal entropy. The decoherence rate can be well described by the
single particle decay rate in our model. Connecting to electroweak baryogenesis
scenarios, we moreover study the effects on the entropy of a changing mass of
the system field. Finally, we compare our correlator approach to existing
approaches to decoherence in the simple quantum mechanical analogue of our
field theoretical model. The entropy following from the perturbative master
equation suffers from physically unacceptable secular growth.
