  The dynamics of a free charged particle, initially described by a coherent
wave packet, interacting with an environment, i.e. the electromagnetic field
characterized by a temperature $T$, is studied. Using the dipole approximation
the exact expressions for the evolution of the reduced density matrix both in
momentum and configuration space and the vacuum and the thermal contribution to
decoherence, are obtained. The time behaviour of the coherence lengths in the
two representations are given. Through the analysis of the dynamic of the field
structure associated to the particle the vacuum contribution is shown to be
linked to the birth of correlations between the single momentum components of
the particle wave packet and the virtual photons of the dressing cloud.
