Classicality of the order parameter during a phase transition

We analyze the quantum to classical transition of the order parameter in second order phase transitions. We consider several toy models in nonrelativistic quantum mechanics. We study the dynamical evolution of a wave packet initially peaked around a local maximum of the potential using variational approximations and also exact numerical results. The influence of the environment on the evolution of the density matrix and the Wigner function is analyzed in great detail. We also discuss the relevance of our results to the analysis of phase transitions in field theory. In particular, we argue that previous results about the classicality of the order parameter in $O(N)$ models may be a consequence of the large N approximation.