  We study the unitary time evolution of the order parameter of a quantum
system after a sudden quench in the parameter driving the transition. By
mapping the dynamics onto the imaginary time path-integral in a film geometry
we derive the full mean-field non-equilibrium phase diagram for a one-component
order parameter. The recently discovered non-equilibrium transition is
identified with the shifted critical point in films and therefore it is
generally expected to occur in more than one spatial dimension. We also find
that anharmonic oscillations of the order parameter are a general feature of
the mean-field quench dynamics.
