  We study collective oscillations of a two-flavor neutrino system with
arbitrary but fixed density. In the vacuum limit, modes with different energies
quickly de-phase (kinematical decoherence), whereas in the limit of infinite
density they lock to each other (synchronization). For intermediate densities,
we find different classes of solutions. There is always a phase transition in
the sense of partial synchronization occurring only above a density threshold.
For small mixing angles, partial or complete decoherence can be induced by a
parametric resonance, introducing a new time scale to the problem, the final
outcome depending on the spectrum and mixing angle. We derive an analytic
relation that allows us to calculate the late-time degree of coherence based on
the spectrum alone.
