Fate of $Z_2$ Symmetric Scalar Field
  The evolution of a coherently oscillating scalar field with $Z_2$ symmetry is
studied in detail. We calculate the dissipation rate of the scalar field based
on the closed time path formalism. Consequently, it is shown that the energy
density of the coherent oscillation can be efficiently dissipated if the
coupling constant is larger than the critical value, even though the scalar
particle is stable due to the $Z_2$ symmetry.
