  There often appear coherently oscillating scalar fields in particle physics
motivated cosmological scenarios, which may have rich phenomenological
consequences. Scalar fields should somehow interact with background thermal
bath in order to decay into radiation at an appropriate epoch, but introducing
some couplings to the scalar field makes the dynamics complicated. We
investigate in detail the dynamics of a coherently oscillating scalar field,
which has renormalizable couplings to another field interacting with thermal
background. The scalar field dynamics and its resultant abundance are
significantly modified by taking account of following effects : (1) thermal
correction to the effective potential, (2) dissipation effect on the scalar
field in thermal bath, (3) non-perturbative particle production events and (4)
formation of non-topological solitons. There appear many time scales depending
on the scalar mass, amplitude, couplings and the background temperature, which
make the efficiencies of these effects non-trivial.
