  Quantum dissipation in thermal environment is investigated, using the path
integral approach. The reduced density matrix of the harmonic oscillator system
coupled to thermal bath of oscillators is derived for arbitrary spectrum of
bath oscillators. Time evolution and the end point of two-body decay of
unstable particles is then elucidated: After early transient times unstable
particles undergo the exponential decay, followed by the power law decay and
finally ending in a mixed state of residual particles containing contributions
from both on and off the mass shell, whose abundance does not suffer from the
Boltzmann suppression.
