  Particle number violating interactions wash out the primordial asymmetry of
particle number density generated by some interaction satisfying Sakharov
conditions for baryogenesis. In this paper, we study how the primordial
asymmetry evolves in time under the presence of particle number violating
interactions and in the environment of expanding universe. We introduce a
complex scalar model with particle number violating mass terms and calculate
the time evolution of the particle number density with non-equilibrium quantum
field theory. We show how the time evolution of the number density depends on
parameters, including the chemical potential related with the particle number,
temperature, size of the particle number violating mass terms, and the
expansion rate of the universe. Depending upon whether the chemical potential
is larger or smaller than the rest mass of the scalar particle, behaviors of
the number density are very different to each other. When the chemical
potential is smaller than the mass, the interference among the contribution of
oscillators with various momenta reduces the number density in addition to the
dilution due to the expansion of universe. In opposite case, the oscillation of
the particle number density lasts for a long time and the cancellation due to
the interference does not occur.
