Influence of interactions on particle production induced by time-varying mass terms

We have investigated effects of interaction terms on non-perturbative particle production.  It is well known that time-varying masses induce abundant particle production, such as the preheating theory.  As our conclusion, it is possible to induce particle production even if the mass term of a particle species is not varying in time.  Such particles are produced through the interactions with other fields, whose mass terms are varying due to a time-dependent background.  The rather general result is that the amount of produced particles without time-dependent masses can be comparable with the particle density produced directly by the varying background if the strength of interaction terms is reasonably large.  In this talk, we will explain with a simple but illustrative model and show analytic and numerical results.