Particle production and dissipative cosmic field

The large amplitude oscillation of a cosmic field that may occur right after inflation and in the decay process of weakly interacting fields gives rise to violent particle production via the parametric resonance. In the large amplitude limit the problem of back reaction against the field oscillation is formulated for Yukawa-type inflaton coupling, and the energy spectrum of created particles is determined in a semiclassical approximation, assuming that interaction among created particles is weak. For a large enough amplitude the field decay is rapid, and the resulting energy spectrum is broadly distributed, implying larger production of high energy particles than what a simple estimate of the reheating temperature due to the Born formula would suggest. {} 1996 The American Physical Society.