A semiclassical calculation of particle production by a scalar field in a potential is performed. We focus on the particular case of production of fermions by a Nambu-Goldstone boson θ. We have derived a (non)local equation of motion for the θ field with the back reaction of the produced particles taken into account. The equation is solved in some special cases, namely, for purely Nambu-Goldstone bosons and for the tilted potential U(θ)∝${\mathit{m}}^2$${\mathrm{\theta }}^2$. Enhanced production of bosons due to parametric resonance is investigated; we argue that the resonance probably disappears when the expansion of the Universe is included. Application of our work on particle production to reheating and an idea for baryogenesis in inflation are mentioned.

• Received 12 October 1994

DOI:https://doi.org/10.1103/PhysRevD.51.2693