Resonance Structure for Preheating with Massless Fields

We extend recent work on the resonance structure for post-inflation reheating, providing an analytic treatment for models in which both the inflaton and the fields into which it decays are massless. Solutions are derived which are valid for either a spatially-flat or spatially-open metric. Closed-form solutions are given for the characteristic exponent, which measures the rate of particle production during preheating. It is demonstrated that in certain regions of parameter space, the maximum values of the characteristic exponent in an open universe are several times greater than the maximum values in a spatially-flat universe. It is further demonstrated that the solutions found here by means of a simple algebraic construction match the two previously-known exact solutions, which were derived in terms of special functions.