Inflaton decay and heavy particle production with negative coupling

We study the decay of the inflaton in a general Z(2)xZ(2) symmetric two scalar theory. Since the dynamics of the system is dominated by states with large occupation numbers which admit a semiclassical description, the decay can be studied by solving the classical equations of motion on the lattice. Of particular interest is the case when the cross coupling between the inflaton and the second scalar field is negative, which is naturally allowed in many realistic models. While the inflaton decays via parametric resonance in the positive coupling cast: we find that fur negative coupling there is a new mechanism of particle production which we call negative coupling instability. Because of this new mechanism the variances of the fields grow significantly larger before the production is shut off by the back reaction of the created particles which could have important consequences for symmetry restoration by nonthermal phase transitions. We also find that heavy particles are produced much more efficiently with negative coupling, which is of prime importance for GUT baryogenesis. Using a simple toy model for baryogenesis and the results of our lattice simulations we show that for natural values of the cross coupling enough 10(14) GeV bosons are created to produce a baryon to entropy ratio consistent with observation. This is to be contrasted with the situation for positive coupling, where the value of the cross coupling required to produce such massive particles is technically unnatural. In addition to our numerical results we obtain analytical estimates for the maximum variances of the fields in atr expanding universe for all cases of interest: massive and massless inflaton, positive and negative cross coupling, with and without significant self-interactions for the second field. [S0556-2821(97)05722-6].