Thermalization after inflation and production of massive stable particles

We discuss thermalization through perturbative inflaton decay at the end of inflation. We find that a thermal plasma should form well before all inflatons have decayed, unless all gauge symmetries are badly broken during that epoch. However, before they thermalize, the very energetic inflaton decay products can contribute to the production of massive stable particles, either through collisions with the thermal plasma, or through collisions with each other. If such reactions exist, the same massive particles can also be produced directly in inflaton decay, once higher--order processes are included. We show that these new, non--thermal production mechanisms often significantly strengthen constraints on the parameters of models containing massive stable particles; for example, stable charged particles with mass below the inflaton mass seem to be essentially excluded.