The Universe after inflation: The wide resonance case

We numerically study the decay of massive and massless inflatons phi into massive excitations X, via a phi(2)X(2) coupling, in the expanding Universe. We find that a wide enough resonance can withstand expansion of the Universe, and we determine the range of parameters in which that happens. For a massive inflaton, the effective production of particles with mass ten times that of the inflaton requires very large values of the resonance parameter q, q greater than or similar to 10(8). For these large q, the maximal size of produced fluctuations is significantly suppressed by back reaction, but at least in the Hartree approximation they are still not negligible. For a massless inflaton with a lambda phi(4)/4 potential, the expansion of the Universe completely prevents resonant production of particles with masses larger than root lambda phi(0) for values of q smaller than 10(6).