Rate of gravitational inflaton decay via gauge trace anomaly
  We analyze decay processes of the inflaton field, phi, during the coherent
oscillation phase after inflation in f(phi)R gravity. It is inevitable that the
inflaton decays gravitationally into gauge fields in the presence of f(phi)R
coupling. We show a concrete calculation of the rate that the inflaton field
decays into a pair of gauge fields via the trace anomaly. Comparing this new
decay channel via the anomaly with the channels from the tree-level analysis,
we find that the branching ratio crucially depends on masses and the internal
multiplicities (flavor quantum number) of decay product particles. While the
inflaton decays exclusively into light fields, heavy fields still play a role
in quantum loops. We argue that this process in principle allows us to
constrain the effects of arbitrary heavy particles in the reheating. We also
apply our analysis to Higgs inflation, and find that the gravitational decay
rate would never exceed gauge interaction decay rates if quantum gravity is
unimportant.
