We study the conditions under which simple relations between the inflaton couplings and CMB observables can be established. The crucial criterion is to avoid feedback effects during reheating, which tend to introduce a complicated dependence of the CMB observables on a large number of microphysical parameters that prohibits the derivation of meaningful constraints on any individual one of them. We find that the inflaton coupling can be measured with cosmological data when the effective potential during reheating can be approximated by a parabola, and when the coupling constants are smaller than an upper bound that it determined by the ratios between the inflaton mass and the Planck mass or the scale of inflation. The power at which these ratios appear is determined by the power at which the inflaton appears in a given interaction term, and the value of the upper bound is largely independent of the type of produced particle. Our results show that next generation CMB observatories may be able to constrain the inflaton couplings for various types of interactions, providing an important clue to understand how a given model of inflation may be embedded into a more fundamental microphysical theory of nature.
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