Measuring the Inflaton Coupling in the CMB

We study the perspectives to extract information about the microphysical parameters that govern the reheating process after cosmic inflation from CMB data. We identify conditions under which the inflaton coupling to other fields can be constrained for a given model of inflation without having to specify the details of the particle physics theory within which this model is realised. This is possible when the effective potential during reheating is approximately parabolic, and when the coupling constants are smaller than an upper bound that is determined by the ratios between the inflaton mass and the Planck mass or the scale of inflation. We consider scalar, Yukawa, and axion-like interactions and estimate that these conditions can be fulfilled if the inflaton coupling is comparable to the electron Yukawa coupling or smaller, and if the inflaton mass is larger than $10^5$ GeV. Pinning down the order of magnitude of the coupling constant requires measuring the scalar-to-tensor ratio at the level of $10^{-3}$, which is possible with future CMB observatories. This would provide an important clue to understand how a given model of inflation may be embedded into a more fundamental theory of nature.Comment: The treatment of feedback effects in absence of a parametric resonance has been substantially improved, and the connection to observables is discussed in more detail. Appendices added, references updated. 24 pages plus appendice