  We show that next generation Cosmic Microwave Background experiments will be
capable of the first ever measurement of the inflaton coupling to other
particles, opening a new window to probe the connection between cosmic
inflation and particle physics.This sensitivity is based on the impact that the
reheating phase after cosmic inflation has on the redshifting of cosmic
perturbations. For our analysis we introduce a simple analytic method to
estimate the sensitivity of future CMB observations to the reheating
temperature and the inflaton coupling. Applying our method to LiteBIRD and
CMB-S4 we find that, within a given model of inflation, these missions have the
potential to impose both an upper and a lower bound on the inflaton coupling.
Further improvement can be achieved if CMB data is combined with optical and
21cm surveys. Our results demonstrate the potential of future observations to
constrain microphysical parameters that can provide an important clue to
understand how a given model of inflation may be embedded in a more fundamental
theory of nature.
