Effective Field Theory of small r

The recent observations from CMB have imposed a very stringent upper-limit on the tensor/scalar ratio $r$ of inflation models, which indicates that the primordial gravitational waves (PGW), even though possible to be detected, should be very tiny. However, current experiments on PGW is ambitious to detect such a tiny signal by improving the accuracy to an even higher level. Whatever their results are, it will give us much information about the early universe, not only from the astrophysical side but also from the theoretical side, e.g., model building. In this paper, we are interested in analyzing what kind of inflation models can be favored by future observations, using the facility of the effective field theory (EFT) approach. We show that $r$ can be expressed in EFT language, and more importantly, by plotting the contours in the parameter spaces, we indicate how $r$ can be affected by model parameters in various concrete examples.