Spectroscopy of Masses and Couplings during Inflation

In this work, we extend the idea of Quasi Single Field inflation to the case of multiple isocurvaton fields with masses of order of Hubble, which are coupled kinetically to the inflaton field and have some interactions among themselves. We consider the effects of these massive modes in both the size and the shape of the bispectrum. We show that the shape of the bispectrum in the squeezed limit is dominated by the lightest field and is the same as in Quasi Single Field inflation. This is a generic feature of multiple isocurvaton fields and is independent of the details of the interactions among the massive fields. When the isocurvaton fields have similar masses, we can potentially distinguish two different shapes in the squeezed limit so that the shape of the bispectrum can act as a particle detector. However, in the presence of hierarchy among the massive fields, the dominant effect is due to the lightest field.