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.
