Higher order statistics of curvature perturbations in IFF model and its
  Planck constraints
  We compute the power spectrum P_\zeta, and non-linear parameters f_nl and
\tau_nl of the curvature perturbation induced during inflation by the
electromagnetic fields in the kinetic coupling model (IFF model). By using the
observational result of P_\zeta, f_nl and \tau_nl reported by the Planck
collaboration, we study the constraint on the model comprehensively.
Interestingly, if the single slow-rolling inflaton is responsible for the
observed P_\zeta, the constraint from \tau_nl is most stringent. We also find a
general relationship between f_nl and \tau_nl generated in this model. Even if
f_nl \sim O(1), a detectable \tau_nl can be produced.
