Power Spectrum of the Density Perturbations From Smooth Hybrid New   Inflation Model
  We numerically investigate density perturbations generated in the smooth hybrid new inflation model, a kind of double inflation model that is designed to reproduce the running spectral index suggested by the WMAP results. We confirm that this model provides the running spectral index within 1sigma range of the three year WMAP result. In addition, we find a sharp and strong peak on the spectrum of primordial curvature perturbation at small scales. This originates from amplification of fluctuation in the first inflaton fields due to parametric resonance, which takes place in the oscillatory phase between two inflationary regime. Formation probability of primordial black holes (PBHs) is discussed as a consequence of such peak. 
Masahiro Kawasaki, Tsutomu Takayama, Masahide Yamaguchi, Jun'ichi   Yokoyama