On the waterfall behavior in hybrid inflation
  We revisit the hybrid inflation model focusing on the dynamics of the waterfall field in an analytical way. It is shown that inflation may last long enough during the waterfall regime for some parameter regions, confirming the claim of Clesse. In this case the scalar spectral index becomes red, and can fall into the best fit range of the WMAP observation. 
Hideo Kodama, Kazunori Kohri, Kazunori Nakayama
astro-ph.CO hep-ph