It has recently been pointed out that a substantial amount of e-folds can occur during the waterfall regime of hybrid inflation .
Moreover , Kodama et.al . have derived analytic approximations for the trajectories of the inflaton and of the waterfall fields .
Based on these , we derive here the consequences for F - and D -term SUSY hybrid inflation : A substantial amount of e-folds may occur in the waterfall regime , provided \kappa \ll M ^ { 2 } / M _ { P } ^ { 2 } , where \kappa is the superpotential coupling , M the scale of symmetry breaking and M _ { P } the reduced Planck mass .
When this condition is amply fulfilled , a number of e-folds much larger than N _ { e } \approx 60 can occur in the waterfall regime and the scalar spectral index is then given by the expression found by Kodama et.al . n _ { s } = 1 - 4 / N _ { e } .
This value may be increased up to unity , if only about N _ { e } e-folds occur during the waterfall regime , such that the largest observable scale leaves the horizon close to the critical point of hybrid inflation , what can be achieved for \kappa \approx 10 ^ { -13 } and M \approx 5 \times 10 ^ { 12 } { GeV } in F -term inflation .
Imposing the normalization of the power spectrum leads to a lower bound on the scale of symmetry breaking .