Waterfall field in hybrid inflation and curvature perturbation
  We study carefully the contribution of the waterfall field to the curvature
perturbation at the end of hybrid inflation. In particular we clarify the
parameter dependence analytically under reasonable assumptions on the model
parameters. After calculating the mode function of the waterfall field, we use
the delta N formalism and confirm the previously obtained result that the power
spectrum is very blue with the index 4 and is absolutely negligible on large
scales. However, we also find that the resulting curvature perturbation is
highly non-Gaussian and hence we calculate the bispectrum. We find that the
bispectrum is at leading order independent of momentum and exhibits its peak at
the equilateral limit, though it is unobservably small on large scales. We also
present the one-point probability distribution function of the curvature
perturbation.
