Primordial black hole formation and hybrid inflation
  We calculate the contribution to the curvature perturbation, that is
generated while the waterfall field grows exponentially (which can occur only
if the waterfall field mass is $m\gg H$). We consider the upper bound on its
spectrum coming from constraints on the abundance of primordial black holes,
taking account of possible non-gaussianity. The constraint is satisfied, but
extrapolation of our result to the regime $m\sim H$ suggests that the
constraint might not be satisfied there.
