Mechanisms for Primordial Black Hole Production in String Theory
  We consider mechanisms for producing a significant population of primordial
black holes (PBHs) within string inspired single field models of inflation. The
production of PBHs requires a large amplification in the power spectrum of
curvature perturbations between scales associated with CMB and PBH formation.
In principle, this can be achieved by temporarily breaking the slow-roll
conditions during inflation. In this work, we identify two string setups that
can realise this process. In string axion models of inflation, subleading
non-perturbative effects can superimpose steep cliffs and gentle plateaus onto
the leading axion potential. The cliffs can momentarily violate the slow-roll
conditions, and the plateaus can lead to phases of ultra slow-roll inflation.
We thus achieve a string motivated model which both matches the Planck
observations at CMB scales and produces a population of light PBHs, which can
account for an order one fraction of dark matter. In DBI models of inflation, a
sharp increase in the speed of sound sourced by a steep downward step in the
warp factor can drive the amplification. In this scenario, discovery of PBHs
could indicate non-trivial dynamics in the bulk, such as flux-antibrane
annihilation at the tip of a warped throat.
