Primordial Black Holes from Thermal Inflation
  We present a novel mechanism for the production of primordial black holes
(PBHs). The mechanism is based on a period of thermal inflation followed by
fast-roll inflation due to tachyonic mass of order the Hubble scale. Large
perturbations are generated at the end of the thermal inflation as the thermal
inflaton potential turns from convex to concave. These perturbations can lead
to copious production of PBHs when the relevant scales re-enter horizon. We
show that such PBHs can naturally account for the observed dark matter in the
Universe when the mass of the thermal inflaton is about $10^6\,$GeV and its
coupling to the thermal bath preexisting the late inflation is of order unity.
We consider also the possibility of forming the seeds of the supermassive black
holes. In this case we find that the mass of the thermal inflaton is about
$1\,$GeV, but its couplings have to be very small, $\sim 10^{-7}$. Finally we
study a concrete realisation of our mechanism through a running mass model.
