Secondary GWs and PBHs in string inflation: formation and detectability
  We derive the spectrum and analyse the detectability prospects of secondary
gravity waves (GWs) associated to primordial black hole (PBH) production in a
class of string inflationary models called Fibre Inflation. The inflationary
potential features a near inflection point that induces a period of ultra
slow-roll responsible for an enhancement of the scalar perturbations which can
lead to PBHs with different masses and contributions to dark matter (DM) in
agreement with current observational bounds, including CMB constraints on the
scalar spectral index and the tensor-to-scalar ratio. This enhancement of the
curvature perturbations sources secondary GWs which can be detected by either
LISA or DECIGO and BBO, depending on the GW frequency but regardless of the
amount of PBH DM since secondary GWs remain detectable even if the PBH
contribution to DM is exponentially suppressed. The possibility to see a
secondary GW signal is instead due to the presence of an ultra slow-roll epoch
between CMB horizon exit and the end of inflation.
