  Primordial blackholes formed in the early Universe via gravitational collapse
of over-dense regions may contribute a significant amount to the present dark
matter relic density. Inflation provides a natural framework for the production
mechanism of primordial blackholes. For example, single field inflation models
with a fine-tuned scalar potential may exhibit a period of ultra-slow roll,
during which the curvature perturbation may be enhanced to become seeds of the
primordial blackholes formed as the corresponding scales reenter the horizon.
In this work, we propose an alternative mechanism for the primordial blackhole
formation. We consider a model in which a scalar field is coupled to the
Gauss-Bonnet term and show that primordial blackholes may be seeded when a
scalar potential term and the Gauss-Bonnet coupling term are nearly balanced.
Large curvature perturbation in this model not only leads to the production of
primordial blackholes but it also sources gravitational waves at the second
order. We calculate the present density parameter of the gravitational waves
and discuss the detectability of the signals by comparing them with sensitivity
bounds of future gravitational wave experiments.
