Primordial black holes from Gauss-Bonnet-corrected single field inflation

Primordial black holes 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 black holes. 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 black holes formed as the corresponding scales reenter the horizon. In this work, we propose an alternative mechanism for the primordial black hole formation. We consider a model in which a scalar field is coupled to the Gauss-Bonnet term and show that primordial black holes 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 black holes, 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.