Mass-gap black holes and the nursery of stars

Young star clusters and open clusters are the nursery of massive stars in the local Universe. Their initial central density is sufficiently high to foster gravitational encounters and collisions, but -unlike globular clusters- they are relatively quickly disrupted by the tidal field of their host galaxies. In this talk, I will review the main dynamical processes leading to the formation of massive binary black holes in young star clusters. Stellar exchanges and collisions are particularly effective in building up massive binary black holes, with primary component mass higher than 40 Msun. Star-star collisions and hierarchical mergers might even give birth to black holes in the pair instability mass gap (~60-120 Msun) or even in the intermediate-mass regime (~100-10'000 Msun). Recent studies predict that ~1% of binary black hole mergers in young star clusters have mass in the pair-instability gap. This result has a variety of implications for current and next-generation ground-based gravitational-wave interferometers.