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With about a hundred mergers of binary black holes (BBHs) detected viagravitational waves by the LIGO-Virgo-KAGRA (LVK) Collaboration, ourunderstanding of the darkest objects in the Universe has taken unparalleledsteps forward. While most of the events are expected to consist of BHs directlyformed from the collapse of massive stars, some may contain the remnants ofprevious BBH mergers. In the most massive globular clusters and in nuclear starclusters, successive mergers can produce second- (2G) or higher-generation BHs,and even form intermediate-mass BHs. Overall, we predict that up to $\sim10\%$, $\sim 1\%$ or $\sim 0.1\%$ of the BBH mergers have one component being a2G, 3G, or 4G BH, respectively. Assuming that $\sim 500$ BBH mergers will bedetected in O4 by LVK, this means that $\sim 50$, $\sim 5$, or $\sim 0.5$events, respectively, will involve a 2G, 3G, or 4G BH, if most sources areproduced dynamically in dense star clusters. With their distinctive signaturesof higher masses and spins, such hierarchical mergers offer an unprecedentedopportunity to learn about the BH populations in the densest stellar systemsand to shed light on the elusive intermediate-mass BHs that may form therein.

Demographics of Hierarchical Black Hole Mergers in Dense Star Clusters