Black Holes with Accretion Disks as High-Energy Particle Colliders

In principle, rapidly rotating black holes (BHs) with accretion disks -- either prograde or retrograde -- could naturally act as high-energy particle colliders (HEPCs) because particles falling in from infinity that collide with particles in the disk near the BH horizon can exhibit center-of-mass (CM) energies beyond the capabilities of terrestrial instruments. However, we show that scattering can moderate the CM energy of near-horizon collisions. We find that rapidly rotating BHs with prograde accretion disks are the most viable candidates for astrophysical HEPCs, whereas BHs with retrograde disks are disfavored, when scattering is incorporated.