Exploring black holes as particle accelerators: hoop-radius, target particles and escaping conditions

The possibility that rotating black holes could be natural particle accelerators has been subject of intense debate. While it appears that for extremal Kerr black holes arbitrarily high center of mass energies could be achieved, several works pointed out that both theoretical as well as astrophysical arguments would severely dampen the attainable energies. In this work we study particle collisions near Kerr black holes, by reviewing and extending the so far proposed scenarios. Most noticeably, we shall focus on the recently advanced target particle scenarios which were claimed to reach arbitrarily high energies even for Schwarzschild black holes. By implementing the hoop conjecture we show that these scenarios involving nearhorizon target particles are in principle able to attain, sub-Planckian, but still ultra-high center of mass energies of the order of 10(23) -10(25) eV even for non-extremal Kerr black holes. Furthermore, analysing the properties of particles produced in such collisions, we find that photons can escape to infinity. However, their energy is only of the order of the energy of the colliding particles and hence relatively low, which is the same conclusion previously reached in the literature about the original Banados-Silk-West process. This finding points towards a general limitation of collisional Penrose processes, at least for what concerns their primary products.

Automated summary

In this study, we investigate particle collisions near Kerr black holes by reviewing and extending proposed scenarios. By implementing the hoop conjecture, we demonstrate that nearhorizon target particle scenarios can attain ultra-high center of mass energies even for non-extremal Kerr black holes. Furthermore, we find that photons produced in these collisions can escape to infinity but have relatively low energy.