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

TL;DR

This paper investigates particle collisions near Kerr black holes, demonstrating that ultra-high energies are achievable with specific target particles, but escaping particles have limited energy, highlighting fundamental constraints of black hole particle acceleration.

Contribution

It extends previous scenarios by applying the hoop conjecture to show that near-horizon target particles can reach ultra-high energies even in non-extremal Kerr black holes.

Findings

Ultra-high energies (~10^{23}-10^{25} eV) are possible near Kerr black holes.

Escaping photons have energies comparable to the original particles.

Limitations exist on the energy of primary products in collisional Penrose processes.

Abstract

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 near-horizon 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 Ba\~nados--Silk--West process. This finding points towards a general limitation of collisional Penrose processes, at least for what concerns their primary products.