Can a nonextremal black hole be a particle accelerator

Abstract

© 2020 American Physical Society. We consider particle collisions in the background of a nonextremal black hole. Two particles fall from infinity, particle 1 is fine-tuned (critical), collision occurs in its turning point. The first example is the Reissner-Nordström (RN) one. If the energy at infinity E1 is big enough, the turning point is close to the horizon. Then, we derive a simple formula according to which Ec.m.~E1?-1/2, where ? is a surface gravity. Thus significant growth of Ec.m. is possible if (i) particle 1 is ultrarelativistic (if both particles are ultrarelativistic, this gives no gain as compared to collisions in flat space-time), (ii) a black hole is near-extremal (small ?). In the scenario of multiple collisions the energy Ec.m. is finite in each individual collision. However, it can grow in subsequent collisions, provided new near-critical particles are heavy enough. For neutral rotating black holes, in case (i) a turning point remains far from the horizon but large Ec.m. is still possible. Case (ii) is similar to that for collisions in the RN metric. We develop a general theoretical scheme, direct astrophysical applications can be a next step to be studied.