Particle collisions on near horizon extremal Kerr background

Recently, Bañados, Silk, and West analyzed a collision of two particles near the horizon of the extremal Kerr black hole and demonstrated that the energy in the center-of-mass frame can be arbitrarily large provided the angular momentum of one of the colliding particles takes a special value. As is known, the vicinity of the extremal Kerr black hole horizon can be viewed as a complete vacuum spacetime in its own right. In this work, we consider a collision of two neutral particles within the context of the near horizon extremal Kerr geometry and demonstrate that the energy in the center-of-mass frame is finite for any admissible value of the particle parameters. An explanation of why the two approaches disagree on the Bañados-Silk-West effect is given.