Consistent analytic approach to the efficiency of collisional Penrose   process
  We propose a consistent analytic approach to the efficiency of collisional Penrose process in the vicinity of a maximally rotating Kerr black hole. We focus on a collision with arbitrarily high center-of-mass energy, which occurs if either of the colliding particles has its angular momentum fine-tuned to the critical value to enter the horizon. We show that if the fine-tuned particle is ingoing on the collision, the upper limit of the efficiency is $(2+\sqrt{3})(2-\sqrt{2})\simeq 2.186$, while if the fine-tuned particle is bounced back before the collision, the upper limit is $(2+\sqrt{3})^{2}\simeq 13.93$. Despite earlier claims, the former can be attained for inverse Compton scattering if the fine-tuned particle is massive and starts at rest at infinity, while the latter can be attained for various particle reactions, such as inverse Compton scattering and pair annihilation, if the fine-tuned particle is either massless or highly relativistic at infinity. We discuss the difference between the present and earlier analyses. 
Tomohiro Harada, Kota Ogasawara, Umpei Miyamoto
gr-qc astro-ph.HE hep-th