Particle collisions in the lower dimensional rotating black hole   space-time with the cosmological constant
  In this paper, we study the effect of ultra-high energy collisions of two particles with different energies near the horizon of a 2+1 dimensional BTZ black hole (BSW effect). We find that the particle with the critical angular momentum could exist inside the outer horizon of BTZ black hole regardless of the particle energy. Therefore, for the non-extremal BTZ black hole, the BSW process is possible on the inner horizon with the fine tuning of parameters which are characterized by the motion of particle. While for the extremal BTZ black hole, the particle with the critical angular momentum could only exist on the degenerate horizon, and the BSW process could also happen there. 
Jie Yang, Yun-Liang Li, Yang Li, Shao-Wen Wei, Yu-Xiao Liu
hep-th gr-qc