BLACK HOLES AND COLLISION ENERGY IN THE CENTER-OF-MASS FRAME

It is shown that in the background of the Ayon-Beato-Garcia-Bronnikov (ABGB) black hole, the center-of-mass (CM) energy of collision for two charged particles falling freely from rest at infinity is infinite at the event horizon if one of the colliding particle has critical charge and the other one has any different value of the charge. For the Einstein-Maxwell-dilaton-axion (EMDA) black hole it is found that the CM energy of collision for two uncharged particles falling freely from rest at infinity remains finite. In the case of Banados-Teitelboim-Zanelli (BTZ) black hole the CM energy of collision for two uncharged particles falling freely from rest at infinity is infinite at the event horizon if there is no rotation. It is observed that in the case of the extremal rotating BTZ black hole, at the event horizon this CM energy remains finite.