Classification of static and homogeneous solutions in exactly solvable models of two-dimensional dilaton gravity

We give the full list of types of static (homogeneous)solutions within a wide family of exactly solvable 2D dilaton gravities with backreaction of conformal fields. It includes previously known solutions as particular cases. Several concrete examples are considered for illustration. They contain a black hole and cosmological horizon in thermal equilibrium, extremal and ultraextremal horizons, etc. In particular, we demonstrate that adS and dS geometries can be \QTR{it}{exact} solutions of semiclassical field equations for a \QTR{it}{nonconstant} dilaton field.Comment: 19 pages. To appear in Int. J. Mod. Phys.

Kinematics of ultra-high energy particle collisions near black holes in the magnetic field

There are different versions of collisions of two particles near black holes with unbound energy $E_{c.m.}$ in the centre of mass frame. The so-called BSW effect arises when a slow fine-tuned "critical" particle hits a rapid "usual" one. We discuss a scenario of collision in the strong magnetic field for which explanation tuns out to be different. Both particles are rapid but the nonzero angle between their velocities (which are both close to $c$, the speed of light) results in a relative velocity close to $c$ and, hence, big $% E_{c.m.}Comment: 8 pages. Final versio

Ultra-high energy collisions in static space-times: single versus multi-black hole cases

We consider collision of two particles near static electrically charged extremal black holes and elucidate the conditions under which the energy in the centre of mass frame E_{c.m.} grows unbounded. For a single black hole, we generalize the results obtained earlier for the Reissner-Nordstr\"om metric, to distorted black holes. In the multi-black hole space-time, qualitatively new features appear. If the point of collision is close to at least two horizons simultaneously, unbounded E_{c.m.} are possible (i) without fine-tuning of particles' parameters, (ii) for an arbitrary mutual orientation of two velocities. Such a combination of properties (i) and (ii) has no analogues in the single black hole case and facilitates the condition of getting unbounded E_{c.m.}. Collisions in the electro-vacuum Majumdar-Papapetrou metric (several extremal black holes in equilibrium) is analyzed explicitly.Comment: 18 pages. Essentially expanded, presentation improved. To appear in GR