  The escape fraction at infinity is evaluated for massless particles produced
in collisions of weakly interacting particles accreted into a density spike
near the particle horizon of an extremal Kerr black hole, for the case of
equatorial orbits. We compare with the Schwarzschild case, and argue that in
the case of extremal black holes, redshifted signatures can be produced that
could potentially explore the physics of particle collisions at centre of mass
energies that extend beyond those of any feasible terrestrial accelerator.
