  We investigate formation of an apparent horizon (AH) in high-energy particle
collisions in four- and higher-dimensional general relativity, motivated by
TeV-scale gravity scenarios. The goal is to estimate the prefactor in the
geometric cross section formula for the black hole production. We numerically
construct AHs on the future light cone of the collision plane. Since this slice
lies to the future of the slice used previously by Eardley and Giddings
(gr-qc/0201034) and by one of us and Nambu (gr-qc/0209003), we are able to
improve the prefactor estimates. The black hole production cross section
increases by 40-70% in the higher-dimensional cases, indicating larger black
hole production rates in future-planned accelerators than previously estimated.
We also determine the mass and the angular momentum of the final black hole
state, as allowed by the area theorem.
