  We study the ultrarelativistic head-on collision of equal mass particles,
modeled as self-gravitating fluid spheres, by numerically solving the coupled
Einstein-hydrodynamic equations. We focus on cases well within the kinetic
energy dominated regime, where between 88-92% ($\gamma=8$ to 12) of the initial
net energy of the spacetime resides in the translation kinetic energy of the
particles. We find that for sufficiently large boosts, black hole formation
occurs. Moreover, near yet above the threshold of black hole formation, the
collision initially leads to the formation of two distinct apparent horizons
that subsequently merge. We argue that this can be understood in terms of a
focusing effect, where one boosted particle acts as a gravitational lens on the
other and vice versa, and that this is further responsible for the threshold
being lower (by a factor of a few) compared to simple hoop conjecture
estimates. Cases slightly below threshold result in complete disruption of the
model particles. The gravitational radiation emitted when black holes form
reaches luminosities of 0.014 $c^5/G$, carrying $16\pm2%$ of the total energy.

General Relativity and Quantum Cosmology;High Energy Astrophysical Phenomena;High Energy Physics - Theory