  Arguments for black hole formation in collisions of high-energy particles
have rested on the emergence of a closed trapped surface in the classical
geometry of two colliding Aichelburg-Sexl solutions. Recent analysis has,
however, shown that curvatures and quantum fluctuations are large on this
apparent horizon, potentially invalidating a semiclassical analysis. We show
that this problem is an artifact of the unphysical classical point-particle
limit: for a particle described by a quantum wavepacket, or for a continuous
matter distribution, trapped surfaces indeed form in a controlled regime.
