  We present theory for coherent effects observed in crystal collimation
experiments that is in good quantitative agreement with RHIC and Tevatron data.
We show that coherent scattering in a bent crystal strongly amplifies beam
diffusion, with an effective radiation length shortened by orders of magnitude
compared to amorphous material. This coherent scattering could replace the
traditional amorphous scattering in accelerator collimation systems. We predict
that crystal collimation for negative particles can be as strong as for
positives, unlike with channeling effect. This opens a principle way for
efficient crystal steering of negative particles at accelerators. It can be
demonstrated with antiproton crystal collimation at the Tevatron. We predict
strong effects for the upcoming Tevatron experiment, for protons and
antiprotons.
