  Experimental systems with a first order phase transition will often exhibit
hysteresis when out of equilibrium. If defects are present, the hysteresis loop
can have different shapes: with small disorder the hysteresis loop has a
macroscopic jump, while for large disorder the hysteresis loop is smooth. The
transition between these two shapes is critical, with diverging length scales
and power laws. We simulate such a system with the zero temperature random
field Ising model, in 2, 3, 4, 5, 7, and 9 dimensions, with systems of up to
1000^3 spins, and find the critical exponents from scaling collapses of several
measurements. The numerical results agree well with the analytical predictions
from a renormalization group calculation.
