Chern-Simons diffusion rate near the electroweak phase transition for m$_{H}$ $\appprox$ m$_{W}$

The rate of B-violation in the standard model at finite temperature is closely related to the diffusion rate \Gamma of Chern-Simons number. We compute this rate for m_H \approx m_W in the classical approximation in an effective SU(2)-Higgs model, using Krasnitz's algorithm. The parameters in the effective hamiltonian are determined by comparison with dimensional reduction. In the high temperature phase we find \Gamma/V (\alpha_W T)^4 \approx 1, neglecting a finite renormalization. In the low temperature phase near the transition we find the rate to be much larger than might be expected from previous analytic calculations based on the sphaleron