A Nonperturbative Measurement of the Broken Phase Sphaleron Rate

We develop a method to compute the sphaleron rate in the electroweak broken phase nonperturbatively. The rate is somewhat slower than a perturbative estimate. In SU(2) X U(1) Higgs theory at the physical value of Theta_W, and assuming that the latent heat of the phase transition reheats the universe to the equilibrium temperature, baryon number erasure after the phase transition is prevented only when lambda/g^2 <= 0.036.