The Weak OPE and Dimension-eight Operators

We discuss recent work which identifies a potential flaw in standard treatments of weak decay amplitudes, including that of epsilon'/epsilon. The point is that (contrary to conventional wisdom) dimension-eight operators contribute to weak amplitudes at order G_F alpha_s and without 1/M_W^2 suppression. The effect of dimension-eight operators is estimated to be at the 100% level in a sum rule determination of the operator Q_7^(6) at mu = 1.5 GeV, suggesting that presently available values of mu are too low to justify the neglect of these effects.