On the Sudakov suppression and enhancement for electroweak reactions
  Accounting for the double-logarithmic (DL) contributions to amplitude of Z to f \bar{f} leads to the Sudakov suppression. In contrast, resumming DL contributions to amplitudes of W to f \bar{f}' results into an exponential with a positive exponent, i.e. into the enhancement. This effect is intrinsic property of the theories with non-Abelian gauge groups. 
B.I. Ermolaev