Relative distributions of W's and Z's at low transverse momenta
  Despite large uncertainties in the $W^\pm$ and $Z^0$ transverse momentum ($q_T$) distributions for $q_T\lsim 10$ GeV, the ratio of the distributions varys little. The uncertainty in the ratio of $W$ to $Z$ $q_T$ distributions is on the order of a few percent, independent of the details of the nonperturbative parameterization. 
M. H. Reno