A Positive-Weight Next-to-Leading Order Monte Carlo Simulation of   Drell-Yan Vector Boson Production
  The positive weight next-to-leading-order (NLO) matching scheme (POWHEG) is applied to Drell-Yan vector boson production in the Herwig++ Monte Carlo event generator. This approach consistently combines the NLO calculation and parton shower simulation, without the production of negative weight events. The simulation includes a full implementation of the truncated shower required to correctly describe soft emissions in an angular-ordered parton shower, for the first time. The results are compared with Tevatron W and Z production data and predictions for the transverse momentum spectrum of gauge bosons at the LHC presented. 
Keith Hamilton, Peter Richardson, Jon Tully