A Positive-Weight Next-to-Leading Order Monte Carlo Simulation of Drell-Yan Vector Boson Production

TL;DR

The authors implement the POWHEG NLO matching scheme for Drell–Yan vector boson production inside the Herwig++ event generator, including a full truncated shower for angular-ordered showers. This approach generates only positive-weight events and preserves NLO accuracy while enabling coherent soft radiation through vetoed and truncated showers. They demonstrate excellent agreement with NLO benchmarks (MCFM) and Tevatron data for Z/W rapidity and transverse momentum spectra, and they provide LHC predictions, highlighting the method's competitive performance relative to MC@NLO and traditional Herwig++. The work also details the practical mapping between real-emission kinematics and parton-shower variables, paving the way for broader application to other processes and advanced matching schemes such as CKKW.

Abstract

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.

Figures (9)

• Figure 1: Comparisons of $\mathrm{d}\sigma/\mathrm{d}y$ for the POWHEG implementation and MCFMCampbell:2000bg for $Z$ and $W^{+}$ production at the Tevatron ($\sqrt{s}=2\mathrm{TeV}$) and the LHC ($\sqrt{s}=14\mathrm{TeV}$).
• Figure 2: The rapidity of a) the electron in $Z$ and b) the positron in $W^+$ production at the Tevatron including the leptonic decay of the gauge boson for the POWHEG implementation and MCFMCampbell:2000bg at the Tevatron ($\sqrt{s}=2\mathrm{TeV}$).
• Figure 3: Rapidity distribution for $Z$ production compared to D0 Run II Tevatron data Abazov:2007jy. The solid line shows the prediction of our POWHEG implementation, the dotted line is the prediction of MC@NLO and the dashed line is the default Herwig++ result.
• Figure 4: Transverse momentum distribution for $Z$ production compared to CDF Run I Tevatron data Affolder:1999jh. The solid line shows the prediction of our POWHEG implementation, the dotted line is the prediction of MC@NLO and the dashed line is the default Herwig++ result. The inset shows an expanded view of the low $p_{T}$ region.
• Figure 5: Transverse momentum distribution for $Z$ production compared to D0 Run II Tevatron data Abazov:2007nt. The solid line shows the prediction of our POWHEG implementation, the dotted line is the prediction of MC@NLO and the dashed line is the default Herwig++ result. The inset shows an expanded view of the low $p_{T}$ region.