W and Z bosons in association with two jets using the POWHEG method
John M. Campbell, R. Keith Ellis, Paolo Nason, Giulia Zanderighi
TL;DR
The paper presents Wjj and Zjj generators within the POWHEG BOX interfaced to parton showers, achieving substantial speed gains by using analytic MCFM matrix elements. It tackles singular Born configurations through generation-cut and Born-suppression strategies, and introduces a MiNLO-based approach to achieve finite, smoothly matched predictions across jet multiplicities. A novel partitioning of real-emission contributions (tilde-d_alpha) mitigates large-weight spikes associated with near-singular underlying Born configurations. Validation against LHC data demonstrates reliability, while improvements to the POWHEG BOX enhance efficiency and flexibility for multi-jet processes and various shower programs.
Abstract
In this work we present the implementation of generators for W and Z bosons in association with two jets interfaced to parton showers using the POWHEG BOX. We incorporate matrix elements from the parton-level Monte Carlo program MCFM in the POWHEG BOX, allowing for a considerable improvement in speed compared to previous implementations. We address certain problems that arise when processes that are singular at the Born level are implemented in a shower framework using either a generation cut or a Born suppression factor to yield weighted events. In such a case, events with very large weights can be generated after the shower through a number of mechanisms. Events with very small transverse momentum at the Born level can develop large transverse momentum either after the hardest emission, after the shower, or after the inclusion of multi-parton interactions. We present a solution to this problem that can be easily implemented in the POWHEG BOX. We also show that a full solution to this problem can only be achieved if the generator maintains physical validity also when the transverse momentum of the emitted partons becomes unresolved. One such scheme is the recently-proposed MiNLO method for the choice of scale and the exponentiation of Sudakov form factors in NLO computations. We present a validation study of our generators, by comparing their output to available LHC data.