approximation. Decays are generated using the algorithm described in [@Richardson:2001df], which ensures spin correlations are correctly treated. The parton shower utilises an approximation that resums the leading collinear and leading-colour soft logarithms [@Buckley:2011ms] and so does not accurately describe QCD radiation in the regions of phase space where the transverse momenta of the emitted partons are high. The Positive Weight Hardest Emission Generator (POWHEG) formalism [@Nason:2004rx] is one method that allows the simulation of high transverse momentum (hard) radiation to be improved upon by using the real-emission matrix element to produce the hardest emission in the shower. This approach affects both the overall cross sections for inclusive processes and results in local changes to the shapes of distributions sensitive to the hardest emission. In particular, local changes to observables such as jet transverse momenta are important since they can impact on the proportion of events passing selection criteria in new physics searches. Since BSM signals often consist of only a few events, this can in turn result in significant changes to the exclusion bounds that can be set.

The POWHEG formalism has been successfully applied to a wide range of hard production processes, for example [@Frixione:2007nw; @Alioli:2008gx; @Hamilton:2008pd; @Alioli:2008tz; @Nason:2009ai; 