A New Method for Combining NLO QCD with Shower Monte Carlo Algorithms
Paolo Nason
TL;DR
The paper addresses the challenge of achieving NLO accuracy for exclusive final states within Shower Monte Carlo frameworks. It introduces a method to generate the hardest emission first using a modified Sudakov form factor, with subsequent emissions constrained by a $p_T$ veto, preserving angular-ordered coherence. This SMC+NLO approach avoids large modifications to existing shower codes and mitigates issues with negative weights seen in prior MC@NLO implementations, while enabling improved matrix-element corrections. The method is compatible with spacelike (initial-state) showers via backward evolution and could be extended to NNLO as calculations become available, with truncated showers accounting for large-angle soft radiation.
Abstract
I show that with simple extensions of the shower algorithms in Monte Carlo programs, one can implement NLO corrections to the hardest emission that overcome the problems of negative weighted events found in previous implementations. Simple variants of the same method can be used for an improved treatment of matrix element corrections in Shower Monte Carlo programs.