Next-to-Leading-Order Event Generators
Paolo Nason, Bryan Webber
TL;DR
The paper addresses the challenge of achieving NLO accuracy in fully exclusive hadronic final states by interfacing fixed-order NLO calculations with parton showers to form NLO+PS event generators. It compares two main methods, MC@NLO and POWHEG, explains their theoretical foundations, similarities and differences, and illustrates them using Higgs production at the LHC. It discusses how NLO+PS relates to ME+PS, addresses issues such as truncated showers, spin correlations, PDFs, and uncertainties, and outlines current and future directions. It emphasizes the practical impact of enabling high-precision, collider-ready simulations with NLO accuracy while preserving shower resummation, along with residual uncertainties to be addressed.
Abstract
We review the methods developed for combining the parton shower approximation to QCD with fixed-order perturbation theory, in such a way as to achieve next-to-leading-order (NLO) accuracy for inclusive observables. This has made it possible to generate fully-simulated hadronic final states with the precision and stability of NLO calculations. We explain the underlying theory of the existing methods, MC@NLO and POWHEG, together with their similarities, differences, achievements and limitations. For illustration we mainly compare results on Higgs boson production at the LHC, with particular emphasis on the residual uncertainties arising from the different treatment of effects beyond NLO. We also briefly summarize the difference between these NLO + parton shower methods and matrix-element + parton shower matching, and current efforts to combine the two approaches.