Next-to-Leading-Order Event Generators

TL;DR

This paper reviews methods for combining parton shower approximations with fixed-order perturbation theory to achieve NLO accuracy in event generators, focusing on MC@NLO and POWHEG, and their application to Higgs production at the LHC.

Contribution

It provides a comprehensive overview of NLO + parton shower methods, comparing MC@NLO and POWHEG, and discusses their achievements, limitations, and the integration with matrix-element matching.

Findings

Achieved NLO accuracy in fully-simulated hadronic events.

Compared results of Higgs production at the LHC using different methods.

Identified residual uncertainties from effects beyond NLO.

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.