Abstract
Abstract In this publication, uncertainties in and differences between the MC@NLO and POWHEG methods for matching next-to-leading order QCD calculations with parton showers are discussed. Implementations of both algorithms within the event generator SHERPA and based on Catani-Seymour subtraction are employed to assess the impact on a representative selection of observables. In the case of MC@NLO a substantial simplification is achieved by using dipole subtraction terms to generate the first emission. A phase space restriction is employed, which allows to vary in a transparent way the amount of non-singular radiative corrections that are exponentiated. Effects on various observables are investigated, using the production of a Higgs boson in gluon fusion, with or without an associated jet, as a benchmark process. The case of H+jet production is presented for the first time in an NLO+PS matched simulation. Uncertainties due to scale choices and non-perturbative effects are explored in the production of W ± and Z bosons in association with a jet. Corresponding results are compared to data from the Tevatron and LHC experiments.
Highlights
K-factor indicates substantial higher-order corrections, such that theoretical predictions should be made at next-to-leading order accuracy or better
Uncertainties due to scale choices and non-perturbative effects are explored in the production of W ± and Z bosons in association with a jet
While MC@NLO relies on a subtraction algorithm based on the parton shower approximation to collinear divergences in real-emission matrix elements, the POWHEG method effectively constructs a one-emission generator, similar to a one-step parton shower, with evolution kernels determined by ratios of real-emission and Born matrix elements
Summary
In order to see how the existing matching algorithms work, consider first the structure of an NLO calculation. This problem has been solved by the traditional matrix-element correction procedure outlined in [14,15,16,17], which is implemented for a number of processes in both HERWIG and PYTHIA The essence of this method lies in the fact that, for the processes it is applied to, the product of Born-level contribution and parton shower evolution kernel K is larger than the corresponding real emission term R in the complete phase space of the extra emission. This allows to correct the first (hardest) emission with a factor R/(B K), leading to the desired distribution in phase space.
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