Abstract
In this article, we present a method to calculate a posteriori event weights at next-to-leading-order (NLO) QCD accuracy for a given jet event defined by the (anti-)$k_t$ algorithm relying on the conventional $2\to 1$ recombination. This is an important extension compared to existing Monte-Carlo tools which generate jet events together with the corresponding weight but do not allow one to calculate the weight for a given event. The method can be used to generate unweighted events distributed according to the fixed-order NLO cross section. In addition, the method allows one to calculate NLO accurate weights for events recorded by experiments. The potential of this ability is illustrated by applying the Matrix Element Method (MEM) to single top-quark events generated with POWHEG in combination with Pythia. For the first time, a systematic study of parton shower effects within the MEM is provided. The method is completely general and can be applied to arbitrary LHC processes.
Highlights
The steadily improving precision achieved in collider experiments like ATLAS and CMS requires an equal precision in the theoretical predictions to make optimal use of the experimental results
A shift in the extracted top-quark mass to lower mass values is expected due to parton-shower effects in the pseudodata which are not taken into account in the matrix element method (MEM): multiple parton emissions lead to a modification of the phase space density, which results in shape differences in differential distributions compared to fixed-order NLO computations
We emphasize that the weight is calculated a posteriori for a given jet event
Summary
The steadily improving precision achieved in collider experiments like ATLAS and CMS requires an equal precision in the theoretical predictions to make optimal use of the experimental results. The evaluation of fully differential event weights incorporating NLO QCD corrections allows the generation of unweighted events following the NLO differential cross sections This possibility has been used already in Refs. While the combinatorial part for the possibilities to cluster the additional radiation is easy to solve, the efficient numerical integration is nontrivial Because of these two complications, the standard approach is to avoid the definition of a fully differential event weight and combine finite phase space regions in terms of histogrammed results as approximations to differential distributions. The basic idea is that the variables should not allow one to reconstruct the invariant mass of the jets since outside soft and collinear regions this precludes a oneto-one correspondence of Born-like virtual corrections and contributions with additional real radiation—which is required to uniquely define an event weight incorporating NLO QCD corrections. Appendix B contains a validation of the calculated event weights
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