[en] The new multipurpose event-generation framework SHERPA, acronym for Simulation for High-Energy Reactions of PArticles, is presented. It is entirely written in the object-oriented programming language C++. In its current form, it is able to completely simulate electron-positron and unresolved photon-photon collisions at high energies. Also, fully hadronic collisions, such as, e.g., proton-anti-proton, proton-proton, or resolved photon-photon reactions, can be described on the signal level. (author)

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[en] We present theoretical predictions for the production of top-quark pairs with up to three jets at the next-to leading order in perturbative QCD. The relevant calculations are performed with Sherpa and OpenLoops. To address the issue of scale choices and related uncertainties in the presence of multiple scales, we compare results obtained with the standard scale H_T/2 at fixed order and the MiNLO procedure. Analyzing various cross sections and distributions for t anti t + 0, 1, 2, 3 jets at the 13 TeV LHC we find a remarkable overall agreement between fixed-order and MiNLO results. The differences are typically below the respective factor-two scale variations, suggesting that for all considered jet multiplicities missing higher-order effects should not exceed the ten percent level. (orig.) 2

[en] In this contribution the new event generation framework SHERPA will be presented. It aims at the full simulation of events at current and future high-energy experiments, in particular the LHC. Some results related to the production of jets at the Tevatron will be discussed

[en] In this paper the current release of the Monte Carlo event generator Sherpa, version 1.1, is presented. Sherpa is a general-purpose tool for the simulation of particle collisions at high-energy colliders. It contains a very flexible tree-level matrix-element generator for the calculation of hard scattering processes within the Standard Model and various new physics models. The emission of additional QCD partons off the initial and final states is described through a parton-shower model. To consistently combine multi-parton matrix elements with the QCD parton cascades the approach of Catani, Krauss, Kuhn and Webber is employed. A simple model of multiple interactions is used to account for underlying events in hadron-hadron collisions. The fragmentation of partons into primary hadrons is described using a phenomenological cluster-hadronisation model. A comprehensive library for simulating tau-lepton and hadron decays is provided. Where available form-factor models and matrix elements are used, allowing for the inclusion of spin correlations; effects of virtual and real QED corrections are included using the approach of Yennie, Frautschi and Suura.

[en] We compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW and MLM schemes using different matrix-element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences. (orig.)

[en] The prediction of backgrounds to new-physics signals in topologies with large missing transverse energy and jets is important to new-physics searches at the LHC. Following a CMS study, we investigate theoretical issues in using measurements of γ+2-jet production to predict the irreducible background to searches for missing energy plus two jets that originates from Z+2-jet production where the Z boson decays to neutrinos. We compute ratios of γ+2-jet to Z+2-jet production cross sections and kinematic distributions at next-to-leading order in α_s, as well as using a parton shower matched to leading-order matrix elements. The former ratios rely on the first next-to-leading-order calculation of γ+2-jet production at a hadron collider. We find that the ratios obtained in the two approximations are quite similar, making γ+2-jet production a theoretically reliable estimator for the missing energy plus two jets background. We employ a Frixione-style photon isolation, but we also show that for isolated prompt-photon production at high transverse momentum the difference between this criterion and the standard-cone isolation used by CMS is small.