NLO QCD predictions for Z+γZ+\gamma + jets production with Sherpa

We present precise predictions for prompt photon production in association with a $Z$ boson and jets. They are obtained within the Sherpa framework as a consistently merged inclusive sample. Leptonic decays of the $Z$ boson are fully included in the calculation with all offshell effects. Virtual matrix elements are provided by OpenLoops and parton shower effects are simulated with a dipole parton shower. Thanks to the NLO QCD corrections included not only for inclusive $Z\gamma$ production but also for the $Z\gamma$ + 1-jet process we find significantly reduced systematic uncertainties and very good agreement with experimental measurements at $\sqrt{s}=8$ TeV. Predictions at $\sqrt{s}=13$ TeV are displayed including a study of theoretical uncertainties. In view of an application of these simulations within LHC experiments, we discuss in detail the necessary combination with a simulation of the $Z$ + jets final state. In addition to a corresponding prescription we introduce recommended cross checks to avoid common pitfalls during the overlap removal between the two samples.Comment: 20 pages, 15 Figure

W+n-jet predictions at NLO matched with a parton shower

The MC@NLO method as implemented in the Sherpa MC generator is presented using the production of W-bosons in conjunction with up to three jets as an example. Corresponding results computed at next-to leading order in QCD and including parton shower corrections are compared to recent experimental data from the Large Hadron Collider.Comment: 4 pages, 2 figures. To appear in the proceedings of DIS 201

Next-to-leading order matrix elements and truncated showers

An algorithm is presented that combines the ME+PS approach to merge sequences of tree-level matrix elements into inclusive event samples with the POWHEG method, which combines exact next-to-leading order matrix elements with parton showers. The quality of the approach and its implementation in Sherpa are exemplified by results for e+e- annihilation into hadrons at LEP, for Drell-Yan lepton-pair production at the Tevatron and for Higgs-boson and W+W- production at LHC energies.Comment: 5 pages, 8 figures; To appear in the proceedings of Physics at the LHC 2010, DESY, Hamburg, 7-12 June 201

Hard photon production and matrix-element parton-shower merging

We present a Monte-Carlo approach to prompt-photon production, where photons and QCD partons are treated democratically. The photon fragmentation function is modelled by an interleaved QCD+QED parton shower. This known technique is improved by including higher-order real-emission matrix elements. To this end, we extend a recently proposed algorithm for merging matrix elements and truncated parton showers. We exemplify the quality of the Monte-Carlo predictions by comparing them to measurements of the photon fragmentation function at LEP and to measurements of prompt photon and diphoton production from the Tevatron experiments.Comment: 18 pages, 5 figures, revised version with minor update

A posteriori inclusion of PDFs in NLO QCD final-state calculations

Any NLO calculation of a QCD final-state observable involves Monte Carlo integration over a large number of events. For DIS and hadron colliders this must usually be repeated for each new PDF set, making it impractical to consider many `error' PDF sets, or carry out PDF fits. Here we discuss ``a posteriori'' inclusion of PDFs, whereby the Monte Carlo run calculates a grid (in x and Q) of cross section weights that can subsequently be combined with an arbitrary PDF. The procedure is numerically equivalent to using an interpolated form of the PDF. The main novelty relative to prior work is the use of higher-order interpolation, which substantially improves the tradeoff between accuracy and memory use. An accuracy of about 0.01% has been reached for the single inclusive cross-section in the central rapidity region |y|<0.5 for jet transverse momenta from 100 to 5000 GeV. This method should facilitate the consistent inclusion of final-state data from HERA, Tevatron and LHC in PDF fits, thus helping to increase the sensitivity of LHC to deviations from standard Model predictions.Comment: contribution to the CERN DESY workshop on "HERA and LHC

Monte-Carlo event generation for the LHC

This thesis discusses recent developments for the simulation of particle physics in the light of the start-up of the Large Hadron Collider. Simulation programs for fully exclusive events, dubbed Monte-Carlo event generators, are improved in areas related to the perturbative as well as non-perturbative regions of strong interactions. A short introduction to the main principles of event generation is given to serve as a basis for the following discussion. An existing algorithm for the correction of parton-shower emissions with the help of exact tree-level matrix elements is revisited and significantly improved as attested by first results. In a next step, an automated implementation of the POWHEG method is presented. It allows for the combination of parton showers with full next-to-leading order QCD calculations and has been tested in several processes. These two methods are then combined into a more powerful framework which allows to correct a parton shower with full next-to-leading order matrix elements and higher-order tree-level matrix elements at the same time. Turning to the non-perturbative aspects of event generation, a tuning of the Pythia event generator within the Monte-Carlo working group of the ATLAS experiment is presented. It is based on early ATLAS minimum bias measurements obtained with minimal model dependence. The parts of the detector relevant for these measurements are briefly explained. Throughout the thesis, results obtained with the improvements are compared to experimental measurements

Next-to-leading order QCD predictions for top-quark pair production with up to three jets

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 HT/2 at fixed order and the MINLO procedure. Analyzing various cross sections and distributions for tt+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.Comment: 10 pages, 6 figures, 2 table

NLO matching for ttbb production with massive b-quarks

Theoretical uncertainties in the simulation of ttbb production represent one of the main obstacles that still hamper the observation of Higgs-boson production in association with top-quark pairs in the H->bb channel. In this letter we present a next-to-leading order (NLO) simulation of ttbb production with massive b-quarks matched to the Sherpa parton shower. This allows one to extend NLO predictions to arbitrary ttbb kinematics, including the case where one or both b-jets arise from collinear g->bb splittings. We find that this splitting mechanism plays an important role for the ttH(bb) analysis.Comment: 4 pages, 3 figures. v2: b-jet momenta replaced by b-quark momenta in the dynamical QCD scale; related technical cut removed; numerical results updated accordingly and conclusions unchanged. References and various comments on the resummation of b-mass logarithms and on the consistency of double-splitting contributions added. Version to appear in Phys. Lett