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# Publications

## NLO matrix elements and truncated showers

- Authors: Stefan Hoeche, Frank Krauss, Marek Schoenherr, Frank Siegert
- In this publication, 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 element results with the parton shower. It was developed in parallel to the MENLOPS technique and has been implemented in the event generator Sherpa. The benefits of this approach are exemplified by some first predictions for a number of processes, namely the production of jets in e+ e- annihilation, in deep-inelastic ep scattering, in association with single W, Z or Higgs bosons, and with vector boson pairs at hadron colliders.
- arXiv:1009.1127 [hep-ph ]

## Automating the POWHEG method in Sherpa

- Stefan Hoeche, Frank Krauss, Marek Schoenherr, Frank Siegert
- A new implementation of the POWHEG method into the Monte-Carlo event generator Sherpa is presented, focusing on processes with a simple colour structure. Results for a variety of reactions, namely e+e- to hadrons, deep-inelastic lepton-nucleon scattering, hadroproduction of single vector bosons and of vector boson pairs as well as the production of Higgs bosons in gluon fusion serve as test cases for the successful realisation. The algorithm is fully automated such that for further processes only virtual matrix elements need to be included.
- arXiv:1008.5399 [hep-ph ]

## Hadronic final states in deep-inelastic scattering with Sherpa

- Authors: Tancredi Carli, Thomas Gehrmann, Stefan Hoeche
- We extend the multi-purpose Monte-Carlo event generator Sherpa to include processes in deeply inelastic lepton-nucleon scattering. Hadronic final states in this kinematical setting are characterised by the presence of multiple kinematical scales, which were up to now accounted for only by specific resummations in individual kinematical regions. Using an extension of the recently introduced method for merging truncated parton showers with higher-order tree-level matrix elements, it is possible to obtain predictions which are reliable in all kinematical limits. Different hadronic final states, defined by jets or individual hadrons, in deep-inelastic scattering are analysed and the corresponding results are compared to HERA data. The various sources of theoretical uncertainties of the approach are discussed and quantified. The extension to deeply inelastic processes provides the opportunity to validate the merging of matrix elements and parton showers in multi-scale kinematics inaccessible in other collider environments. It also allows to use HERA data on hadronic final states in the tuning of hadronisation models.
- Eur.Phys.J.C67 (2010) 73

## Hard photon production and matrix-element parton-shower merging

- Authors: Stefan Hoeche, Steffen Schumann, Frank Siegert
- 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.
- Phys.Rev.D81 (2010) 034026

## QCD matrix elements and truncated showers

- Authors: S. Hoeche, F. Krauss, S. Schumann, F. Siegert
- We derive an improved prescription for the merging of matrix elements with parton showers, extending the CKKW approach. A flavour-dependent phase space separation criterion is proposed. We show that this new method preserves the logarithmic accuracy of the shower, and that the original proposal can be derived from it. One of the main requirements for the method is a truncated shower algorithm. We outline the corresponding Monte Carlo procedures and apply the new prescription to QCD jet production in e+e- collisions and Drell-Yan lepton pair production. Explicit colour information from matrix elements obtained through colour sampling is incorporated in the merging and the influence of different prescriptions to assign colours in the large N_C limit is studied. We assess the systematic uncertainties of the new method.
- JHEP05 (2009) 053

## Event generation with SHERPA 1.1.

- Authors: T. Gleisberg, S. Hoeche, F. Krauss, M. Schoenherr, S. Schumann, F. Siegert, J. Winter
- 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.
- This is meant to be the standard reference if you are using versions of Sherpa > 1.1.
- JHEP02 (2009) 007

## Soft Photon Radiation in Particle Decays in SHERPA

- Authors: Marek Schoenherr, Frank Krauss
- In this paper the Yennie-Frautschi-Suura approach is used to simulate real and virtual QED corrections in particle decays. It makes use of the universal structure of soft photon corrections to resum the leading logarithmic QED corrections to all orders, and it allows a systematic correction of this approximate result to exact fixed order results from perturbation theory. The approach has been implemented as a Monte Carlo algorithm, which a posteriori modifies decay matrix elements through the emission of varying numbers of photons. The corresponding computer code is incorporated into the SHERPA event generator framework.
- JHEP12 (2008) 018

## Comix, a new matrix element generator

- Authors: T. Gleisberg, S. Hoeche
- We present a new tree-level matrix element generator, based on the colour dressed Berends-Giele recursive relations. We discuss two new algorithms for phase space integration, dedicated to be used with large multiplicities and colour sampling.
- JHEP12 (2008) 039

## How to calculate colourful cross sections efficiently

- Authors: T. Gleisberg, S. Hoeche, F. Krauss, R. Matyskiewicz
- Different methods for the calculation of cross sections with many QCD particles are compared. To this end, CSW vertex rules, Berends-Giele recursion and Feynman-diagram based techniques are implemented as well as various methods for the treatment of colours and phase space integration. We find that typically there is only a small window of jet multiplicities, where the CSW technique has efficiencies comparable or better than both of the other two methods.
- arXiv:0808.3672 [hep-ph ]

## Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions

- Authors: J. Alwall, S. Hoeche, F. Krauss, N. Lavesson, L. Lonnblad, F. Maltoni, M.L. Mangano, M. Moretti, C.G. Papadopoulos, F. Piccinini, S. Schumann, M. Treccani, J. Winter, M. Worek
- 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.
- the long and published version of the extensive generator comparison
- Eur.Phys.J.C53 (2008) 473-500

## Matching parton showers and matrix elements

- Authors: S. Hoeche, F. Krauss, N. Lavesson, L. Lonnblad, M. Mangano, A. Schaelicke, S. Schumann
- 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 scheme and one based on the so-called MLM scheme using different matrix element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.
- hep-ph/0602031

## Supersymmetry simulations with off-shell effects for LHC and ILC

- Authors: K. Hagiwara, W. Kilian, F. Krauss, T. Ohl, T. Plehn, D. Rainwater, J. Reuter, S. Schumann
- At the LHC and at an ILC, serious studies of new physics benefit from a proper simulation of signals and backgrounds. Using supersymmetric sbottom pair production as an example, we show how multi-particle final states are necessary to properly describe off-shell effects induced by QCD, photon radiation, or by intermediate on-shell states. To ensure the correctness of our findings we compare in detail the implementation of the supersymmetric Lagrangian in MadGraph?, Sherpa and Whizard. As a future reference we give the numerical results for several hundred cross sections for the production of supersymmetric particles, checked with all three codes.
- Phys.Rev.D73 (2005) 055005

## Studying W+W- production at the Fermilab Tevatron with SHERPA

- Authors: T. Gleisberg, F. Krauss, A. Schaelicke, S. Schumann, J.C. Winter
- The merging procedure of tree-level matrix elements with the subsequent parton shower as implemented in SHERPA will be studied for the example of W boson pair production at the Fermilab Tevatron. Comparisons with fixed order calculations at leading and next-to-leading order in the strong coupling constant and with other Monte Carlo simulations validate once more the impact and the quality of the merging algorithm and its implementation.
- Phys.Rev.D72 (2005) 034028

## Implementing the ME+PS merging Algorithm.

- Authors: A. Schaelicke, F. Krauss
- The method to merge matrix elements for multi particle production and parton showers in electron-positron annihilations and hadronic collisions and its implementation into the new event generator SHERPA is described in detail. Examples highlighting different aspects of it are thoroughly discussed, some results for various cases are presented. In addition, a way to extend this method to general electroweak interactions is presented.
- JHEP 0507 (2005) 018

## Simulating W/Z + jets production at the CERN LHC.

- Authors: F. Krauss, A. Schaelicke, S. Schumann, G.Soff
- The merging procedure of tree-level matrix elements and the subsequent parton shower as implemented in the new event generator SHERPA will be validated for the example of single gauge boson production at the LHC. The validation includes consistency checks and comparisons to results obtained from other event generators. In particular, comparisons with full next-to-leading order QCD calculations prove SHERPA's ability to correctly account for additional hard QCD radiation present in these processes.
- Phys.Rev.D72 (2005) 054017

## APACIC++ 2.0: A PArton Cascade In C++.

- Authors: A. Schaelicke, F. Krauss, G. Soff
- The new version of the parton shower module APACIC++ for the SHERPA event generator framework is presented. It incorporates some features, that are specific for the consistent merging with multi-particle matrix elements at tree-level. This publication also includes some exemplary results and a short description of the upgraded class structure of APACIC++, version 2.0.
- Comput.Phys.Commun.174 (2006) 876

## Simulating W/Z + jets production at the Tevatron.

- Authors: F. Krauss, A. Schaelicke, S. Schumann, G.Soff
- The merging procedure of tree-level matrix elements and the subsequent parton shower as implemented in the new event generator SHERPA will be validated for the example of W/Z+jets production at the Tevatron. Comparisons with results obtained from other approaches and programs and with experimental results clearly show that the merging procedure yields relevant and correct results at both the hadron and parton levels.
- Phys.Rev.D70 (2004) 114009

## Cross sections for multi-particle final states at a linear collider.

- Authors: T. Gleisberg, F. Krauss, C.G. Papadopoulos, A. Schaelicke, S. Schumann
- In this paper total cross sections for signals and backgrounds of top- and Higgs-production channels in electron-positron collisions at a future linear collider are presented. All channels considered are characterized by the emergence of six-particle final states. The calculation takes into account the full set of tree-level amplitudes in each process. Two multi-purpose parton level generators, HELAC/PHEGAS and AMEGIC++ are used, their results are found to be in perfect agreement.
- Eur.Phys.J.C34 (2004) 173-180

## SHERPA 1.alpha, a proof-of-concept version.

- Authors: T. Gleisberg, S. Hoeche, F. Krauss, A. Schaelicke, S. Schumann, J. Winter
- 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.
- JHEP02 (2004) 056

## A modified cluster hadronization model.

- Authors: J. Winter, F. Krauss, G. Soff
- A new phenomenological cluster-hadronization model is presented. Its specific features are the incorporation of soft colour reconnection, a more general treatment of diquarks including their spin and giving rise to clusters with baryonic quantum numbers, and a dynamic separation of the regimes of clusters and hadrons according to their masses and flavours. The distinction between the two regions automatically leads to different cluster decay and transformation modes. Additionally, these aspects require an extension of individual cluster-decay channels that were available in previous versions of such models.
- Eur.Phys.J.C36 (2004) 381-395

## Helicity formalism for spin-2 particles

- Authors: T. Gleisberg, F. Krauss, K. Matchev, A. Schaelicke, S. Schumann, G. Soff
- We develop the helicity formalism for spin-2 particles and apply it to the case of gravity in flat extra dimensions. We then implement the large extra dimensions scenario of Arkani-Hamed, Dimopoulos and Dvali in the program AMEGIC++, allowing for an easy calculation of arbitrary processes involving the emission or exchange of gravitons. We complete the set of Feynman rules derived by Han, Lykken and Zhang, and perform several consistency checks of our implementation.
- JHEP09 (2003) 001

## Matrix elements and parton showers in hadronic interactions

- Authors: F. Krauss
- A method is suggested to combine tree level QCD matrix for the production of multi jet final states and the parton shower in hadronic interactions. The method follows closely an algorithm developed recently for the case of $e
^{+e}-$ annihilations. - JHEP08 (2002) 015

## Implementing initial state radiation for lepton induced processes in Amegic++

- Authors: A. Schalicke, F. Krauss, R. Kuhn, G. Soff
- We have implemented the method of Yennie, Frautschi, and Suura up to first order in alpha for the simulation of QED Initial State Radiation in lepton induced processes in AMEGIC++. We consider s-channel processes via the exchange of scalar or vector resonances at electron and muon colliders.
- JHEP12 (2002) 013

## AMEGIC++ 1.0: A Matrix Element Generator In C++

- Authors: F. Krauss, R. Kuhn, G. Soff
- The new matrix element generator AMEGIC++ is introduced, dedicated to describe multi-particle production in high-energy particle collisions. It automatically generates helicity amplitudes for the processes under consideration and constructs suitable, efficient integration channels for the multi-channel phase space integration. The corresponding expressions for the amplitudes and the integrators are stored in library files to be linked to the main program.
- JHEP02 (2002) 044

## QCD Matrix elements + parton showers

- Authors: S. Catani, F. Krauss, R. Kuhn, B.R. Webber
- We propose a method for combining QCD matrix elements and parton showers in Monte Carlo simulations of hadronic final states in $e
^{+e}-$ annihilation. The matrix element and parton shower domains are separated at some value $y_{ini}$ of the jet resolution, defined according to the $k_T$-clustering algorithm. The matrix elements are modified by Sudakov form factors and the parton showers are subjected to a veto procedure to cancel dependence on $y_{ini}$ to next-to-leading logarithmic accuracy. The method provides a leading-order description of hard multi-jet configurations together with jet fragmentation, while avoiding the most serious problems of double counting. We present first results of an approximate implementation using the eventgenerator APACIC++. - JHEP11 (2001) 063

## APACIC++: A PArton Cascade In C++, version 1.0

- Authors: R. Kuhn, F. Krauss, B. Ivanyi, G. Soff
- APACIC++ is a Monte-Carlo event-generator dedicated for the simulation of electron-positron annihilations into jets. Within the framework of APACIC++, the emergence of jets is identified with the perturbative production of partons as governed by corresponding matrix elements. In addition to the build-in matrix elements describing the production of two and three jets, further programs can be linked allowing for the simultaneous treatment of higher numbers of jets. APACIC++ hosts a new approach for the combination of arbitrary matrix elements for the production of jets with the parton shower, which in turn models the evolution of these jets. For the evolution, different ordering schemes are available, namely ordering by virtualities or by angles. At the present state, the subsequent hadronization of the partons is accomplished by means of the Lund-string model as provided within Pythia. An appropriate interface is provieded. The program takes full advantage of the object-oriented features provided by C++ allowing for an equally abstract and transparent programming style.
- Comput. Phys. Commun.134 (2001) 223