# Publications¶

**A practical guide to event generation for prompt photon production**

Authors: Frank Siegert

The production of prompt photons is one of the most relevant scattering processes studied at hadron-hadron colliders in recent years. This article will give an overview of the different approaches used to simulate prompt photon production in the Sherpa event generator framework. Special emphasis is placed on a complete simulation of this process including fragmentation configurations. As a novel application a merged simulation of γγ and γγ+jet production at NLO accuracy is presented and compared to measurements from the ATLAS experiment.

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

Authors: Stefan Höche, Philipp Maierhoefer, Niccolo Moretti, Stefano Pozzorini, Frank Siegert

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.

**The midpoint between dipole and parton showers**

Authors: Stefan Hoeche, Stefan Prestel

We present a new parton-shower algorithm. Borrowing from the basic ideas of dipole cascades, the evolution variable is judiciously chosen as the transverse momentum in the soft limit. This leads to a very simple analytic structure of the evolution. A weighting algorithm is implemented, that allows to consistently treat potentially negative values of the splitting functions and the parton distributions. We provide two independent, publicly available implementations for the two event generators Pythia and Sherpa.

**Beyond Standard Model calculations with Sherpa**

Authors: Stefan Hoeche, Silvan Kuttimalai, Steffen Schumann, Frank Siegert

We present a fully automated framework as part of the Sherpa event generator for the computation of tree-level cross sections in beyond Standard Model scenarios, making use of model information given in the Universal FeynRules? Output format. Elementary vertices are implemented into C++ code automatically and provided to the matrix-element generator Comix at runtime. Widths and branching ratios for unstable particles are computed from the same building blocks. The corresponding decays are simulated with spin correlations. Parton showers, QED radiation and hadronization are added by Sherpa, providing a full simulation of arbitrary BSM processes at the hadron level.

**Soft evolution of multi-jet final states**

Authors: Erik Gerwick, Stefan Hoeche, Simone Marzani, Steffen Schumann

We present a new framework for computing resummed and matched distributions in processes with many hard QCD jets. The intricate color structure of soft gluon emission at large angles renders resummed calculations highly non-trivial in this case. We automate all ingredients necessary for the color evolution of the soft function at next-to-leading-logarithmic accuracy, namely the selection of the color bases and the projections of color operators and Born amplitudes onto those bases. Explicit results for all QCD processes with up to 2→5 partons are given. We also devise a new tree-level matching scheme for resummed calculations which exploits a quasi-local subtraction based on the Catani-Seymour dipole formalism. We implement both resummation and matching in the Sherpa event generator. As a proof of concept, we compute the resummed and matched transverse-thrust distribution for hadronic collisions.

**Higgs-boson production through gluon fusion at NNLO QCD with parton showers**

Authors: Stefan Hoeche, Ye Li, Stefan Prestel

We discuss how the UN2LOPS scheme for matching NNLO calculations to parton showers can be applied to processes with large higher-order perturbative QCD corrections. We focus on Higgs-boson production through gluon fusion as an example. We also present an NNLO fixed-order event generator for this reaction.

**Drell-Yan lepton pair production at NNLO QCD with parton showers**

Authors: Stefan Hoeche, Ye Li, Stefan Prestel

We present a simple approach to combine NNLO QCD calculations and parton showers, based on the UNLOPS technique. We apply the method to the computation of Drell-Yan lepton-pair production at the Large Hadron Collider. We comment on possible improvements and intrinsic uncertainties.

**Triple vector boson production through Higgs-Strahlung with NLO multijet merging**

Authors: Stefan Hoeche, Frank Krauss, Stefano Pozzorini, Marek Schoenherr, Jennifer M. Thompson, Korinna C. Zapp

Triple gauge boson hadroproduction, in particular the production of three W-bosons at the LHC, is considered at next-to leading order accuracy in QCD. The NLO matrix elements are combined with parton showers. Multijet merging is invoked such that NLO matrix elements with one additional jet are also included. The studies here incorporate both the signal and all relevant backgrounds for VH production with the subsequent decay of the Higgs boson into W- or τ-pairs. They have been performed using Sherpa+OpenLoops? in combination with Collier.

**Next-to-leading order QCD predictions for top-quark pair production with up to two jets merged with a parton shower**

Authors: Stefan Hoeche, Frank Krauss, Philipp Maierhoefer, Stefano Pozzorini, Marek Schonherr, Frank Siegert

We present differential cross sections for the production of top-quark pairs in conjunction with up to two jets, computed at next-to leading order in perturbative QCD and consistently merged with a parton shower in the Sherpa+OpenLoops? framework. Top quark decays including spin correlation effects are taken into account at leading order accuracy. The calculation yields a unified description of top-pair plus multi-jet production, and detailed results are presented for various key observables at the Large Hadron Collider. A large improvement is found for the total transverse energy spectrum, which plays a prominent role in searches for physics beyond the Standard Model.

**Uncertainties in MEPS@NLO calculations of h+jets**

Authors: Stefan Hoeche, Frank Krauss, Marek Schoenherr

Uncertainties in the simulation of Higgs boson production with up to two jets at next-to leading order accuracy are investigated. Traditional uncertainty estimates based on scale variations are extended employing different functional forms for the central scale, and the impact of details in the implementation of the parton shower is discussed.

**NLO matching for ttbb production with massive b-quarks**

Authors: Fabio Cascioli, Philipp Maierhoefer, Niccolo Moretti, Stefano Pozzorini, Frank Siegert

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.

*Precise Higgs-background predictions: merging NLO QCD and squared quark-loop corrections to four-lepton + 0,1 jet prod* uction**

Authors: Fabio Cascioli, Stefan Hoeche, Frank Krauss, Philipp Maierhofer, Stefano Pozzorini, Frank Siegert

We present precise predictions for four-lepton plus jets production at the LHC obtained within the fully automated Sherpa+OpenLoops? framework. Off-shell intermediate vector bosons and related interferences are consistently included using the complex-mass scheme. Four-lepton plus 0- and 1-jet final states are described at NLO accuracy, and the precision of the simulation is further increased by squared quark-loop NNLO contributions in the gg -> 4l, gg -> 4l+g, gq -> 4l+q, and qq -> 4l+g channels. These NLO and NNLO contributions are matched to the Sherpa parton shower, and the 0- and 1-jet final states are consistently merged using the MEPS@NLO technique. Thanks to Sudakov resummation, the parton shower provides improved predictions and uncertainty estimates for exclusive observables. This is important when jet vetoes or jet bins are used to separate four-lepton final states arising from Higgs decays, diboson production, and top-pair production. Detailed predictions are presented for the ATLAS and CMS H->WW analyses at 8 TeV in the 0- and 1-jet bins. Assessing renormalisation-, factorisation- and resummation-scale uncertainties, which reflect also unknown subleading Sudakov logarithms in jet bins, we find that residual perturbative uncertainties are as small as a few percent.

**Zero and one jet combined NLO analysis of the top quark forward-backward asymmetry**

Authors: Stefan Hoeche, Junwu Huang, Gionata Luisoni, Marek Schoenherr, Jan Winter

We present an analysis of the forward-backward asymmetry in the production of top quark pairs at the Tevatron collider. We use novel Monte Carlo methods for merging matrix elements and parton showers to combine NLO QCD predictions for tt and tt+jet production. Theoretical uncertainties are quantified in detail. We find agreement with experimental data on the transverse momentum dependence of the asymmetry.

**Uncertainties in NLO + parton shower matched simulations of inclusive jet and dijet production**

Authors: Stefan Hoeche, Marek Schonherr

We quantify uncertainties in the Monte-Carlo simulation of inclusive and dijet final states, which arise from using the MC@NLO technique for matching next-to-leading order parton level calculations and parton showers. We analyse a large variety of data from early measurements at the LHC. In regions of phase space where Sudakov logarithms dominate over high-energy effects, we observe that the main uncertainty can be ascribed to the free parameters of the parton shower. In complementary regions, the main uncertainty stems from the considerable freedom in the simulation of underlying events.

**NLO QCD matrix elements + parton showers in e+e- -> hadrons**

Authors: Thomas Gehrmann, Stefan Hoeche, Frank Krauss, Marek Schonherr, Frank Siegert

We present a new approach to combine multiple NLO parton-level calculations matched to parton showers into a single inclusive event sample. The method provides a description of hard multi-jet configurations at next-to leading order in the perturbative expansion of QCD, and it is supplemented with the all-orders resummed modelling of jet fragmentation provided by the parton shower. The formal accuracy of this technique is discussed in detail, invoking the example of electron-positron annihilation into hadrons. We focus on the effect of renormalisation scale variations in particular. Comparison with experimental data from LEP underlines that this novel formalism describes data with a theoretical accuracy that has hitherto not been achieved in standard Monte Carlo event generators.

**QCD matrix elements + parton showers: The NLO case**

Authors: Stefan Hoeche, Frank Krauss, Marek Schonherr, Frank Siegert

We present a process-independent technique to consistently combine next-to-leading order parton-level calculations of varying jet multiplicity and parton showers. Double counting is avoided by means of a modified truncated shower scheme. This method preserves both the fixed-order accuracy of the parton-level result and the logarithmic accuracy of the parton shower. We discuss the renormalisation and factorisation scale dependence of the approach and present results from an automated implementation in the Sherpa event generator using the test case of W-boson production at the Large Hadron Collider. We observe a dramatic reduction of theoretical uncertainties compared to existing methods which underlines the predictive power of our novel technique.

**W+n-jet predictions with MC@NLO in Sherpa**

Authors: Stefan Hoeche, Frank Krauss, Marek Schonherr, Frank Siegert

Results for the production of W-bosons in conjunction with up to three jets including parton shower corrections are presented and compared to recent LHC data. These results consistently incorporate the full next-to-leading order QCD corrections through the MC@NLO method, as implemented in the Sherpa event generator, with the virtual corrections obtained from the BlackHat? library.

**A critical appraisal of NLO+PS matching methods**

Authors: Stefan Hoeche, Frank Krauss, Marek Schonherr, Frank Siegert

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 are employed to assess the impact on a representative selection of observables. In the MC@NLO approach a phase space restriction has been added to subtraction and parton shower, 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.

**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.

**Automating the POWHEG method in Sherpa**

Authors: 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.

**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.

**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.

**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.

**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.

**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.

**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.

**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.

**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

**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.

**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.

**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.

**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.

**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.

**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.

**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.

**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.

**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.

**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.

**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.

**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.

**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.

**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.

**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++.

**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.