Frank Krauss

Event generation with SHERPA 1.1

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.

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

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.

QCD Matrix Elements + Parton Showers

We propose a method for combining QCD matrix elements and parton showers in Monte Carlo simulations of hadronic flnal states in e + e i annihilation. The matrix element and parton shower domains are separated at some value yini of the jet resolution, deflned according to the kT-clustering algorithm. The matrix elements are modifled by Sudakov form factors and the parton showers are subjected to a veto procedure to cancel dependence on yini to next-to-leading logarithmic accuracy. The method provides a leading-order description of hard multi-jet conflgurations together with jet fragmentation, while avoiding the most serious problems of double counting. We present flrst results of an approximate implementation using the event generator APACIC++.

A parton shower algorithm based on Catani-Seymour dipole factorisation

In this publication the implementation of a new parton shower model based on the Catani-Seymour dipole factorisation, as first suggested by [1,?2], is discussed. First results obtained with the new algorithm are compared with experimental data.

General-purpose event generators for LHC physics

We review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider. Topics included are: the generation of hardscattering matrix elements for processes of interest, at both leading and nextto-leading QCD perturbative order; their matching to approximate treatments of higher orders based on the showering approximation; the parton and dipole shower formulations; parton distribution functions for event generators; non-perturbative aspects such as soft QCD collisions, the underlying event and diractive processes; the string and cluster models for hadron formation; the treatment of hadron and tau decays; the inclusion of QED radiation and beyond-Standard-Model processes. We describe the principal features of the Ariadne, Herwig++, Pythia 8 and Sherpa generators, together with the Rivet and Professor validation and tuning tools, and discuss the physics philosophy behind the proper use of these generators and tools. This review is aimed at phenomenologists wishing to understand better how parton-level predictions are translated into hadron-level events as well as experimentalists wanting a deeper insight into the tools available for signal and background simulation at the LHC.

QCD Matrix Elements + Parton Showers: The NLO Case

A bstractWe 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.

SHERPA 1., a proof-of-concept version

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.

NLO matrix elements and truncated showers

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 [1] with the Powheg method, which combines exact next-to-leading order matrix element results with the parton shower [2, 3]. It was developed in parallel to the MEnloPS technique discussed in [4] and has been implemented in the event generator Sherpa [5, 6]. 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.

Matrix elements and parton showers in hadronic interactions

A method is suggested to combine tree level QCD matrix elements 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 \cite{Catani:2001cc}.

AMEGIC++ 1.0: A Matrix element generator in C++

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.