Event generation with SHERPA 1.1 (0811.4622v1)

Abstract: 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.

• The paper details Sherpa 1.1’s modular design that simulates both Standard Model and beyond Standard Model processes with high precision.
• It employs advanced techniques, including matrix element generation and DGLAP-based parton showers, to realistically model LHC collision dynamics.
• The framework’s flexible architecture supports future upgrades, such as enhanced NLO accuracy and increased automation for complex scattering calculations.

An Overview of the Sherpa Event Generator (Version 1.1)

The described paper provides an extensive examination of Sherpa, a versatile Monte Carlo event generator designed for simulating particle collisions in high-energy physics experiments. This paper details the latest iteration of Sherpa, version 1.1, outlining its architecture, modular components, and the physics processes it can simulate, particularly in the context of the Large Hadron Collider (LHC) environment.

Key Features and Contributions

Sherpa stands out due to its modularity and comprehensive capability to simulate complex physics processes. It encompasses several key computational modules:

1. Matrix Element Generator: Sherpa's matrix-element generator supports a wide range of processes in the Standard Model (SM) and several beyond the Standard Model (BSM) theories. It uses Feynman diagrams translated into helicity amplitudes and integrates over phase spaces to compute cross sections.
2. Parton Shower Models: Implementing both initial-state and final-state radiation, Sherpa uses DGLAP-based algorithms for parton showering, ensuring a realistic simulation of QCD radiation effects. The algorithm maintains coherence by angular ordering of emissions, enhancing its precision in partonic evolution.
3. Multi-Parton Interactions: The generator models multiple parton interactions (MPI) to account for underlying events in hadron-hadron collisions, a necessary feature for fidelity in simulating LHC events.
4. Hadronization and Decays: Sherpa includes a cluster-based hadronization model, transitioning colored partons into hadronized states while preserving phenomenological realism. The framework for hadron and τ-lepton decays is robust, accommodating various decay channels with spin correlations.
5. QED Radiation: Through the Yennie-Frautschi-Suura algorithm, Sherpa incorporates corrections due to photon radiation, crucial for adjustments in particle production and decay cross-sections.

Numerical Results

Sherpa 1.1 has demonstrated strong performance through rigorous testing against other established generators and alignment with experimental data from LHC and beyond. The consistent agreement in predictions for SM processes such as W/Z production in association with jets and t̅t production exhibits its reliability and precision.

Implications and Future Directions

Sherpa provides a crucial toolset for the phenomenological community, offering detailed insights into both SM backgrounds and potential new physics at collider experiments. Its ability to incorporate and test new physics models modularly makes it a valuable resource for particle physics research.

Potential future directions indicated by the paper include enhancements towards next-to-leading order (NLO) accuracy and increased automation in handling complex scattering amplitudes. The continuous effort in improving module efficiency and expanding its particle physics model repertoire will potentially allow researchers to make increasingly precise predictions at future experimental conditions, such as those anticipated at the High-Luminosity LHC.

In conclusion, Sherpa 1.1 establishes itself as a powerful, flexible tool essential for analyzing and understanding high-energy particle interactions. Its comprehensive framework provides a reliable simulation environment essential for the success of current and future collider research endeavors.