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

TL;DR

SHERPA 1.alpha introduces a modular, object-oriented framework in C++ for comprehensive high-energy event generation, integrating matrix elements, parton showers, and hadronisation through abstract interfaces. It provides two matrix-element engines (AMEGIC++ and EXTRA_XS) and a blob-based event record managed by a cohesive workflow, including beam spectra and PDFs for realistic initial states. The alpha version demonstrates electron-positron, gamma-gamma, and hadronic collisions, while outlining plans for cluster hadronisation (AHADIC++), underlying-event modeling, and spin-correlation improvements. This work delivers a flexible platform to prototype and compare advanced event-generation strategies, enabling systematic testing and integration of new physics models with experimental analyses.

Abstract

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.

Figures (2)

• Figure 1: Pictorial representation of the event record. In the left picture, a hadron--hadron collision is exhibited. Clearly, apart from the hard signal subprocess followed by hard decays of two heavy unstable particles, it also contains two more hard parton interactions, all of them shown as thick blobs. The partons are dressed with secondary radiation as well, before the parton ensemble is transformed into primary hadrons which then decay further. On the right this is translated into the language of Blobs. Here, each hard matrix-element Blob (red) is equipped with merging Blobs (green) in the initial and final state which define initial conditions for the parton shower. All extra partons emitted during the shower evolution are combined in individual shower Blobs (blue). In the hadronisation Blobs (magenta) colour singlet chains formed by incoming partons are translated into primary hadrons which might decay further. Each such hadron decay is represented by an extra Blob.
• Figure 2: The Event_Phase_Handlers and their interfaces, all of which are implemented up to now in SHERPA.