A multi-event interface for next-to-leading order calculations in MadGraph5_aMC@NLO

TL;DR

This paper introduces a multi-event interface and multithreaded implementation for NLO calculations in MadGraph5_aMC@NLO, enhancing computational efficiency and paving the way for data parallelism in QCD event generation.

Contribution

It presents a novel multi-event interface and multithreaded implementation for NLO calculations, optimizing amplitude evaluations for parallel processing.

Findings

Tree-level amplitude evaluations dominate NLO QCD event generation runtime.

The multi-event interface enables efficient evaluation of multiple phase space points.

Parallelization significantly improves computational performance.

Abstract

We detail the implementation of a multi-event interface for next-to-leading order (NLO) calculations in MadGraph5_aMC@NLO, allowing tree-level scattering amplitudes for multiple phase space points to be evaluated in each call to the integrated NLO differential cross section during event generation. Additionally, a multithreaded implementation based on this multi-event interface where tree-level amplitudes are evaluated in parallel across multiple CPU threads is presented for the Monte Carlo generation of quantum chromodynamical (QCD) events. Although this work primarily concerns the implemented code, some algorithmic changes involving the order of the application of phase-space cuts and calls to different scattering amplitudes are included. The codebase currently supports multi-threaded execution, but these changes pave the way for continued data parallelism in the form of on-CPU SIMD instructions or SIMT GPU offloading. A study in the runtime fraction spent in different diagrammatic contributions across various processes suggests that NLO QCD event generation are computationally dominated by tree-level scattering amplitude evaluations, which we show are perfectly suited for data parallelisation.