HELAC-PHEGAS: automatic computation of helicity amplitudes and cross sections

TL;DR

The paper presents HELAC-PHEGAS, a FORTRAN-based framework for automatic, efficient calculation of tree-level helicity amplitudes and cross sections across arbitrary SM and QCD processes. It combines Dyson–Schwinger recursion for amplitudes with diagram-informed phase-space mappings and multichannel Monte Carlo integration to overcome factorial growth in event generation. Key contributions include a scalable amplitude computation method, a color-aware QCD treatment, and a phase-space strategy that enables reliable handling of multiparticle final states, all with optional extended precision. The work lays a foundation for higher-order corrections and beyond-Standard-Model extensions within a unified, automatic toolkit.

Abstract

HELAC-PHEGAS is a FORTRAN based package that is able to compute automatically and efficiently tree-order helicity amplitudes and cross sections for arbitrary scattering processes within the standard electroweak theory and QCD. The algorithm for the amplitude computation, HELAC, exploits the virtues of the Dyson-Schwinger equations. The phase-space generation algorithm, PHEGAS, constructs all possible kinematical mappings dictated by the amplitude under consideration. Combined with mutichannel self-optimized Monte Carlo integration it results to efficient cross section evaluation.