The Implementation of the Minimal Supersymmetric Standard Model in FeynArts and FormCalc

TL;DR

This work implements the Minimal Supersymmetric Standard Model within the FeynArts/FormCalc framework, enabling nearly fully automatic loop calculations. It introduces two main components: a flexible MSSM model file (MSSM.mod and MSSMQCD.mod) for diagram generation and a FormCalc-based Fortran initializer (mssm_ini.F) that derives MSSM parameters from a compact input set, including complex parameters. The methodology combines a SUSY-preserving algebraic simplification, a robust diagram-generation strategy via the flipping-rule, and detailed numerical treatment of the Higgs, chargino, neutralino, and sfermion sectors, with validation against established results. The work provides ready-to-use tools and documentation for researchers performing MSSM loop calculations, with open-source availability and specified requirements.

Abstract

We describe the implementation of the MSSM in the diagram generator FeynArts and the calculational tool FormCalc. This extension allows to perform loop calculations of MSSM processes almost fully automatically. The actual implementation has two aspects: The MSSM Feynman rules are specified in a new model file for FeynArts. The computation of the parameters in the MSSM Lagrangian from the input parameters is realized as a Fortran subroutine in the framework of FormCalc. The model file does not depend on the latter, however, and can be used even if one does not want to continue the calculation with FormCalc. The Feynman rules have been entered in a very generic way to allow e.g. scenarios with complex parameters, and have been tested extensively by reproducing known results for several non-trivial scattering processes.