LanHEP - a package for automatic generation of Feynman rules from the Lagrangian. Updated version 3.2

We present a new version 3.2 of the LanHEP software package. New features include UFO output, color sextet particles and new substutution techniques which allow to define new routines.

💡 Research Summary

This paper presents the updated version 3.2 of the LanHEP software package, a dedicated tool for the automatic generation of Feynman rules from a Lagrangian written in a compact, publication-style notation. The new version introduces several significant features that expand its interoperability, physical scope, and user customization capabilities.

The most prominent addition is the support for output in the UFO (Universal FeynRules Output) format. By using the command-line option -ufo, users can now generate model files compatible with this widely adopted standard, allowing for seamless integration of LanHEP-generated rules into modern Monte Carlo event generator chains like MadGraph5_aMC@NLO, thereby greatly enhancing the utility of the developed models for phenomenological studies.

A major extension in the physics scope is the native implementation of color sextet particles. LanHEP now supports the definition of particles belonging to the 6-dimensional representation of the SU(3) color group (and its conjugate) using the color c6 and color c6b keywords. Corresponding invariant tensors (k_c6, k_c6b for sextet-triplet interactions and l_c6 for the sextet-gluon interaction) are introduced, enabling the description of the covariant derivative for such particles. This feature facilitates the study of Beyond the Standard Model theories involving exotic color structures, though it is currently supported only for CalcHEP output.

To aid in the implementation of specific loop-induced processes and effective operators, a new coefficient extraction function CoefVrt has been introduced. This function allows users to isolate the numerical factor corresponding to a specific tensor structure (e.g., scalar 1, pseudoscalar γ5, vector γμ, or momentum-dependent pμ) from a generated vertex. This is particularly valuable for separately implementing the contributions of different particle species to vertices like Hγγ or Hgg, which arise at the loop level.

Finally, the update brings a powerful and generalized Alias system, which acts as a macro processor for model definitions. Aliases can define substitution rules applied to all subsequent statements, ranging from simple parameter renaming to complex, argument-taking routines that generate new particle or parameter declarations. Features like string concatenation (using #), the definition of local temporary symbols, and control over alias application (using the unary + operator) provide a high degree of abstraction. This allows users to create custom, concise syntax for repetitive definitions, dramatically improving the readability and maintainability of complex model files.

In summary, LanHEP v3.2 evolves from a specialized Feynman rule generator into a more versatile and user-friendly model-building framework. By embracing the standard UFO format, extending support to non-standard color representations, providing tools for precise coefficient manipulation, and offering sophisticated macro capabilities, it significantly empowers theorists in constructing, testing, and deploying new physics models for computational analysis.