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## The FeynArts interface

### by C. Duhr

FeynArts is a Mathematica package for the generation and visualization of Feynman diagrams and amplitudes. The source code can be downloaded from www.feynarts.de.

#### Generating a new FeynArts model with FeynRules

The FeynRules interface to FeynArts enables the user to write out a FeynArts model file for is new physics model just by knowing the Lagrangian and the FeynRules model file. As the syntax of the FeynRules model files is similar to the one used in FeynArts, most of the information is just copied from one model file to the other.In addition, the FeynArts model file contains the object `M$CouplingMatrices`, which contains all the interactions and vertices for this model.

The FeynArts interface is called via the command

WriteFeynArtsOutput[L1, L2, L3,..., options]

where `L1`, `L2`, `L3`, ... denote the Lagrangians which should be used to calculate Feynman rules from. `WriteFeynArtsOutput` shares all the options of `FeynmanRules`. In addition, it has thefollowing two options:

`Output`: The name of the FeynArts model file written. The default is`M$ModelName`with the extension ".mod" appended.`CouplingRename`: Whether to rename vertices with a new name and store the definitions in`M$FACouplings`. The default is`True`.

For further details on the FeynArts interface, have a look at the user manual.

This creates the FeynArts model file `file.mod` in the current working directory. This model file can then be copied to the FeynArts `/Model/` subdirectory, and the model can be used in the usual way.

#### Features and limitations

- The FeynArts interface allows for the use of ghost fields, and in particular for the use of the Feynman gauge in the Standard model. The SM lagrangian in the Feynman gauge is obtained by putting the variable
`FeynmanGauge`to`True`after the model file has been loaded. - Currently, the only generic couplings supported are the standard generic couplings of
`Lorentz.gen`. - The model files created by FeynRules are compliant with FormCalc.