The MadGraph interface (C. Duhr, M. Herquet)

MadGraph /MadEvent is a software that allows you to generate amplitudes and events for any process (with up to 9 external particles) in any model. The source code can be downloaded from ​madgraph.phys.ucl.ac.be.

Generating a new MadGraph model with FeynRules

The FeynRules interface to MadGraph enables the user to implement his new physics model into MadGraph just by knowing the Lagrangian and the FeynRules model file. The interface is called via

WriteMGOutput[L1, L2, L3,...]

where L1, L2, L3, ... denote the lagrangians which should be used to calculate Feynman rules from.

WriteMGOutput has two options:

• Output:The name of the directory where the MadGraph model files should be written. The default is M$ModelName with the extension " MG" appended.
• DecomposeGluonVertex: If False, the four-gluon vertex is not decomposed into three-point vertices. The default value is True.

Notice that by default the MadGraph interface decomposes the four-gluon vertex using an auxiliary tensor particle. This property can be turned off by putting the option DecomposeGluonVertex to False. In particular, this decomposition allows to include five-dimensional operators describing the effective coupling of (pseudo-)scalars to gluons in the large top-mass limit (See the user manual for more details).

Running the interface creates in the current working directory a new subdirectory ModelNameMG, where ModelName is the name of the the FeynRules model file. This directory contains all the files needed for the implementation of the new model into MadGraph. After this directory is copied into the MadGraph /Models/ subdirectory, it can be used just like any other MadGraph model.

Let us note that the FeynRules-generated models contain a test program called 'testprog' that can be run to test that it is working. This can be run with the command:

make testprog
./testprog

If successful, the output will be a listing of the numerical values of all the couplings and masses in the model. The model can now be used in MadGraph in the usual way.

Features and limitations

• The MadGraph interface follows the MC conventions described in the FeynRules user manual.
• The four-gluon vertex is by default decomposed into three-point vertices using an auxiliary tensor particle.
• Effective operators describing the coupling of (pseudo-)scalars to the gluons in the large top-mass limit is supported. This is especially useful to model gluon fusion processes.
• If the interface is run under Windows, the files obtained cannot be immediately compiled under Unix, because Windows uses a different ASCII coding for the line breaks. In this case, the files have to be copy/pasted and saved again under Unix.