## MadGraph 5 Processes

By far the easiest way to implement new processes into PYTHIA 8 is by using the matrix-element generator MadGraph 5. This program has an option to output the results of a matrix-element calculation as a set of PYTHIA 8 C++ classes (plus further auxiliary code), that can then be linked and used as semi-internal processes, meaning they are handled identically with normal internal ones. This way, MadGraph 5 can be used to implement processes from any model that can be written in terms of a Lagrangian. Any 2 → 1, 2 → 2 and 2 → 3 processes can be implemented, the limit being set by the absence of efficient phase space generator algorithms for higher multiplicities in PYTHIA. Features such as s-channel resonances are automatically implemented in the process classes. Besides the process library and necessary model files, also an example main program is generated for each set of processes, which can be easily modified to perform the desired analyses.

In order to create a PYTHIA 8 process library with MadGraph 5, first download the MadGraph 5 package from https://launchpad.net/madgraph5, and untar the package. You can then specify the location of your `pythia81xx` directory in the file `input/mg5_configuration.txt`:
`pythia8_path = ./pythia81xx`
The location can be either relative (to the directory `MadGraph5_v_x_x_x/.`) or absolute.

For any model that is already implemented in the MadGraph 5 package, you can directly use the model. Start the MadGraph 5 interface `bin/mg5`, and do:

    import model model_name
    generate your_process_in_mg5_syntax
    add process your_next_process_in_mg5_syntax
    ...
    output pythia8 [path_to_pythia81xx_directory]

For examples of MG5 process syntax, please see http://madgraph.phys.ucl.ac.be/EXAMPLES/example_mg5.html or type `help generate`. If you specified the path to the `pythia81xx` directory in the `mg5_configuration` file, you do not need to enter it in the `output` command.

If your preferred model is found on the FeynRules model wiki page, http://feynrules.irmp.ucl.ac.be/wiki/ModelDatabaseMainPage, download the UFO (Universal FeynRules Output) tar file for the model, untar in the `models/` directory, and use as above.

If you want to implement a new model which has not yet been implemented, you can do this either using the Mathematica package FeynRules (see http://feynrules.irmp.ucl.ac.be/) or directly edit the UFO model files of the most similar model in the `models/` directory.

The resulting output from the `output pythia8` command is:

- A process directory `Processes_modelname` with the model information and the files needed for all processes defined for this model, placed in the `pythia81xx` main directory. The model files are `Parameters_modelname.h/cc` and `HelAmps_modelname.h/cc`, and the process files for each process class (with the same mass, spin and color of the initial/final state particles) are called `Sigma_modelname_processname.h/cc`. The directory also contains a `makefile` and a model parameter file `param_card_modelname.dat`.
- An example main program in the directory `examples/` (in the `pythia81xx` main directory) called `main_modelname_N.cc` and a corresponding makefile `Makefile_modelname_N`. This main program links in the process classes in the process directory described above. To run the example main program, just go to the `examples/` directory and run
  `make -f Makefile_modelname_N`
  or run `launch` directly inside the MadGraph 5 command line interface.

Note that in order for PYTHIA 8 to be able to automatically decay any new particles, it is necessary to specify the branching ratios of the particles in the `param_card` file, see [Ska04,Alw07] for details.

For further technical details, please see the MadGraph 5 release paper [Alw11] and the semi-internal processes page.

Of course, as with MadGraph 4, MadGraph 5 can also output files of parton-level events according to the LHEF standard, that can be read in and processed further by PYTHIA 8. The advantage is that then the MadGraph 5 phase space generator can be used, which opens up for processes with more than three particles in the final state. The disadvantages are that it is less easy to mix and match with existing PYTHIA processes, and that one needs to regenerate and store large LHEF files for different kinematics cuts or parameter values.

Please cite the MadGraph 5 release paper [Alw11] if you use MadGraph 5 to generate process libraries for PYTHIA 8.