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1<html>
2<head>
3<title>MadGraph 5 Processes</title>
4<link rel="stylesheet" type="text/css" href="pythia.css"/>
5<link rel="shortcut icon" href="pythia32.gif"/>
6</head>
7<body>
8
9<h2>MadGraph 5 Processes</h2>
10
11By far the easiest way to implement new processes into PYTHIA 8 is
12by using the matrix-element generator MadGraph 5. This program has
13an option to output the results of a matrix-element calculation
14as a set of PYTHIA 8 C++ classes (plus further auxiliary code),
15that can then be linked and used as
16<a href="SemiInternalProcesses.html" target="page">semi-internal</a> processes,
17meaning they are handled identically with normal internal ones.
18This way, MadGraph 5 can be used to implement processes from
19any model that can be written in  terms of a Lagrangian. Any
20<i>2 -> 1</i>, <i>2 -> 2</i> and <i>2 -> 3</i> processes
21can be implemented, the limit being set by the absence of efficient
22phase space generator algorithms for higher multiplicities in
23PYTHIA. Features such as <i>s</i>-channel resonances are
24automatically implemented in the process classes.  Besides the process
25library and necessary model files, also an example main program is
26generated for each set of processes, which can be easily modified to
27perform the desired analyses.
28
29<p/>
30In order to create a PYTHIA 8 process library with MadGraph 5, first
31download the MadGraph 5 package from
32<a href="https://launchpad.net/madgraph5" target="page">
33https://launchpad.net/madgraph5</a>, and untar the package. You can
34then specify the location of your <code>pythia81xx</code> directory
35in the file <code>input/mg5_configuration.txt</code>:
36<br/><code>pythia8_path = ./pythia81xx</code>
37<br/>The location can be either relative (to the directory
38<code>MadGraph5_v_x_x_x/.</code>) or absolute.
39
40<p/>
41For any model that is already implemented in the MadGraph 5 package,
42you can directly use the model. Start the MadGraph 5 interface
43<code>bin/mg5</code>, and do:
44<pre>
45import model model_name 
46generate your_process_in_mg5_syntax
47add process your_next_process_in_mg5_syntax
48... 
49output pythia8 [path_to_pythia81xx_directory]
50</pre>
51
52<p/>
53For examples of MG5 process syntax, please see
54<a href="http://madgraph.phys.ucl.ac.be/EXAMPLES/example_mg5.html" 
55target="page">http://madgraph.phys.ucl.ac.be/EXAMPLES/example_mg5.html</a> 
56or type <code>help generate</code>. If you specified the path to the
57<code>pythia81xx</code> directory in the <code>mg5_configuration</code> 
58file, you do not need to enter it in the <code>output</code> command.
59
60<p/>
61If your preferred model is found on the FeynRules model wiki page,
62<a href="http://feynrules.irmp.ucl.ac.be/wiki/ModelDatabaseMainPage" 
63target="page">http://feynrules.irmp.ucl.ac.be/wiki/ModelDatabaseMainPage</a>,
64download the UFO (Universal FeynRules Output) tar file for the model,
65untar in the <code>models/</code> directory, and use as above.
66
67<p/>
68If you want to implement a new model which has not yet been implemented,
69you can do this either using the Mathematica package FeynRules (see
70<a href="http://feynrules.irmp.ucl.ac.be/" target="page">
71http://feynrules.irmp.ucl.ac.be/</a>) or directly edit the UFO model
72files of the most similar model in the <code>models/</code> directory.
73
74<p/>
75The resulting output from the <code>output pythia8</code> command is:
76<ul>
77<li>A process directory <code>Processes_modelname</code> with the
78model information and the files needed for all processes defined for
79this model, placed in the <code>pythia81xx</code> main directory. 
80The model files are <code>Parameters_modelname.h/cc</code> and
81<code>HelAmps_modelname.h/cc</code>, and the process files for each
82process class (with the same mass, spin and color of the initial/final
83state particles) are called <code>Sigma_modelname_processname.h/cc</code>.
84The directory also contains a <code>makefile</code> and a model parameter
85file <code>param_card_modelname.dat</code>.</li> 
86<li>An example main program in the directory <code>examples/</code> 
87(in the <code>pythia81xx</code> main directory) called
88<code>main_modelname_N.cc</code> and a corresponding makefile
89<code>Makefile_modelname_N</code>. This main program links in the
90process classes in the process directory described above. To run the
91example main program, just go to the <code>examples/</code> 
92directory and run
93<br/><code>make -f Makefile_modelname_N</code> 
94<br/>or run <code>launch</code> directly inside the MadGraph 5
95command line interface.</li>
96</ul>
97
98<p/>
99Note that in order for PYTHIA 8 to be able to automatically decay any
100new particles, it is necessary to specify the branching ratios of the
101particles in the <code>param_card</code> file, see
102[<a href="Bibliography.html" target="page">Ska04,Alw07</a>] for details.
103
104<p/>
105For further technical details, please see the MadGraph 5 release paper
106[<a href="Bibliography.html" target="page">Alw11</a>] and the
107<a href="SemiInternalProcesses.html" target="page">semi-internal</a> processes page.
108
109<p/>
110Of course, as with MadGraph 4, MadGraph 5 can also output
111files of parton-level events according to the
112<a href="LesHouchesAccord.html" target="page">LHEF</a> standard,
113that can be read in and processed further by PYTHIA 8.
114The advantage is that then the MadGraph 5 phase space generator
115can be used, which opens up for processes with more than three
116particles in the final state. The disadvantages are that it is less
117easy to mix and match with existing PYTHIA processes, and that one
118needs to regenerate and store large LHEF files for different
119kinematics cuts or parameter values.
120
121<p/>
122Please cite the MadGraph 5 release paper [<a href="Bibliography.html" target="page">Alw11</a>] if you use
123MadGraph 5 to generate process libraries for PYTHIA 8.
124
125</body>
126</html>
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