3 <title>MadGraph 5 Processes</title>
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30 <h2>MadGraph 5 Processes</h2>
32 By far the easiest way to implement new processes into PYTHIA 8 is
33 by using the matrix-element generator MadGraph 5. This program has
34 an option to output the results of a matrix-element calculation
35 as a set of PYTHIA 8 C++ classes (plus further auxiliary code),
36 that can then be linked and used as
37 <?php $filepath = $_GET["filepath"];
38 echo "<a href='SemiInternalProcesses.php?filepath=".$filepath."' target='page'>";?>semi-internal</a> processes,
39 meaning they are handled identically with normal internal ones.
40 This way, MadGraph 5 can be used to implement processes from
41 any model that can be written in terms of a Lagrangian. Any
42 <i>2 -> 1</i>, <i>2 -> 2</i> and <i>2 -> 3</i> processes
43 can be implemented, the limit being set by the absence of efficient
44 phase space generator algorithms for higher multiplicities in
45 PYTHIA. Features such as <i>s</i>-channel resonances are
46 automatically implemented in the process classes. Besides the process
47 library and necessary model files, also an example main program is
48 generated for each set of processes, which can be easily modified to
49 perform the desired analyses.
52 In order to create a PYTHIA 8 process library with MadGraph 5, first
53 download the MadGraph 5 package from
54 <a href="https://launchpad.net/madgraph5" target="page">
55 https://launchpad.net/madgraph5</a>, and untar the package. You can
56 then specify the location of your <code>pythia81xx</code> directory
57 in the file <code>input/mg5_configuration.txt</code>:
58 <br/><code>pythia8_path = ./pythia81xx</code>
59 <br/>The location can be either relative (to the directory
60 <code>MadGraph5_v_x_x_x/.</code>) or absolute.
63 For any model that is already implemented in the MadGraph 5 package,
64 you can directly use the model. Start the MadGraph 5 interface
65 <code>bin/mg5</code>, and do:
67 import model model_name
68 generate your_process_in_mg5_syntax
69 add process your_next_process_in_mg5_syntax
71 output pythia8 [path_to_pythia81xx_directory]
75 For examples of MG5 process syntax, please see
76 <a href="http://madgraph.phys.ucl.ac.be/EXAMPLES/example_mg5.html"
77 target="page">http://madgraph.phys.ucl.ac.be/EXAMPLES/example_mg5.html</a>
78 or type <code>help generate</code>. If you specified the path to the
79 <code>pythia81xx</code> directory in the <code>mg5_configuration</code>
80 file, you do not need to enter it in the <code>output</code> command.
83 If your preferred model is found on the FeynRules model wiki page,
84 <a href="http://feynrules.irmp.ucl.ac.be/wiki/ModelDatabaseMainPage"
85 target="page">http://feynrules.irmp.ucl.ac.be/wiki/ModelDatabaseMainPage</a>,
86 download the UFO (Universal FeynRules Output) tar file for the model,
87 untar in the <code>models/</code> directory, and use as above.
90 If you want to implement a new model which has not yet been implemented,
91 you can do this either using the Mathematica package FeynRules (see
92 <a href="http://feynrules.irmp.ucl.ac.be/" target="page">
93 http://feynrules.irmp.ucl.ac.be/</a>) or directly edit the UFO model
94 files of the most similar model in the <code>models/</code> directory.
97 The resulting output from the <code>output pythia8</code> command is:
99 <li>A process directory <code>Processes_modelname</code> with the
100 model information and the files needed for all processes defined for
101 this model, placed in the <code>pythia81xx</code> main directory.
102 The model files are <code>Parameters_modelname.h/cc</code> and
103 <code>HelAmps_modelname.h/cc</code>, and the process files for each
104 process class (with the same mass, spin and color of the initial/final
105 state particles) are called <code>Sigma_modelname_processname.h/cc</code>.
106 The directory also contains a <code>makefile</code> and a model parameter
107 file <code>param_card_modelname.dat</code>.</li>
108 <li>An example main program in the directory <code>examples/</code>
109 (in the <code>pythia81xx</code> main directory) called
110 <code>main_modelname_N.cc</code> and a corresponding makefile
111 <code>Makefile_modelname_N</code>. This main program links in the
112 process classes in the process directory described above. To run the
113 example main program, just go to the <code>examples/</code>
115 <br/><code>make -f Makefile_modelname_N</code>
116 <br/>or run <code>launch</code> directly inside the MadGraph 5
117 command line interface.</li>
121 Note that in order for PYTHIA 8 to be able to automatically decay any
122 new particles, it is necessary to specify the branching ratios of the
123 particles in the <code>param_card</code> file, see
124 [<a href="Bibliography.php" target="page">Ska04,Alw07</a>] for details.
127 For further technical details, please see the MadGraph 5 release paper
128 [<a href="Bibliography.php" target="page">Alw11</a>] and the
129 <?php $filepath = $_GET["filepath"];
130 echo "<a href='SemiInternalProcesses.php?filepath=".$filepath."' target='page'>";?>semi-internal</a> processes page.
133 Of course, as with MadGraph 4, MadGraph 5 can also output
134 files of parton-level events according to the
135 <?php $filepath = $_GET["filepath"];
136 echo "<a href='LesHouchesAccord.php?filepath=".$filepath."' target='page'>";?>LHEF</a> standard,
137 that can be read in and processed further by PYTHIA 8.
138 The advantage is that then the MadGraph 5 phase space generator
139 can be used, which opens up for processes with more than three
140 particles in the final state. The disadvantages are that it is less
141 easy to mix and match with existing PYTHIA processes, and that one
142 needs to regenerate and store large LHEF files for different
143 kinematics cuts or parameter values.
146 Please cite the MadGraph 5 release paper [<a href="Bibliography.php" target="page">Alw11</a>] if you use
147 MadGraph 5 to generate process libraries for PYTHIA 8.
152 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->