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1<html>
2<head>
3<title>ROOT usage</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>ROOT usage</h2>
10
11Many PYTHIA users wish to use <a href="http://root.cern.ch/">ROOT</a> 
12to produce histograms, or even to run PYTHIA as a plugin to ROOT.
13This is possible. It is not a task supported by the PYTHIA team,
14however. All issues involving ROOT usage should be directed to the
15ROOT team, or to the local support team of your collaboration.
16Below some helpful hints have been collected. The text is based on
17contributions by Rene Brun, Andreas Morsch and Axel Naumann.
18Another example may be found in the
19<a href="http://home.fnal.gov/~skands/vincia/">VINCIA</a> 
20add-on program for parton showers, but this should also work for
21a PYTHIA standalone run.
22<br/><br/>
23Note that in all that follows, a Linux-type system with a Bash shell
24and GNU Make is assumed. In particular, for Mac OS X, the
25<code>LD_LIBRARY_PATH</code> should be replaced with
26<code>DYLD_LIBRARY_PATH</code> and the extension for shared libraries
27<code>.so</code> should be replaced with <code>.dylib</code>.
28
29<br/><br/><hr/>
30<h3>Standalone usage</h3>
31
32One can perform the generation and analysis of events in a completely
33standalone fashion, and only use ROOT to process the completed events.
34Some example programs are provided in the <code>rootexamples</code>
35directory, with details provided below and in the <code>README</code>
36file.
37
38The examples assume that you have already compiled the PYTHIA library,
39ROOT is installed, and that all paths have been setup correctly (e.g.
40PATH and LD_LIBRARY_PATH). If these paths are not already set, ROOT
41typically comes with a shell script to set these paths, which can be
42run with
43<pre>
44    source /path_to_ROOT_installation/bin/thisroot.sh
45</pre>
46
47<h4>Histogramming with ROOT</h4>
48
49An example of histogramming with ROOT is provided in
50<code>rootexamples/hist.cc</code>. It may be compiled and run
51with the following commands
52<pre>
53    make hist
54    ./hist.exe
55</pre>
56After PYTHIA has run, a ROOT histogram of the charged multiplicity in
57the events will be shown. This is now stored in the
58<code>hist.root</code> file.
59
60<h4>Storing PYTHIA events in ROOT trees</h4>
61
62Instead of only generating histograms, it is possible to store entire
63PYTHIA events in ROOT trees. The <code>tree</code> example in the
64<code>rootexamples</code> subdirectory provides an example of this and
65is comprised of the following files
66<ul>
67  <li><code>tree.cc</code> is the main example program showing how
68  PYTHIA events can be stored in ROOT trees.</li>
69  <li><code>pythiaLinkdef.h</code> is used by Makefile to generate the
70  dictionary for all PYTHIA classes involved in the IO, as needed for
71  the example.</li>
72  <li><code>pythiaROOT.h</code> is a small include declaring the
73  <code>Pythia8</code> namespace as default.</li>
74</ul>
75
76<br/>
77The example may be compiled and run with
78<pre>
79    make tree
80    ./tree.exe
81</pre>
82After the example has run, the <code>pytree.root</code> file will now
83be present containing the PYTHIA events. Note that files can become
84quite large when many events are generated.
85
86<br/><br/><hr/>
87<h3>PYTHIA as a plugin to ROOT</h3>
88
89In more ROOT-centric applications, PYTHIA can be run as a ROOT plug-in.
90This requires a version of ROOT that has been
91<a href="http://root.cern.ch/drupal/content/installing-root-source">
92installed from source</a>. The reason is that the interfaces depend on
93PYTHIA header files that are not distributed with ROOT. Installing ROOT
94is not more difficult than the PYTHIA installation, and some
95guidelines are provided below.
96
97<h4>Installation</h4>
98
99To be run as a plugin, PYTHIA must be compiled as a shared library.
100This is achieved by running the PYTHIA <code>configure</code> script
101with the <code>--enable-shared</code> option before <code>make</code>
102is run.<br/><br/>
103
104Define an environment variable for the path to your
105PYTHIA installation directory
106<pre>
107    export PYTHIA8=path_to_PYTHIA8_installation
108</pre>
109Before compiling ROOT,
110<a href="http://root.cern.ch/drupal/content/installing-root-source">
111configure ROOT</a> by running the <code>configure</code> command
112including the following options
113<pre>
114    --enable-pythia8
115    --with-pythia8-incdir=$PYTHIA8/include
116    --with-pythia8-libdir=$PYTHIA8/lib
117</pre>
118In case ROOT has already been compiled before, it will only recompile
119the PYTHIA module and build the library <code>libEGPythia8</code>.
120
121<h4>Interfaces</h4>
122
123When running PYTHIA as a plugin, the exact interface structure becomes
124very relevant. ROOT provides two simple interfaces (wrappers) for
125PYTHIA 8. The code for these interfaces are located in
126<pre>
127    path_to_ROOT_source/montecarlo/pythia8
128</pre>
129<br/>
130The two interfaces are
131<ul>
132  <li><a href="http://root.cern.ch/root/html/TPythia8.html">
133  <code>TPythia8</code></a> is an implementation of the
134  <a href="http://root.cern.ch/root/html/TGenerator.html">
135  <code>TGenerator</code></a> interface for PYTHIA 8.<br/>
136  It allows you to use PYTHIA within a ROOT macro or as a plug-in
137  for a general-purpose particle generator based on this interface. The
138  main methods of the interface are
139  <ul>
140    <li><code>GenerateEvent()</code> which triggers the
141    generation of the next event, and </li>
142    <li><code>ImportParticles(TClonesArray* particles)</code>
143    which copies the native PYTHIA stack into a
144    <a href="http://root.cern.ch/root/html/TClonesArray.html">
145    <code>TClonesArray</code></a> of
146    <a href="http://root.cern.ch/root/html/TParticle.html">
147    <code>TParticles</code></a>.
148  </ul>
149
150  In addition, some methods that are directly related to corresponding
151  PYTHIA methods are implemented
152  <ul>
153    <li><code>ReadString(const char* string)</code> ->
154    <code>readString(...)</code></li>
155    <li><code>ReadConfigFile(const char* string)</code> ->
156    <code>readFile(...)</code></li>
157    <li><code>Initialize(int idAin, int idBin, double ecms)</code> ->
158    <code>init(...)</code></li>
159    <li><code>EventListing()</code> ->
160    <code>event.list()</code></li>
161    <li><code>PrintStatistic()</code> ->
162    <code>statistics()</code></li>
163  </ul>
164
165  These methods provide already the basic PYTHIA functionality
166  interactively from the ROOT command line. However, this does not mean
167  that the usage of PYTHIA from within ROOT is restricted to these methods.
168  In compiled code, one can always obtain a pointer to the
169  <code>Pythia</code> instance e.g.
170  <pre>
171    TPythia8        *tp = new TPythia8();
172    Pythia8::Pythia *p  = tp->Pythia8();</pre>
173  giving access to the full PYTHIA functionality. To access this
174  functionality in the CINT interpreter see the "Advanced usage"
175  section below.</li>
176
177  <li><a href="http://root.cern.ch/root/html/TPythia8Decayer.html">
178  <code>TPythia8Decayer</code></a> is an implementation of the
179  <a href="http://root.cern.ch/root/html/TVirtualMCDecayer.html">
180  <code>TVirtualMCDecayer</code></a> interface.<br/>
181  It allows you to use PYTHIA as a plug-in decayer for simulation
182  frameworks based on the Virtual Monte Carlo
183  (<a href="http://root.cern.ch/drupal/content/vmc">VMC</a>) interface
184  classes. The main methods of the interface are
185  <ul>
186    <li><code>TPythia8Decayer::Init()</code> for initialisation,</li>
187    <li><code>TPythia8Decayer::Decay(Int_t pdg, TLorentzVector* p)</code> 
188    to decay a particle with PDG code <code>pdg</code> and
189    <a href="http://root.cern.ch/root/html/TLorentzVector.html">
190    4-momentum</a> <code>p</code>, and </li>
191    <li><code>ImportParticles(TClonesArray* particles)</code> 
192    to retrieve the decay products as
193    <a href="http://root.cern.ch/root/html/TParticle.html">
194    <code>TParticles</code></a> in the
195    <code><a href="http://root.cern.ch/root/html/TClonesArray.html">
196    TClonesArray</a> particles</code>.</li>
197  </ul></li>
198</ul>
199
200<h4>An example</h4>
201
202A <a href="http://root.cern.ch/root/html/tutorials/pythia/pythia8.C.html"> 
203basic example</a> for generating minimum-bias events with PYTHIA 8 inside
204a ROOT macro, and filling some histograms with the kinematics of the
205final-state particles is provided in either of the locations below
206<pre>
207    /path_to_ROOT_source/tutorials/pythia/pythia8.C
208    /path_to_ROOT_installation/share/doc/root/tutorials/pythia/pythia8.C
209</pre>
210<br/>
211Note that before executing this script
212<ul>
213  <li>the environment variables <code>PYTHIA8</code> and
214  <code>PYTHIA8DATA</code> must be setup correctly e.g.
215  <pre>
216    export PYTHIA8=/path_to_PYTHIA_installation
217    export PYTHIA8DATA=$PYTHIA8/xmldoc
218  <pre></li>
219  <li>your LD_LIBRARY_PATH must contain the location of the
220  PYTHIA 8 shared library, e.g.
221  <pre>
222    export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:path_to_PYTHIA8_installation/lib
223  </pre>
224  </li>
225</ul>
226The script can then be run with ROOT
227<pre>
228    root pythia8.C
229</pre>
230After execution, ROOT will display some histograms from the event
231generation.
232
233<h4>Advanced usage</h4>
234
235To access the full PYTHIA functionality from the CINT interpreter,
236a ROOT dictionary must be created. An example of creating this
237dictionary is contained in the <code>rootexamples</code> directory.
238The <code>pythiaDict.so</code> library may be created by running
239<pre>
240    make dict
241</pre>
242
243This may then be loaded in ROOT giving full access to the full PYTHIA 8
244functionality, e.g. in an interactive session
245<pre>
246    gSystem->Load("path_to_PYTHIA8_installation/rootexamples/pythiaDict");
247    Pythia8::Pythia *p = new Pythia8::Pythia();
248    p->readString("SoftQCD:minBias = on");
249</pre>
250 
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