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1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>3.6.  Event Generator Interface</title><link rel="stylesheet" href="../xml/XSLCustomizationLayer/G4HTMLStylesheet.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.71.1"><link rel="start" href="index.html" title="Geant4 User's Guide for Application Developers"><link rel="up" href="ch03.html" title="Chapter 3.  Toolkit Fundamentals"><link rel="prev" href="ch03s05.html" title="3.5.  Event"><link rel="next" href="ch03s07.html" title="3.7.  Event Biasing Techniques"><script language="JavaScript">
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8</script></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">3.6. 
9Event Generator Interface
10</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch03s05.html"><img src="AllResources/IconsGIF/prev.gif" alt="Prev"></a> </td><th width="60%" align="center">Chapter 3. 
11Toolkit Fundamentals
12</th><td width="20%" align="right"> <a accesskey="n" href="ch03s07.html"><img src="AllResources/IconsGIF/next.gif" alt="Next"></a></td></tr></table><hr></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="sect.EventGen"></a>3.6. 
13Event Generator Interface
14</h2></div></div></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.EventGen.StructPriEvt"></a>3.6.1. 
15Structure of a primary event
16</h3></div></div></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.EventGen.StructPriEvt.VtxPart"></a>3.6.1.1. 
17Primary vertex and primary particle
18</h4></div></div></div><p>
19The <span class="emphasis"><em>G4Event</em></span> class object should have a set of primary
20particles when it is sent to <span class="emphasis"><em>G4EventManager</em></span> via
21<code class="literal">processOneEvent()</code> method. It is the mandate of your
22<span class="emphasis"><em>G4VUserPrimaryGeneratorAction</em></span> concrete class to send primary
23particles to the <span class="emphasis"><em>G4Event</em></span> object.
24</p><p>
25The <span class="emphasis"><em>G4PrimaryParticle</em></span> class represents a primary particle
26with which Geant4 starts simulating an event. This class object has
27information on particle type and its three momenta. The positional
28and time information of primary particle(s) are stored in the
29<span class="emphasis"><em>G4PrimaryVertex</em></span> class object and, thus, this class object
30can have one or more <span class="emphasis"><em>G4PrimaryParticle</em></span> class objects which
31share the same vertex. As shown in Fig.?.?, primary vertexes and
32primary particles are associated with the <span class="emphasis"><em>G4Event</em></span> object by
33a form of linked list.
34</p><p>
35A concrete class of <span class="emphasis"><em>G4VPrimaryGenerator</em></span>, the
36<span class="emphasis"><em>G4PrimaryParticle</em></span> object is constructed with either a
37pointer to <span class="emphasis"><em>G4ParticleDefinition</em></span> or an integer number which
38represents P.D.G. particle code. For the case of some artificial
39particles, e.g., geantino, optical photon, etc., or exotic nuclear
40fragments, which the P.D.G. particle code does not cover, the
41<span class="emphasis"><em>G4PrimaryParticle</em></span> should be constructed by
42<span class="emphasis"><em>G4ParticleDefinition</em></span> pointer. On the other hand, elementary
43particles with very short life time, e.g., weak bosons, or
44quarks/gluons, can be instantiated as <span class="emphasis"><em>G4PrimaryParticle</em></span>
45objects using the P.D.G. particle code. It should be noted that,
46even though primary particles with such a very short life time are
47defined, Geant4 will simulate only the particles which are defined
48as <span class="emphasis"><em>G4ParticleDefinition</em></span> class objects. Other primary
49particles will be simply ignored by <span class="emphasis"><em>G4EventManager</em></span>. But it
50may still be useful to construct such "intermediate" particles for
51recording the origin of the primary event.
52</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.EventGen.StructPriEvt.ForcedDecay"></a>3.6.1.2. 
53Forced decay channel
54</h4></div></div></div><p>
55The <span class="emphasis"><em>G4PrimaryParticle</em></span> class object can have a list of its
56daughter particles. If the parent particle is an "intermediate"
57particle, which Geant4 does not have a corresponding
58<span class="emphasis"><em>G4ParticleDefinition</em></span>, this parent particle is ignored and
59daughters are assumed to start from the vertex with which their
60parent is associated. For example, a Z boson is associated with a
61vertex and it has positive and negative muons as its daughters,
62these muons will start from that vertex.
63</p><p>
64There are some kinds of particles which should fly some
65reasonable distances and, thus, should be simulated by Geant4, but
66you still want to follow the decay channel generated by an event
67generator. A typical case of these particles is B meson. Even for
68the case of a primary particle which has a corresponding
69<span class="emphasis"><em>G4ParticleDefinition</em></span>, it can have daughter primary
70particles. Geant4 will trace the parent particle until it comes to
71decay, obeying multiple scattering, ionization loss, rotation with
72the magnetic field, etc. according to its particle type. When the
73parent comes to decay, instead of randomly choosing its decay
74channel, it follows the "pre-assigned" decay channel. To conserve
75the energy and the momentum of the parent, daughters will be
76Lorentz transformed according to their parent's frame.
77</p></div></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.EventGen.InterPriGen"></a>3.6.2. 
78Interface to a primary generator
79</h3></div></div></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.EventGen.InterPriGen.HEPEvt"></a>3.6.2.1. 
80G4HEPEvtInterface
81</h4></div></div></div><p>
82Unfortunately, almost all event generators presently in use,
83commonly are written in FORTRAN. For Geant4, it was decided to not
84link with any FORTRAN program or library, even though the C++
85language syntax itself allows such a link. Linking to a FORTRAN
86package might be convenient in some cases, but we will lose many
87advantages of object-oriented features of C++, such as robustness.
88Instead, Geant4 provides an ASCII file interface for such event
89generators.
90</p><p>
91<span class="emphasis"><em>G4HEPEvtInterface</em></span> is one of <span class="emphasis"><em>G4VPrimaryGenerator</em></span>
92concrete class and thus it can be used in your
93<span class="emphasis"><em>G4VUserPrimaryGeneratorAction</em></span> concrete class.
94<span class="emphasis"><em>G4HEPEvtInterface</em></span> reads an ASCII file produced by an event
95generator and reproduces <span class="emphasis"><em>G4PrimaryParticle</em></span> objects
96associated with a <span class="emphasis"><em>G4PrimaryVertex</em></span> object. It reproduces a
97full production chain of the event generator, starting with primary
98quarks, etc. In other words, <span class="emphasis"><em>G4HEPEvtInterface</em></span> converts
99information stored in the <code class="literal">/HEPEVT/</code> common block to an
100object-oriented data structure. Because the <code class="literal">/HEPEVT/</code>
101common block is commonly used by almost all event generators
102written in FORTRAN, <span class="emphasis"><em>G4HEPEvtInterface</em></span> can interface to
103almost all event generators currently used in the HEP community.
104The constructor of <span class="emphasis"><em>G4HEPEvtInterface</em></span> takes the file name.
105<a href="ch03s06.html#programlist_EventGen_1" title="Example 3.3. 
106An example code for G4HEPEvtInterface
107">Example 3.3</a> shows an example how to use
108<span class="emphasis"><em>G4HEPEvtInterface</em></span>. Note that an event generator is not
109assumed to give a place of the primary particles, the interaction
110point must be set before invoking <span class="emphasis"><em>GeneratePrimaryVertex()</em></span>
111method.
112</p><div class="example"><a name="programlist_EventGen_1"></a><p class="title"><b>Example 3.3. 
113An example code for <span class="emphasis"><em>G4HEPEvtInterface</em></span>
114</b></p><div class="example-contents"><pre class="programlisting">
115#ifndef ExN04PrimaryGeneratorAction_h
116#define ExN04PrimaryGeneratorAction_h 1
117
118#include "G4VUserPrimaryGeneratorAction.hh"
119#include "globals.hh"
120
121class G4VPrimaryGenerator;
122class G4Event;
123
124class ExN04PrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction
125{
126 public:
127 ExN04PrimaryGeneratorAction();
128 ~ExN04PrimaryGeneratorAction();
129
130 public:
131 void GeneratePrimaries(G4Event* anEvent);
132
133 private:
134 G4VPrimaryGenerator* HEPEvt;
135};
136
137#endif
138
139
140#include "ExN04PrimaryGeneratorAction.hh"
141
142#include "G4Event.hh"
143#include "G4HEPEvtInterface.hh"
144
145ExN04PrimaryGeneratorAction::ExN04PrimaryGeneratorAction()
146{
147 HEPEvt = new G4HEPEvtInterface("pythia_event.data");
148}
149
150ExN04PrimaryGeneratorAction::~ExN04PrimaryGeneratorAction()
151{
152 delete HEPEvt;
153}
154
155void ExN04PrimaryGeneratorAction::GeneratePrimaries(G4Event* anEvent)
156{
157 HEPEvt-&gt;SetParticlePosition(G4ThreeVector(0.*cm,0.*cm,0.*cm));
158 HEPEvt-&gt;GeneratePrimaryVertex(anEvent);
159}
160</pre></div></div><br class="example-break"></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.EventGen.InterPriGen.FormASCII"></a>3.6.2.2. 
161Format of the ASCII file
162</h4></div></div></div><p>
163An ASCII file, which will be fed by <span class="emphasis"><em>G4HEPEvtInterface</em></span>
164should have the following format.
165
166</p><div class="itemizedlist"><ul type="disc" compact><li><p>
167 The first line of each primary event should be an integer which
168 represents the number of the following lines of primary
169 particles.
170 </p></li><li><p>
171 Each line in an event corresponds to a particle in the
172 <code class="literal">/HEPEVT/</code> common. Each line has <code class="literal">ISTHEP, IDHEP,
173 JDAHEP(1), JDAHEP(2), PHEP(1), PHEP(2), PHEP(3), PHEP(5).</code>
174 Refer to the <code class="literal">/HEPEVT/</code> manual for the meanings of these
175 variables.
176 </p></li></ul></div><p>
177</p><p>
178<a href="ch03s06.html#programlist_EventGen_2" title="Example 3.4. 
179A FORTRAN example using the /HEPEVT/ common.
180">Example 3.4</a> shows an example FORTRAN code to
181generate an ASCII file.
182</p><div class="example"><a name="programlist_EventGen_2"></a><p class="title"><b>Example 3.4. 
183A FORTRAN example using the <code class="literal">/HEPEVT/</code> common.
184</b></p><div class="example-contents"><pre class="programlisting">
185***********************************************************
186 SUBROUTINE HEP2G4
187*
188* Convert /HEPEVT/ event structure to an ASCII file
189* to be fed by G4HEPEvtInterface
190*
191***********************************************************
192 PARAMETER (NMXHEP=2000)
193 COMMON/HEPEVT/NEVHEP,NHEP,ISTHEP(NMXHEP),IDHEP(NMXHEP),
194 &gt;JMOHEP(2,NMXHEP),JDAHEP(2,NMXHEP),PHEP(5,NMXHEP),VHEP(4,NMXHEP)
195 DOUBLE PRECISION PHEP,VHEP
196*
197 WRITE(6,*) NHEP
198 DO IHEP=1,NHEP
199 WRITE(6,10)
200 &gt; ISTHEP(IHEP),IDHEP(IHEP),JDAHEP(1,IHEP),JDAHEP(2,IHEP),
201 &gt; PHEP(1,IHEP),PHEP(2,IHEP),PHEP(3,IHEP),PHEP(5,IHEP)
20210 FORMAT(4I10,4(1X,D15.8))
203 ENDDO
204*
205 RETURN
206 END
207</pre></div></div><br class="example-break"></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.EventGen.InterPriGen.FutureInt"></a>3.6.2.3. 
208Future interface to the new generation generators
209</h4></div></div></div><p>
210Several activities have already been started for developing
211object-oriented event generators. Such new generators can be easily
212linked and used with a Geant4 based simulation. Furthermore, we
213need not distinguish a primary generator from the physics processes
214used in Geant4. Future generators can be a kind of physics process
215plugged-in by inheriting <span class="emphasis"><em>G4VProcess</em></span>.
216</p></div></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.EventGen.EvtOverlap"></a>3.6.3. 
217Event overlap using multiple generators
218</h3></div></div></div><p>
219Your <span class="emphasis"><em>G4VUserPrimaryGeneratorAction</em></span> concrete class can have
220more than one <span class="emphasis"><em>G4VPrimaryGenerator</em></span> concrete class. Each
221<span class="emphasis"><em>G4VPrimaryGenerator</em></span> concrete class can be accessed more than
222once per event. Using these class objects, one event can have more
223than one primary event.
224</p><p>
225One possible use is the following. Within an event, a
226<span class="emphasis"><em>G4HEPEvtInterface</em></span> class object instantiated with a minimum
227bias event file is accessed 20 times and another
228<span class="emphasis"><em>G4HEPEvtInterface</em></span> class object instantiated with a signal
229event file is accessed once. Thus, this event represents a typical
230signal event of LHC overlapping 20 minimum bias events. It should
231be noted that a simulation of event overlapping can be done by
232merging hits and/or digits associated with several events, and
233these events can be simulated independently. Digitization over
234multiple events will be mentioned in
235<a href="ch04s05.html" title="4.5. 
236Digitization
237">Section 4.5</a>.
238</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch03s05.html"><img src="AllResources/IconsGIF/prev.gif" alt="Prev"></a> </td><td width="20%" align="center"><a accesskey="u" href="ch03.html"><img src="AllResources/IconsGIF/up.gif" alt="Up"></a></td><td width="40%" align="right"> <a accesskey="n" href="ch03s07.html"><img src="AllResources/IconsGIF/next.gif" alt="Next"></a></td></tr><tr><td width="40%" align="left" valign="top">3.5. 
239Event
240 </td><td width="20%" align="center"><a accesskey="h" href="index.html"><img src="AllResources/IconsGIF/home.gif" alt="Home"></a></td><td width="40%" align="right" valign="top"> 3.7. 
241Event Biasing Techniques
242</td></tr></table></div></body></html>
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