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21<FONT SIZE="-1" COLOR="#238E23">
22<B>Geant4 User's Guide</B>
23<BR>
24<B>For Application Developers</B>
25<BR>
26<B>Toolkit Fundamentals</B>
27</FONT>
28</TD>
29</TR></TABLE>
30<BR><BR>
31
32<P ALIGN="Center">
33<FONT SIZE="+3" COLOR="#238E23">
34<B>3.6 Event Generator Interface</B>
35</FONT>
36<BR><BR>
37
38<HR ALIGN="Center" SIZE="7%">
39<p>
40
41<a name="3.6.1">
42<H2>3.6.1 Structure of a primary event</H2></a>
43
44<b>Primary vertex and primary particle</b>
45<p>
46 The <i>G4Event</i> class object should have a set of primary particles
47 when it is sent to <i>G4EventManager</i> via <tt>processOneEvent()</tt>
48 method. It is the mandate of your <i>G4VUserPrimaryGeneratorAction</i>
49 concrete class to send primary particles to the <i>G4Event</i>
50 object.
51 <p>
52 The <i>G4PrimaryParticle</i> class represents a primary particle
53 with which Geant4 starts simulating an event. This class object
54 has information on particle type and its three momenta. The
55 positional and time information of primary particle(s) are stored
56 in the <i>G4PrimaryVertex</i> class object and, thus, this class object
57 can have one or more <i>G4PrimaryParticle</i> class objects which
58 share the same vertex. As shown in Fig.?.?, primary vertexes and
59 primary particles are associated with the <i>G4Event</i> object by
60 a form of linked list.
61 <p>
62 A concrete class of <i>G4VPrimaryGenerator</i>, the
63 <i>G4PrimaryParticle</i> object is constructed with either
64 a pointer to <i>G4ParticleDefinition</i> or an integer number
65 which represents P.D.G. particle code. For the case of some
66 artificial particles, e.g., geantino, optical photon, etc.,
67 or exotic nuclear fragments, which the P.D.G. particle code does
68 not cover, the <i>G4PrimaryParticle</i> should be constructed
69 by <i>G4ParticleDefinition</i> pointer. On the other hand,
70 elementary particles with very short life time, e.g., weak
71 bosons, or quarks/gluons, can be instantiated as <i>G4PrimaryParticle</i>
72 objects using the P.D.G. particle code. It should be noted that,
73 even though primary particles with such a very short life time
74 are defined, Geant4 will simulate only the particles which
75 are defined as <i>G4ParticleDefinition</i> class objects. Other
76 primary particles will be simply ignored by <i>G4EventManager</i>.
77 But it may still be useful to construct such "intermediate"
78 particles for recording the origin of the primary event.
79<p>
80<b>Forced decay channel</b>
81<p> 
82 The <i>G4PrimaryParticle</i> class object can have a list of its
83 daughter particles. If the parent particle is an "intermediate"
84 particle, which Geant4 does not have a corresponding <i>G4ParticleDefinition</i>,
85 this parent particle is ignored and daughters
86 are assumed to start from the vertex with which their parent is associated.
87 For example, a Z boson is associated with a vertex and it has
88 positive and negative muons as its daughters, these muons will
89 start from that vertex.
90 <p>
91 There are some kinds of particles which should fly some reasonable
92 distances and, thus, should be simulated by Geant4, but you still
93 want to follow the decay channel generated by an event generator.
94 A typical case of these particles is B meson. Even for the case of
95 a primary particle which has a corresponding <i>G4ParticleDefinition</i>,
96 it can have daughter primary particles. Geant4 will trace the
97 parent particle until it comes to decay, obeying multiple
98 scattering, ionization loss, rotation with the magnetic field, etc.
99 according to its particle type. When the parent comes to decay,
100 instead of randomly choosing its decay channel, it follows the
101 "pre-assigned" decay channel. To conserve the energy and the momentum
102 of the parent, daughters will be Lorentz transformed according to
103 their parent's frame.
104<p>
105
106<HR>
107<a name="3.6.2">
108<H2>3.6.2 Interface to a primary generator</H2></a>
109
110<b><i>G4HEPEvtInterface</i></b>
111<p>
112 Unfortunately, almost all event generators presently in use,
113 commonly are written in FORTRAN. For Geant4, it was decided to not link with any
114 FORTRAN program or library, even though the C++ language syntax itself
115 allows such a link. Linking to a FORTRAN package might be convenient in
116 some cases, but we will lose many advantages of object-oriented
117 features of C++, such as robustness. Instead, Geant4 provides
118 an ASCII file interface for such event generators.
119 <p>
120 <i>G4HEPEvtInterface</i> is one of <i>G4VPrimaryGenerator</i>
121 concrete class and thus it can be used in your
122 <i>G4VUserPrimaryGeneratorAction</i> concrete class.
123 <i>G4HEPEvtInterface</i> reads an ASCII file produced by
124 an event generator and reproduces <i>G4PrimaryParticle</i> objects
125 associated with a <i>G4PrimaryVertex</i> object. It reproduces
126 a full production chain of the event generator, starting with
127 primary quarks, etc. In other words, <i>G4HEPEvtInterface</i>
128 converts information stored in the <tt>/HEPEVT/</tt> common block to
129 an object-oriented data structure. Because the <tt>/HEPEVT/</tt> common block
130 is commonly used by almost all event generators written in
131 FORTRAN, <i>G4HEPEvtInterface</i> can interface to almost
132 all event generators currently used in the HEP community.
133 The constructor of <i>G4HEPEvtInterface</i> takes the file name.
134 Source listing 3.6.1 shows an example how to use <i>G4HEPEvtInterface</i>.
135 Note that an event generator is not assumed to give a place of the
136 primary particles, the interaction point must be set before 
137 invoking <i>GeneratePrimaryVertex()</i> method.
138 <p>
139<center>
140<table border=2 cellpadding=10>
141<tr>
142<td>
143<PRE>
144<FONT FACE="Courier New">
145#ifndef ExN04PrimaryGeneratorAction_h
146#define ExN04PrimaryGeneratorAction_h 1
147
148#include "G4VUserPrimaryGeneratorAction.hh"
149#include "globals.hh"
150
151class G4VPrimaryGenerator;
152class G4Event;
153
154class ExN04PrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction
155{
156  public:
157    ExN04PrimaryGeneratorAction();
158    ~ExN04PrimaryGeneratorAction();
159
160  public:
161    void GeneratePrimaries(G4Event* anEvent);
162
163  private:
164    G4VPrimaryGenerator* HEPEvt;
165};
166
167#endif
168
169
170#include "ExN04PrimaryGeneratorAction.hh"
171
172#include "G4Event.hh"
173#include "G4HEPEvtInterface.hh"
174
175ExN04PrimaryGeneratorAction::ExN04PrimaryGeneratorAction()
176{
177  HEPEvt = new G4HEPEvtInterface("pythia_event.data");
178}
179
180ExN04PrimaryGeneratorAction::~ExN04PrimaryGeneratorAction()
181{
182  delete HEPEvt;
183}
184
185void ExN04PrimaryGeneratorAction::GeneratePrimaries(G4Event* anEvent)
186{
187  HEPEvt->SetParticlePosition(G4ThreeVector(0.*cm,0.*cm,0.*cm));
188  HEPEvt->GeneratePrimaryVertex(anEvent);
189}
190</font>
191</pre>
192</td>
193</tr>
194<tr>
195<td align=center>
196 Source listing 3.6.1<BR>
197 An example code for <i>G4HEPEvtInterface</i>.
198</td>
199</tr>
200</table></center>
201
202
203<p>
204<b>Format of the ASCII file</b>
205<p>
206 An ASCII file, which will be fed by <i>G4HEPEvtInterface</i>
207 should have the following format.
208 <ul>
209 <li>The first line of each primary event should be an integer
210  which represents the number of the following lines of primary
211  particles.
212 <li>Each line in an event corresponds to a particle in the
213  <tt>/HEPEVT/</tt> common. Each line has <tt>ISTHEP, IDHEP, JDAHEP(1), JDAHEP(2),
214  PHEP(1), PHEP(2), PHEP(3), PHEP(5).</tt> Refer to the <tt>/HEPEVT/</tt> manual for
215  the meanings of these variables.
216 </ul>
217 Source listing 3.6.2 shows an example FORTRAN code to generate an ASCII file.
218<p>
219<center>
220<table border=2 cellpadding=10>
221<tr>
222<td>
223<pre>
224***********************************************************
225      SUBROUTINE HEP2G4
226*
227* Convert /HEPEVT/ event structure to an ASCII file
228* to be fed by G4HEPEvtInterface
229*
230***********************************************************
231      PARAMETER (NMXHEP=2000)
232      COMMON/HEPEVT/NEVHEP,NHEP,ISTHEP(NMXHEP),IDHEP(NMXHEP),
233     >JMOHEP(2,NMXHEP),JDAHEP(2,NMXHEP),PHEP(5,NMXHEP),VHEP(4,NMXHEP)
234      DOUBLE PRECISION PHEP,VHEP
235*
236      WRITE(6,*) NHEP
237      DO IHEP=1,NHEP
238       WRITE(6,10)
239     >  ISTHEP(IHEP),IDHEP(IHEP),JDAHEP(1,IHEP),JDAHEP(2,IHEP),
240     >  PHEP(1,IHEP),PHEP(2,IHEP),PHEP(3,IHEP),PHEP(5,IHEP)
24110     FORMAT(4I10,4(1X,D15.8))
242      ENDDO
243*
244      RETURN
245      END
246</pre>
247</td>
248</tr>
249<tr>
250<td align=center>
251 Source listing 3.6.2<BR>
252 A FORTRAN example using the <tt>/HEPEVT/</tt> common.
253</td>
254</tr>
255</table></center>
256<p>
257<b>Future interface to the new generation generators</b>
258<p>
259 Several activities have already been started for developing
260 object-oriented event generators. Such new generators can
261 be easily linked and used with a Geant4 based simulation.
262 Furthermore, we need not distinguish a primary generator from
263 the physics processes used in Geant4. Future generators can be
264 a kind of physics process plugged-in by inheriting
265 <i>G4VProcess</i>.
266<p>
267
268<HR>
269<a name="3.6.3">
270<H2>3.6.3 Event overlap using multiple generators</H2></a>
271
272 Your <i>G4VUserPrimaryGeneratorAction</i> concrete class can
273 have more than one <i>G4VPrimaryGenerator</i> concrete class.
274 Each <i>G4VPrimaryGenerator</i> concrete class can be accessed
275 more than once per event. Using these class objects, one
276 event can have more than one primary event.
277 <p>
278 One possible use is the following. Within an event,
279 a <i>G4HEPEvtInterface</i> class object instantiated with a
280 minimum bias event file is accessed 20 times and another
281 <i>G4HEPEvtInterface</i> class object instantiated with a
282 signal event file is accessed once. Thus, this event represents
283 a typical signal event of LHC overlapping 20 minimum bias
284 events. It should be noted that a simulation of event
285 overlapping can be done by merging hits and/or digits
286 associated with several events, and these events can be
287 simulated independently. Digitization over multiple events
288 will be mentioned in <a href="../Detector/digitization.html">Section 4.5</a>.
289
290<BR><BR>
291<HR>
292<A HREF="../../../../Authors/html/subjectsToAuthors.html">
293<I>About the authors</I></A>
294
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