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| 2 | <TITLE>
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| 3 | </TITLE>
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| 4 | <!-- Changed by: Katsuya Amako, 21-Sep-1998 -->
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| 5 | <!-- Changed by: Dennis Wright, 25-Jun-2002 -->
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| 6 | <!-- Proof read by: Joe Chuma, 28-Jun-1999 -->
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| 7 |
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| 8 | <BODY>
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| 9 | <TABLE WIDTH="100%"><TR>
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| 10 | <TD>
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| 11 |
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| 12 |
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| 13 | <A HREF="index.html">
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| 14 | <IMG SRC="../../../../resources/html/IconsGIF/Contents.gif" ALT="Contents"></A>
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| 15 | <A HREF="event.html">
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| 16 | <IMG SRC="../../../../resources/html/IconsGIF/Previous.gif" ALT="Previous"></A>
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| 17 | <A HREF="biasing.html">
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| 18 | <IMG SRC="../../../../resources/html/IconsGIF/Next.gif" ALT="Next"></A>
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| 19 | </TD>
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| 20 | <TD ALIGN="Right">
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| 21 | <FONT SIZE="-1" COLOR="#238E23">
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| 22 | <B>Geant4 User's Guide</B>
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| 23 | <BR>
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| 24 | <B>For Application Developers</B>
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| 25 | <BR>
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| 26 | <B>Toolkit Fundamentals</B>
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| 27 | </FONT>
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| 28 | </TD>
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| 29 | </TR></TABLE>
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| 30 | <BR><BR>
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| 31 |
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| 32 | <P ALIGN="Center">
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| 33 | <FONT SIZE="+3" COLOR="#238E23">
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| 34 | <B>3.6 Event Generator Interface</B>
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| 35 | </FONT>
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| 36 | <BR><BR>
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| 37 |
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| 38 | <HR ALIGN="Center" SIZE="7%">
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| 39 | <p>
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| 40 |
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| 41 | <a name="3.6.1">
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| 42 | <H2>3.6.1 Structure of a primary event</H2></a>
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| 43 |
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| 44 | <b>Primary vertex and primary particle</b>
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| 45 | <p>
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| 46 | The <i>G4Event</i> class object should have a set of primary particles
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| 47 | when it is sent to <i>G4EventManager</i> via <tt>processOneEvent()</tt>
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| 48 | method. It is the mandate of your <i>G4VUserPrimaryGeneratorAction</i>
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| 49 | concrete class to send primary particles to the <i>G4Event</i>
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| 50 | object.
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| 51 | <p>
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| 52 | The <i>G4PrimaryParticle</i> class represents a primary particle
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| 53 | with which Geant4 starts simulating an event. This class object
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| 54 | has information on particle type and its three momenta. The
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| 55 | positional and time information of primary particle(s) are stored
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| 56 | in the <i>G4PrimaryVertex</i> class object and, thus, this class object
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| 57 | can have one or more <i>G4PrimaryParticle</i> class objects which
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| 58 | share the same vertex. As shown in Fig.?.?, primary vertexes and
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| 59 | primary particles are associated with the <i>G4Event</i> object by
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| 60 | a form of linked list.
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| 61 | <p>
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| 62 | A concrete class of <i>G4VPrimaryGenerator</i>, the
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| 63 | <i>G4PrimaryParticle</i> object is constructed with either
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| 64 | a pointer to <i>G4ParticleDefinition</i> or an integer number
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| 65 | which represents P.D.G. particle code. For the case of some
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| 66 | artificial particles, e.g., geantino, optical photon, etc.,
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| 67 | or exotic nuclear fragments, which the P.D.G. particle code does
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| 68 | not cover, the <i>G4PrimaryParticle</i> should be constructed
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| 69 | by <i>G4ParticleDefinition</i> pointer. On the other hand,
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| 70 | elementary particles with very short life time, e.g., weak
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| 71 | bosons, or quarks/gluons, can be instantiated as <i>G4PrimaryParticle</i>
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| 72 | objects using the P.D.G. particle code. It should be noted that,
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| 73 | even though primary particles with such a very short life time
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| 74 | are defined, Geant4 will simulate only the particles which
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| 75 | are defined as <i>G4ParticleDefinition</i> class objects. Other
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| 76 | primary particles will be simply ignored by <i>G4EventManager</i>.
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| 77 | But it may still be useful to construct such "intermediate"
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| 78 | particles for recording the origin of the primary event.
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| 79 | <p>
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| 80 | <b>Forced decay channel</b>
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| 81 | <p>
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| 82 | The <i>G4PrimaryParticle</i> class object can have a list of its
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| 83 | daughter particles. If the parent particle is an "intermediate"
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| 84 | particle, which Geant4 does not have a corresponding <i>G4ParticleDefinition</i>,
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| 85 | this parent particle is ignored and daughters
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| 86 | are assumed to start from the vertex with which their parent is associated.
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| 87 | For example, a Z boson is associated with a vertex and it has
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| 88 | positive and negative muons as its daughters, these muons will
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| 89 | start from that vertex.
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| 90 | <p>
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| 91 | There are some kinds of particles which should fly some reasonable
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| 92 | distances and, thus, should be simulated by Geant4, but you still
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| 93 | want to follow the decay channel generated by an event generator.
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| 94 | A typical case of these particles is B meson. Even for the case of
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| 95 | a primary particle which has a corresponding <i>G4ParticleDefinition</i>,
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| 96 | it can have daughter primary particles. Geant4 will trace the
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| 97 | parent particle until it comes to decay, obeying multiple
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| 98 | scattering, ionization loss, rotation with the magnetic field, etc.
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| 99 | according to its particle type. When the parent comes to decay,
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| 100 | instead of randomly choosing its decay channel, it follows the
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| 101 | "pre-assigned" decay channel. To conserve the energy and the momentum
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| 102 | of the parent, daughters will be Lorentz transformed according to
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| 103 | their parent's frame.
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| 104 | <p>
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| 105 |
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| 106 | <HR>
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| 107 | <a name="3.6.2">
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| 108 | <H2>3.6.2 Interface to a primary generator</H2></a>
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| 109 |
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| 110 | <b><i>G4HEPEvtInterface</i></b>
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| 111 | <p>
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| 112 | Unfortunately, almost all event generators presently in use,
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| 113 | commonly are written in FORTRAN. For Geant4, it was decided to not link with any
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| 114 | FORTRAN program or library, even though the C++ language syntax itself
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| 115 | allows such a link. Linking to a FORTRAN package might be convenient in
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| 116 | some cases, but we will lose many advantages of object-oriented
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| 117 | features of C++, such as robustness. Instead, Geant4 provides
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| 118 | an ASCII file interface for such event generators.
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| 119 | <p>
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| 120 | <i>G4HEPEvtInterface</i> is one of <i>G4VPrimaryGenerator</i>
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| 121 | concrete class and thus it can be used in your
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| 122 | <i>G4VUserPrimaryGeneratorAction</i> concrete class.
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| 123 | <i>G4HEPEvtInterface</i> reads an ASCII file produced by
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| 124 | an event generator and reproduces <i>G4PrimaryParticle</i> objects
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| 125 | associated with a <i>G4PrimaryVertex</i> object. It reproduces
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| 126 | a full production chain of the event generator, starting with
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| 127 | primary quarks, etc. In other words, <i>G4HEPEvtInterface</i>
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| 128 | converts information stored in the <tt>/HEPEVT/</tt> common block to
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| 129 | an object-oriented data structure. Because the <tt>/HEPEVT/</tt> common block
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| 130 | is commonly used by almost all event generators written in
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| 131 | FORTRAN, <i>G4HEPEvtInterface</i> can interface to almost
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| 132 | all event generators currently used in the HEP community.
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| 133 | The constructor of <i>G4HEPEvtInterface</i> takes the file name.
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| 134 | Source listing 3.6.1 shows an example how to use <i>G4HEPEvtInterface</i>.
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| 135 | Note that an event generator is not assumed to give a place of the
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| 136 | primary particles, the interaction point must be set before
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| 137 | invoking <i>GeneratePrimaryVertex()</i> method.
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| 138 | <p>
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| 139 | <center>
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| 140 | <table border=2 cellpadding=10>
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| 141 | <tr>
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| 142 | <td>
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| 143 | <PRE>
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| 144 | <FONT FACE="Courier New">
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| 145 | #ifndef ExN04PrimaryGeneratorAction_h
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| 146 | #define ExN04PrimaryGeneratorAction_h 1
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| 147 |
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| 148 | #include "G4VUserPrimaryGeneratorAction.hh"
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| 149 | #include "globals.hh"
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| 150 |
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| 151 | class G4VPrimaryGenerator;
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| 152 | class G4Event;
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| 153 |
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| 154 | class ExN04PrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction
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| 155 | {
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| 156 | public:
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| 157 | ExN04PrimaryGeneratorAction();
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| 158 | ~ExN04PrimaryGeneratorAction();
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| 159 |
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| 160 | public:
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| 161 | void GeneratePrimaries(G4Event* anEvent);
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| 162 |
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| 163 | private:
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| 164 | G4VPrimaryGenerator* HEPEvt;
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| 165 | };
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| 166 |
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| 167 | #endif
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| 168 |
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| 169 |
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| 170 | #include "ExN04PrimaryGeneratorAction.hh"
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| 171 |
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| 172 | #include "G4Event.hh"
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| 173 | #include "G4HEPEvtInterface.hh"
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| 174 |
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| 175 | ExN04PrimaryGeneratorAction::ExN04PrimaryGeneratorAction()
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| 176 | {
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| 177 | HEPEvt = new G4HEPEvtInterface("pythia_event.data");
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| 178 | }
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| 179 |
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| 180 | ExN04PrimaryGeneratorAction::~ExN04PrimaryGeneratorAction()
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| 181 | {
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| 182 | delete HEPEvt;
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| 183 | }
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| 184 |
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| 185 | void ExN04PrimaryGeneratorAction::GeneratePrimaries(G4Event* anEvent)
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| 186 | {
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| 187 | HEPEvt->SetParticlePosition(G4ThreeVector(0.*cm,0.*cm,0.*cm));
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| 188 | HEPEvt->GeneratePrimaryVertex(anEvent);
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| 189 | }
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| 190 | </font>
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| 191 | </pre>
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| 192 | </td>
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| 193 | </tr>
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| 194 | <tr>
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| 195 | <td align=center>
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| 196 | Source listing 3.6.1<BR>
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| 197 | An example code for <i>G4HEPEvtInterface</i>.
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| 198 | </td>
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| 199 | </tr>
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| 200 | </table></center>
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| 201 |
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| 202 |
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| 203 | <p>
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| 204 | <b>Format of the ASCII file</b>
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| 205 | <p>
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| 206 | An ASCII file, which will be fed by <i>G4HEPEvtInterface</i>
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| 207 | should have the following format.
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| 208 | <ul>
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| 209 | <li>The first line of each primary event should be an integer
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| 210 | which represents the number of the following lines of primary
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| 211 | particles.
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| 212 | <li>Each line in an event corresponds to a particle in the
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| 213 | <tt>/HEPEVT/</tt> common. Each line has <tt>ISTHEP, IDHEP, JDAHEP(1), JDAHEP(2),
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| 214 | PHEP(1), PHEP(2), PHEP(3), PHEP(5).</tt> Refer to the <tt>/HEPEVT/</tt> manual for
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| 215 | the meanings of these variables.
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| 216 | </ul>
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| 217 | Source listing 3.6.2 shows an example FORTRAN code to generate an ASCII file.
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| 218 | <p>
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| 219 | <center>
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| 220 | <table border=2 cellpadding=10>
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| 221 | <tr>
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| 222 | <td>
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| 223 | <pre>
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| 224 | ***********************************************************
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| 225 | SUBROUTINE HEP2G4
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| 226 | *
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| 227 | * Convert /HEPEVT/ event structure to an ASCII file
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| 228 | * to be fed by G4HEPEvtInterface
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| 229 | *
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| 230 | ***********************************************************
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| 231 | PARAMETER (NMXHEP=2000)
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| 232 | COMMON/HEPEVT/NEVHEP,NHEP,ISTHEP(NMXHEP),IDHEP(NMXHEP),
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| 233 | >JMOHEP(2,NMXHEP),JDAHEP(2,NMXHEP),PHEP(5,NMXHEP),VHEP(4,NMXHEP)
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| 234 | DOUBLE PRECISION PHEP,VHEP
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| 235 | *
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| 236 | WRITE(6,*) NHEP
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| 237 | DO IHEP=1,NHEP
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| 238 | WRITE(6,10)
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| 239 | > ISTHEP(IHEP),IDHEP(IHEP),JDAHEP(1,IHEP),JDAHEP(2,IHEP),
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| 240 | > PHEP(1,IHEP),PHEP(2,IHEP),PHEP(3,IHEP),PHEP(5,IHEP)
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| 241 | 10 FORMAT(4I10,4(1X,D15.8))
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| 242 | ENDDO
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| 243 | *
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| 244 | RETURN
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| 245 | END
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| 246 | </pre>
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| 247 | </td>
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| 248 | </tr>
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| 249 | <tr>
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| 250 | <td align=center>
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| 251 | Source listing 3.6.2<BR>
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| 252 | A FORTRAN example using the <tt>/HEPEVT/</tt> common.
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| 253 | </td>
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| 254 | </tr>
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| 255 | </table></center>
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| 256 | <p>
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| 257 | <b>Future interface to the new generation generators</b>
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| 258 | <p>
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| 259 | Several activities have already been started for developing
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| 260 | object-oriented event generators. Such new generators can
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| 261 | be easily linked and used with a Geant4 based simulation.
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| 262 | Furthermore, we need not distinguish a primary generator from
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| 263 | the physics processes used in Geant4. Future generators can be
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| 264 | a kind of physics process plugged-in by inheriting
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| 265 | <i>G4VProcess</i>.
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| 266 | <p>
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| 267 |
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| 268 | <HR>
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| 269 | <a name="3.6.3">
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| 270 | <H2>3.6.3 Event overlap using multiple generators</H2></a>
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| 271 |
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| 272 | Your <i>G4VUserPrimaryGeneratorAction</i> concrete class can
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| 273 | have more than one <i>G4VPrimaryGenerator</i> concrete class.
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| 274 | Each <i>G4VPrimaryGenerator</i> concrete class can be accessed
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| 275 | more than once per event. Using these class objects, one
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| 276 | event can have more than one primary event.
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| 277 | <p>
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| 278 | One possible use is the following. Within an event,
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| 279 | a <i>G4HEPEvtInterface</i> class object instantiated with a
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| 280 | minimum bias event file is accessed 20 times and another
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| 281 | <i>G4HEPEvtInterface</i> class object instantiated with a
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| 282 | signal event file is accessed once. Thus, this event represents
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| 283 | a typical signal event of LHC overlapping 20 minimum bias
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| 284 | events. It should be noted that a simulation of event
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| 285 | overlapping can be done by merging hits and/or digits
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| 286 | associated with several events, and these events can be
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| 287 | simulated independently. Digitization over multiple events
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| 288 | will be mentioned in <a href="../Detector/digitization.html">Section 4.5</a>.
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| 289 |
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| 290 | <BR><BR>
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| 291 | <HR>
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| 292 | <A HREF="../../../../Authors/html/subjectsToAuthors.html">
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| 293 | <I>About the authors</I></A>
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| 294 |
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| 295 | </BODY>
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| 296 | </HTML>
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